effect of relative humidity and co2 concentration on the ... · yaroslav bilan master of applied...

Effect of Relative Humidity and CO2 Concentration on the Properties of Carbonated

Reactive MgO Cement Based Materials

by

Yaroslav Bilan

A thesis submitted in conformity with the requirements

for the degree of Master of Science

Graduate Department of Civil Engineering

University of Toronto

© Copyright by Yaroslav Bilan (2014)

ii

Effect of Relative Humidity and CO2 Concentration on the Properties of Reactive MgO Cement

Based Materials

Yaroslav Bilan

Master of Applied Science

Graduate Department of Civil Engineering

University of Toronto

2014

Abstract

Sustainability of modern concrete industry recently has become an important topic of scientific

discussion, and consequently there is an effort to study the potential of the emerging new

supplementary materials. This study has a purpose to investigate the effect of reactive magnesia

(reactive MgO) as a replacement for general use (GU) Portland Cements and the effect of

environmental factors (CO2 concentrations and relative humidity) on accelerated carbonation

curing results, The findings of this study revealed that improvement of physical properties is

related directly to the increase in CO2 concentrations and inversely to the increase in relative

humidity and also depend much on %MgO in the mixture. The conclusions of this study helped

to clarify the effect of variable environmental factors and the material replacement range on

carbonation of reactive magnesia concrete materials, as well as providing an assessment of the

optimal conditions for the effective usage of the material.

iii

Acknowledgments

This important study would have not be possible without the financial support provided by

Ministry of Economic Development and Innovation for support by Professor Daman Panesar's

Early Researcher Award (ERA) which provided the funding for this research.

In addition to this, the author of this study wants to express his most sincere appreciation to the

support, advice and guidance of Professor Daman Panesar, who was a research supervisor for

this study. The council of other Concrete Material Group Professors, namely Prof. Doug

Hooton, and Karl Peterson has been also immensely important for the successful outcome of

this research.

The support of the whole Concrete Materials Group was surely significant and is much

appreciated. Finally the author wants to thank deeply from his heart to the selfless and

dedicated support and care from Olga Perebatova. Without a doubt, this study would not be

possible without her critical help during the long and hard work in a laboratories when

conducting the tests for this study.

iv

Table of Contents

Effect of Relative Humidity and CO2 Concentration on the Properties of Reactive MgO Cement

Based Materials i

Abstract ii

Acknowledgements iii

Table of contents iv

List of Figures vii

List of Tables ix

1.Introduction 1

1.1 Motivation for Study 2

1.2 Research Objectives 4

2. Literature Review 6

2.1 Conventional Cement-Based Materials and Carbonation 6

2.2 Reactive Magnesia Cements 12

2.2.1 Hydration, Chemical Composition and Physical Properties 12

2.2.2 Mechanics of Carbonation of Reactive MgO as Cement Replacement and the Effect on

Compressive Strength, Porosity and Durability Properties of the Material 14

2.3 Competition Between Hydration, Carbonation and Influence of Environmental and Material

Conditions 15

3.Experimental Program 18

3.1 Materials and Mix Design 18

3.2 Sample Preparation and Curing 18

3.3 Testing Procedure 20

3.3.1 Sample Preparation and Drying Procedures 20

3.3.2 Carbonation Front 21

3.3.3 Compressive Strength 21

v

3.3.4 Chemical Composition Tests 21

3.3.5. Mercury Intrusion Porosimetry (MIP) 21

4. Results and Discussion 22

4.1 Chemical Analysis – XRD, DT and TGA 22

4.1.1 Carbonation Front 22

4.1.2 X-ray Diffraction Results 25

4.1.3 DTA/TG Results 36

4.2 Mechanical Properties 41

4.2.1 Effect of Reactive MgO on Compressive Strength 41

4.2.2 Strength-Porosity Correlation 48

4.2.3 Effect of CO2 on Compressive Strength 51

4.2.4 Effect of Relative Humidity on Compressive Strength 56

4.3 Porosity 56

5. Interplay between Chemical Analysis and Mechanical Properties 61

5.1 Interplay between Chemical and Physical properties and the Effect of CO2 Concentration

61

5.1.1 Compressive strength 61

5.1.2 Porosity 64

5.1.3 Carbonation front 65

5.2 Interplay between Chemical and Physical properties and the Effect of Relative Humidity

65

5.2.1 Compressive Strength 65

5.2.2 Carbonation and Chemistry of the Carbonation Zone 66

5.2.3 Carbonation and Porosity 67

6. Conclusions 79

vi

7. Recommendation for Future Research 82

8. Appendix 84

9. References 96

vii

List of Figures

Figure 2.1: Compressive Strength- Total Porosity Relationship for Conventional Concrete

9

Figure 2.2: Compressive Strength- Total Porosity Relationship for Conventional Paste and

Mortar 10

Figure 2.3: Compressive Strength- Permeability Relationship for Conventional Concrete

10

Figure 2.4: Compressive Strength- Total Porosity Relationship for Carbonated

Concrete 11

Figure 4.1: Condition [75%RH, 50%CO2] XRD Pattern of Mortar a) 3d, b) 28d 32

Figure 4.2: Condition [75%RH, 50%CO2] XRD Pattern of Paste a) 3d, b) 28d 33



Figure 4.5: Condition: [75%RH, 99%CO2] Influence of MgO on DT at Day 3 38

Figure 4.6: Condition: [75%RH, 99%CO2] Influence of MgO on DT at Day 28 39

Figure 4.7: Condition: [75%RH, 99%CO2] Influence of MgO on TGA at Day 3 39

Figure 4.8: Condition: [75%RH, 99%CO2] Influence of MgO on TGA at Day 28 40

Figure 4.9: Condition: [75%RH, 50%CO2] Effect of MgO on 3, 7, and 28d compressive

strength 45


strength 45


strength 46


strength 46


strength 47

viii


strength 47

Figure 4.19: Correlation between Compressive Strength and Total Porosity for 0%MgO mortar

49

Figure 4.20: Correlation between Compressive Strength and Total Porosity for 20%MgO

mortar 51


mortar 52


mortar 52

Figure 4.23: Effect of CO2 Concentration on Compressive Strength Development of 0%MgO

mortar, a) Relative Humidity - 75%, b) Relative Humidity - 50% 52







Figure 5.1: Correlation between Compressive Strength and Carbonated Area for 0%MgO

mortar 62


mortar 63


mortar 63


mortar 64

Figure B.1: Condition: [75%RH, 50%CO2] Effect of MgO on 3, 7, and 28d carbonated area

76


76

ix


77


77


78


78

Figure C.1: Condition: [75%RH, 50%CO2] Effect of MgO on 3, 7, and 28d total porosity

79


79


80


80

Figure C.5: Condition: [50%RH, 75%CO2] Effect of MgO on 3, 7, and 28d total

porosity 81

Figure C.6: Condition: [50%RH, 99%CO2] Effect of MgO on 3, 7, and 28d total

porosity 81

Figure D.1: Condition [50%RH, 50%CO2] XRD Pattern of Paste a) 3d, b) 28d 84



Figure D.4: Condition: [50%RH, 50%CO2] Influence of MgO on TGA at a) Day 3, b) Day 28

87


88


89

x

Figure E.1: Effect of Relative Humidity on Compressive for Mixture with 0% of MgO a) 50%

CO2, b) 75% CO2, c) 99% CO2 91


CO2, b) 75% CO2, c) 99% CO2 92


CO2, b) 75% CO2, c) 99% CO2 94


CO2, b) 75% CO2, c) 99% CO2 95

xi

List of Tables

Table 3.1: Chemical Composition of Cementitious Materials ...................................................18

Table 3.2: Mortar and Paste Mixture Design Proportions ........................................................19

Table 3.3: Carbonation Curing Scenarios ............................................................................20

Table 4.1: Carbonated Area of Specimens cured in 75%RH ..................................................23

Table 4.2: Carbonated Area of Specomes cured in 50%RH .....................................................24

Table 4.3: X-ray Diffraction Main Peaks Data .....................................................................26

Table 4.4: Influence of Age, Percentage of MgO and CO2 Concentration on X-ray Diffraction

Results ............................................................................................................................27

Table 4.5: DTA/TG data for Separate Peaks, Condition [75%RH, 99%CO2] .........................38

Table 4.6: Mean Compressive Strength (f'c) and Coefficient of Variation (COV) for 75% RH

.........................................................................................................................................42

Table 4.7: Mean Compressive Strength (f'c) and Coefficient of Variation (COV) for 50% RH

.......................................................................................................................................43

Table 4.8: Percentage Change in Compressive Strength Due to MgO used as Cement

Replacement ....................................................................................................................44

Table 4.9: Percentage Change in Compressive Strength Due to MgO used as Cement

Replacement .......................................................................................................................57

Table 4.10: Total Paste Intruded Porosity ...........................................................................58

Table 4.11: Percentage Change in Total Mortar Porosity Due to MgO used as Cement

Replacement .....................................................................................................................59

Table 4.12: Percentage Change in Total Paste Porosity Due to MgO used as Cement

Replacement ...................................................................................................................60

Table A.1: Percentage Change in Compressive Strength Due to MgO used as Cement

Replacement Compared to 0%MgO Mixes ...........................................................................74

Table A.2: Percentage Change in Compressive Strength Due to MgO used as Cement

Replacement (Incremental) ..................................................................................................75

Table C.1: Percentage Change in Total Mortar Porosity Due to MgO used as Cement

Replacement (Incremental) .................................................................................................82

xii

Table C.2: Percentage Change in Total Paste Porosity Due to MgO used as Cement

Replacement (Incremental) ................................................................................................83

1

1. Introduction

Reactive magnesia (r-MgO) is a relatively new binder type that has cementitious properties,

similarly to Portland Cement (PC) under certain conditions. This new binder can potentially

partially replace ordinary PC cements, which are known for their significant carbon dioxide

(CO2) emissions during production. This material is not to be confused with ordinary magnesia

(also known as periclase) that can be found in low-quality PC and is known for its low

reactivity that leads to material's dimensional instability.

Reactive MgO has a relatively smaller (~120 microns, and less) particle size compared to

conventional magnesia and resultantly a larger surface area. Hence, the reactivity of reactive

magnesia is improved drastically compared to periclase and is also whilst lower is still

relatively comparable to that of PC. This process is discussed more in depth in section 2.1.1

The binder is produced at lower (650-750°C) temperature, and it means that CO2 emissions for

a mass unit are reduced. (Thomas et al, 2007). Reactive magnesia-based materials do not just

have lower carbon dioxide emission costs, but can also be used to sequester CO2 inside its

structure more effectively, thus sealing and removing this major greenhouse gas from the

atmosphere. While the overall emissions during the production of ordinary magnesia are around

1.4 t for 1 t of material produced (almost double of 0.85 t PC production emissions), new

production technologies together with sequestration can potentially decrease the emissions. The

environmental benefits from such solutions should not be underestimated (Liska and Al-Tabbaa

2008; Taylor 1990).

Sequestration of CO2 inside concrete material is possible through carbonation reactions. This in

turn changes concrete's properties such as compressive strength, density, permeability but also

its alkalinity. As the carbonation reaction is exothermic, it also affects hydration kinetics, and

consequently strength gain of the material. It is possible to use these features of the reaction to

cure the concrete. This curing regime is known as accelerated carbonation curing. Using higher

than normal atmospheric (~400 ppm) carbon dioxide environment, controlling relative humidity

and temperature, the carbonation is facilitated significantly. However, it should be noted that

several researchers who have examined carbonation curing of reactive MgO, have used various

CO2 concentrations, relative humidity and temperatures. Mo and Panesar (2012, 2013) studied

reactive MgO pastes in conditions of 99% CO2, and 98% relative humidity. Vanderper and Al-

2

Tabbaa (2007) used carbonation environments of 0.04, 5, and 20% CO2, and 65 and 95%

relative humidity for reactive MgO pastes. Liska and Al-Tabbaa (2008) studied masonry

concrete containing up to 10% reactive MgO and cured samples at 40°C, 98% relative humidity

and 20% CO2. Very recently, Unluer and Al-Tabbaa (2014) reported on reactive MgO concrete

blends with fly ash and they cured the specimens in CO2 ranging from 5 to 20%, and a relative

humidity ranging from 55 to 98%. Therefore, to date there is no consensus on the most

desirable curing conditions in terms of CO2 concentration and relative humidity in order to

achieve optimum material mechanical and transport properties. This is currently a recognized

gap in the knowledge reported in published literature.

Reactive magnesia materials are specifically interesting in this regard, as the carbonation

reaction has a number of features that are significantly different from ordinary PC-based

materials. These features affect resulting properties such as amount of CO2 sequestrated,

carbonation speed, porosity and the resulting strength and in generally may have a greater

practical usefulness.

A potential of such practical application largely depends not just on the properties of reactive

MgO, but also on different environmental variables in accelerated carbonation process. While

the carbonation of ordinary PC materials has been a subject of very many different research

publications, reactive MgO material's carbonation and especially the effect of the

aforementioned variables remains being covered very lightly.

1.1 Motivation for study

The environmental effect of CO2 emissions on the atmosphere temperature and ocean acidity

has been and is a very important subject in many spheres of science (Scott and Levine 2006).

The concerns of global warming and ocean acidification were brought back again as a new

serious warning has been received when in May 2013 the atmospheric CO2 concentration have

passed the 400 ppm (parts per million) point, and became the highest in the whole 55 years

history of measurement, and possibly even more than 3 to 20 million years. (Tans 2013). Being

a major greenhouse gas, CO2 content increase is believed to be a main reason of recent

temperature increase, commonly known as global warming. As this correlates with both human-

3

made CO2 emission increase and glaciers retreat, it is indicated by many analyses that a climate

change on a global scale is indeed a possible scenario (Stocker, 2013).

It is now widely accepted that the driver of such sudden change are carbon fossil fuel, cement

production and also deforestation. Thus, there is an international initiative to reduce man-made

carbon dioxide impact through various means. Emission control is the primary solution to

mitigate the impact. However, as industry is without a doubt remains a vital part of civilization,

and as the industry currently largely depends on fossil fuels, this reduction has a negative

impact on economy, especially in developing countries (Ghosh and Kanjilal, 2014). This

means that if any other emission process, such as cement production, can be controlled, the

pressure on the industry, and thus on economy of developing countries may be reduced

significantly. Apart of that, sustainability doctrines almost inevitably lead to the more effective

use of available resources that may in its turn improves the stability of economical

development.

The cement industry, as mentioned, is responsible for at least 5% of global man-made CO2

emissions (Mukherjee and Cass. 2012). The amount of Portland cement produced today

exceeds 2.5 billion tonnes per year (Taylor 1990) It is estimated that for each ton of cement

produced there is 0.85 t of CO2 emitted (Taylor 1990). As sustainability becomes one of the

most important tasks that lies before the scientific community in this century, a concrete in the

environmental perspective has become the subject of major discussions around the world. One

of the solutions to this problem was found in reducing the amount of conventional Portland

cement concrete used by replacing it with the so-called supplementary cementitious materials -

mostly byproducts of different industries. These products include pulverized fly ash (PFA),

ground-granulated blast furnace slag (GBFS) and silica fumes (Johari et al. 2011). In addition to

these, there are other binders that can potentially replace a part of PC in use, thus further

reducing CO2 output. One of such binders is reactive magnesia that as many other current and

potential supplementary materials may actually improve the physical properties of the concrete

in addition to being more sustainable alternative to conventional Portland cement concretes.

As mentioned above in section 1.0, accelerated carbonation curing is another potential solution

towards more sustainable construction materials. It allows to rationally use the available CO2

to facilitate carbonation, hydration of the concrete, but also to seal a gas inside its structure.

4

This essentially removes this greenhouse gas from the atmosphere and outer environment. The

effectiveness of this process would largely depend on the amount of CO2 being captured (Shao

et al., 2014; Caijun and Yanzhong, 2009). This leads to the conclusion, that it is possible to

effectively combine both material replacement and accelerated carbonation, and, possibly,

reaching truly effective sustainability results.

1.2 Research Objectives

The overall purpose of this research is to investigate the effect of carbonation curing conditions

(relative humidity and carbon dioxide concentration) on the chemistry (formation of

carbonation and hydration products); and physical properties (microstructure and compressive

strength) of cement based materials containing up to 60% of reactive MgO. Outcomes of this

study will reveal optimum carbonation curing conditions for reactive MgO products and the

sensitivity of the carbonation curing conditions on the reactive MgO materials properties

In order to achieve these research goals the following sub-tasks are identified:

1) Create a controlled curing chamber with appropriate curing conditions in which

concrete materials can be subjected to the specified environment necessary for

accelerated environment;

2) Examine the effect of six different curing conditions based on relative humidity and

carbon dioxide concentration on sequestration efficiency, chemical and physical

properties;

3) Study the chemical composition of a carbonated material and assess the hydration and

carbonation reactions products using X-ray diffraction and DT\TGA techniques;

4) Establish the most optimal environmental conditions and material replacement ranges

for the practical use. In addition the optimal size and shape of concrete elements can be

assessed based on carbonation depth achieved.

The conclusions on the most effective setup are essential to facilitate the integration of reactive

MgO products as a construction building material such as in industries that are dealing with

precast concrete elements - porous masonry blocks, bricks and other similar products.

5

2. Literature review

The subject of carbonation of conventional Portland Cement based materials, and those blended

with Supplementary Cementitious Materials (commonly known as SCMs) are well published in

scientific publications (Johari et al, 2011). In contrast carbonation of cement materials

containing reactive magnesia remains less understood, since relatively few studies have been

conducted on the subject.

When comparing Portland cement and reactive magnesia in terms of their carbonation

processes one must identify the key differences in the carbonation reaction mechanics.

Furthermore, it is necessary to understand the implications of carbonation on other properties,

such as hydration kinetics, initial porosity and chemical composition, and also how do all those

properties affect and compete with each other in the process must be studied extensively.

2.1 Conventional Cement-Based Materials and Carbonation

A topic of concrete carbonation is known to be a recurring theme of many building materials-

related studies. Today, the outcomes and conclusions of these studies are taken into account in

building materials development. However, carbonation of concrete is deemed to be such an

important topic because of its degrading effect on a steel reinforcement. As the bulk material

carbonates a major drop in alkalinity occurs, leading to depassivation of oxide protective film

on the steel reinforcement. As it is established, pH level drops from around 13 down to 9 and

less as a result of CaCO3 formation and further oxidation occurs, degrading the metal rebar.

The resulting corrosion leads to the loss of mechanical properties of the reinforcement, but also

to the formation of rust that induces local stresses and consequently damages the concrete. Such

a loss of predesigned properties that can potentially occur because of carbonation process was a

reason why for most of the time carbonation studies were concentrated on its negative

tendencies.

Carbonation processes have been well researched and it is well recognized that they have

potential to alter the microstructure of hydrated OPC (Johannesson and Utgenannt 2001). The

extent to which carbonation alters the microstructure is significant largely because of the well

recognized implications it could have on mechanical and durability properties. The general

6

strength-porosity relationship of solids is very well established. For basic homogeneous

materials it is commonly expressed as (Mehta and Montiero 2006):

S=Soe-kp

Where, S = strength of the material which has a given porosity, p

So = intrinsic strength at zero porosity

k = constant

For cement based materials, several decades ago, Powers (1958) found that the 28-day

compressive strength was related to the ratio between the solid hydration products (gel) and the

total space in the system (space) (gel/space ratio). Certainly, the exact compressive strength to

porosity relationship when plotted for paste, mortar, or concrete of different mix proportions,

materials, curing regime etc. will vary, however the general trend of increasing compressive

strength with decreasing porosity holds. This is apparent in Figure 2.1, which presented the

concrete compressive strength to total porosity as measured by mercury intrusion porosimetry

(MIP) by Das et al (2012), Kumar and Bhaltacharjee (2003), and Poon et al (2006). Figure 2.2

shows the compressive strength vs total porosity relationship for pasted and mortars using

Portland cement (Guneyisi et al 2008, Chindaprasirt et al 2005, Rossler and Odler 1985, Li et al

2006, and Kuo et al 2006). In all cases it is apparent that strength increases with reduced

porosity. Recognizing that durability performance is largely controlled by permeability and not

total porosity, many researchers also evaluate permeability and incontext with its relationship to

compressive strength it is generally recognized that the less connected the capillary pores, the

lower the strength and this is apparent in Figure 2.3 based on the study by Das et al (2012).

However, it should be noted that the relationship is not linear.

Carbonation processes alter the microstructure of hydrated OPC as a result of the reaction of

CO2 with the Ca(OH)2 present in hydrated paste or mortar (Johannesson and Utgenannt 2001).

The formation of CaCO3 product results in a densification of the paste’s microstructure, a

decrease in capillary pore volume and a decrease in total porosity. Several studies have

validated this observation as a result of various types of tests to measure changes in the

microstructural form such as: scanning electron microscopy images, mercury intrusion

7

porosimetry measurements, desorption isotherm determinations and chloride penetration tests

(Johannesson and Utgenannt 2001, Tumidajski and Chan 1996, Kropp and Hilsdorf 1995).

Figure 2.4 presented the compressive strength vs. total porosity relationship for carbonated

ordinary Portland cement concrete based on (Chang and Chen, 2005 and Liska and Al-Tabbaa

2008). In general it is observed that the strength-porosity relationship holds.

The ability of carbonation processes to occur and the implications of these process is influenced

by the type of cementing material used and its chemical composition. For example, considering

the surface layer of concrete containing GGBFS, the occurrence of carbonation reactions yields

an increase in capillary porosity in particular, the volume of very large capillary pores (>100

nm). The increase in porosity has been identified to be due to the formation of soluble

metastable calcium carbonates (Stark and Ludwig 1997, Tumidajski and Chan 1996).

Beyond the vulnerability of reinforced concrete elements to be subjected to higher likelihood of

corrosion due to the effect on the pH of the pore solution, other durability mechanisms can also

be affected such as freeze-thaw and de-icer salt scaling damage (Stark and Ludwig 1997,

Tumidajski and Chan 1996). Carbonation of ordinary Portland cement paste is expected to

decrease the amount of freezable capillary pore water owing to the more refined microstructure

whereas mixtures containing GGBFS will permit a greater amount of freezable pore water in

comparison to a non-carbonated specimen (Stark and Ludwig 1997).

The details of the chemistry of the carbonation processes are described. One of hydration

products that is affected by CO2 in the concrete material is 𝐶𝑎(𝑂𝐻)2, which comprises 25 to

50% wt of the total hydrated product (Muntean et al. 2008). In its simplest, the reaction can be

written as the following:

𝐶𝑎(𝑂𝐻)2 + 𝐶𝑂2 → 𝐶𝑎𝐶𝑂3 + 𝐻2𝑂

The nature of this physiochemical reaction is more complicated. First, CO2 dissolves in water,

forming carbonic acid - 𝐻𝐶𝑂3 :

𝐻2𝑂 + 𝐶𝑂2= 𝐻𝐶𝑂3− + H+

8

The resulting acid can react with calcium hydroxide alkali, and the following neutralization

reaction occurs:

𝐶𝑎(𝑂𝐻)2 + 2H+ + 𝐶𝑂32−

= 𝐶𝑎𝐶𝑂3 + 2𝐻2𝑂

The reaction’s main product is calcite. Since neutralization reactions are exothermic in nature

the standard free energy Gor and volume expansion of solid V are:

Gor = -64.62 kJ.mol-1

V = 3.22 moles

The reaction is highly energy intensive, and results in densification of the hydrated paste (Guo

et al. 2013) It must be taken into account although that this reaction effectively removes water,

and while this facilitates further carbonation, it also results in carbonation shrinkage (Garcia-

Gonzales et al. 2006).

In addition to this the primary carbonation reaction, CSH and unreacted cement also react with

CO2 to form carbonate compounds:

(3𝐶𝑎𝑂 ∗ 2𝑆𝑖𝑂2 ∗ 3𝐻2𝑂) + 3𝐶𝑂2 -> (3𝐶𝑎𝐶𝑂3*2𝑆𝑖𝑂2*3𝐻2𝑂)

and

(x 𝐶𝑎𝑂 *𝑆𝑖𝑂2) +2𝐶𝑂2 +n𝐻2𝑂 -> (𝑆𝑖𝑂2*n𝐻2𝑂) +2𝐶𝑎𝐶𝑂3

where x is number of moles (2 and 3 for 𝐶2𝑆 and 𝐶3𝑆 respectively), and n is a variable molar

content of the water. These reactions contribute to around 30% of CO2 sequestrated during the

carbonation and have much smaller impact compared to the primary reaction (Muntean et al.

2008).

In general, the carbonation process follows a pattern:

1) Calcium bearing compounds undergo decalcination during which they are gradually

dissoluted, and Ca+ ions start to fill the water in pore solution;

2) 𝐶𝑂2 is absorbed into water and both carbonate (𝐶𝑂32−

and bicarbonate(𝐻𝐶𝑂3−) ions form;

9

3) As these ions form a supersaturated solution of 𝐶𝑎𝐶𝑂3, calcite starts precipitating, filling the

pore space gradually;

As mentioned earlier, as the 𝐶𝑂2 penetration front forms calcite layers, the resulting

densification of microstructure gradually obstructs the intake and in turn slows the carbonation

reaction (Rostami et al, 2012).

Total Porosity (%)

0 2 4 6 8 10 12 14 16 18

Concr

ete

Co

mpre

ssiv

e S

tren

gth

(M

Pa)

0

20

40

60

80

100

120

Das (2012)

Kumar (2008)

Poon (2006)

Trend Line

Figure 2.1: Compressive Strength- Total Porosity Relationship for Conventional Concrete

10

Total Porosity (%)

0 5 10 15 20 25 30 35 40

Co

mpre

ssiv

e S

tren

gth

(M

Pa)

0

20

40

60

80

100

120

140

Guneyisi (2008)

Chindaprasirt (2005)

Rossler (1984)

Li (2006)

Kuo (2006)

Trend Line

Figures 2.2: Compressive Strength- Total Porosity Relationship for Conventional Paste and Mortar

Permeability (index)

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Concr

ete

Com

pre

ssiv

e S

tren

gth

(M

Pa)

0

10

20

30

40

50

60Das (2012)

Figure 2.3: Compressive Strength- Permeability Relationship for Conventional Concrete

11

Total Porosity (%)

0 2 4 6 8 10 12 14 16 18

Concr

ete

Co

mpre

ssiv

e S

tren

gth

(M

Pa)

0

10

20

30

40

50

60

70ACI-102-M36

Liska (2008)

Trendline

Figure 2.4: Compressive Strength- Total Porosity Relationship for Carbonated Concrete

2.2 Reactive magnesia cements

2.2.1 Hydration, Chemical Composition and Physical Properties.

Reactive magnesia is known to react with water and harden into a solid bulk material. When

magnesia reacts with water, the following reaction occurs:

𝑀𝑔𝑂 + 𝐻2𝑂 ->𝑀𝑔(𝑂𝐻)2

While brucite is the only product of this reaction, it has been reported that a formation of

brucite hydrate (Mg(OH)2 ∗ nH2O ) may occur if a sufficiently high amount of water is

available for the reaction (Harrison, 2003). In addition to these reactions, if magnesia is used in

conjunction with Portland Cements the formation of M-S-H gel can happen, however it is

influenced by alkalinity of the material (Zhang et al. 2011). The overall effect of brucite on

strength development was found to be negative, both because of bigger water-to-cement ration

needed for the reaction to occur (that in its turn increases the amount of water needed for a

12

specific workability), and also because of slower hydration rate of magnesia (Cwirzen and

Habermehl-Cwirzen, 2013).

Various crack-producing dimensional instabilities are traditionally linked with brucite

formation in concrete. Indeed, the presence of dead-burn magnesia in cement, also known as

periclase has been found to affect mechanical properties of concrete in a degrading way

(Nokken 2010). This primarily occurs because of very low reactivity of periclase, that means

that most of reaction-produced expansion happens at later ages. This expansion induces tensile

stresses in the concrete material, which may lead to uncontrolled cracking and overall material

deterioration. On the other hand, reactive magnesia has a hydration rate similar to that of PC,

and thus the mentioned instabilities can be avoided.

The topic of reactivity of various reactive magnesia products and the extent of various factors

influencing that has been a subject of major debates and discussion in the literature due to its

importance in refractory applications (Salomao et al, 2007). It has been defined that the

hydration of magnesia is crucially dependent on surface area of a particle that is undergoing

solvation when contacting with water molecules (Salomao and Pandolfelli, 2007; Harrison,

2010; Pera and Soudee, 2001). Therefore, in a bulk hydrating material the degree of saturation

and thus the speed of a reaction is also dependent on how quickly various magnesia particles

react with water. The effort has been done to define the effect of surface area, and thus specific

surface area or SSA has been used to explain the process (Stumm, 1992). SSA takes into

account the available surface for the dissolution and is affected not just by the size of a particle

but also structural defects and disorderly packing of the molecules in the crystalline structure.

Other factors, such as pH have also been identified to influence both rate and mechanism of the

reaction (Souza et al, 2014; Filippou et al, 1999).

Various factors affecting the initial SSA have been defined in the research. Most important of

these are the calcining temperature (e.g. a temperature at which magnesite is decomposed

during the production process) and grinding size (Suvorov and Nazmiev, 2007). The relevance

and importance of these factors was a subject of several scientific studies. It was found that the

effect of calcining temperature is a dominant in this regard. That is primarily because of a

different entropy and enthalpy energy induced into a forming magnesia crystal (Harrison, 2010;

Salomao et al, 2007; Rocha et al, 2004). Higher temperatures are giving in excess energy, and

13

this allows more orderly crystalline lattice to form. Since a highly ordered lattice of periclase

crystal has high (>3795 Kj*mol-1) lattice energy this kinetic barrier is is significantly harder to

overcome by the energy of solvation. Several studies have underlined the importance of low

calcining temperatures, and how is this factor more relevant in comparison to grind size

(Harrison, 2010; Blaha 1997).

2.2.2 Mechanics of carbonation of reactive MgO as Cement Replacement and the effect

on compressive strength, porosity and durability properties of the material.

Carbonation in r-MgO based mixtures is more complex and depends on several of variables.

The primary carbonation reaction in materials containing only r-MgO yields nesquehonite:

𝑀𝑔(𝑂𝐻)2 + 𝐶𝑂2 ->𝑀𝑔𝐶𝑂3 ∗3𝐻2𝑂

Same as with PC analogue, the reaction requires water to be present for the reaction to occur. In

contrast though it directly binds water molecules, instead of just removing them from solid

phases. This directly means that the reaction is considerably more water demanding

(Vandeperre and Al-Tabbaa, 2007). The reaction is much different in comparison to its PC

counterpart in other aspects as well. It is much less energy intensive. In addition the solid

volume change is significantly larger:

Gor = -38.73 kJ.mol-1

V = 50.47 moles

Apart from nesquehonite, many other compounds can form. They include landsfordite

( 𝑀𝑔𝐶𝑂3*5𝐻2𝑂), hydromagnesite (𝑀𝑔2𝐶𝑂3(𝑂𝐻)2*3𝐻2𝑂) and others (Harrison, 2003). These

numerous compounds may only form in certain specific relative humidity, temperature, partial

CO2 pressure among them. The carbonation proceeds in a similar to PC pattern, when brucite

(Mg(OH)2) dissolves and yields Mg2+ ions to the solution, which then reacts with CO2 based

ions in the water. However, magnesium hydroxide is considerably less soluble (0.009/100 ml

compared to CH 0.185/100 ml) in water, so in normal conditions the amount of material

carbonated will most likely be comparatively low (Mo and Panesar 2012). This itself is also

14

influenced by acidity of the pore solution, that may also regulate the overall speed of the

reaction (De Silva at al, 2009).

A special case is represented by Ca-Mg carbonates that readily form in r-MgO-PC mixes. Main

product of such reaction is magnesium calcite:

𝐶𝑎(𝑂𝐻)2 + 𝑀𝑔(𝑂𝐻)2 + 𝐶𝑂2 → (𝐶𝑎,𝑀𝑔)𝐶𝑂3

Because of low solubility of brucite means that under normal conditions only a fraction of

magnesium ions will precipitate in the solution, similarly as in nesquehonite formation.

However as the amount of MgO increases, this changes considerably (Mo and Panesar 2013,

Liska and Al-Tabbaa 2008). This compound is often taking most of Mg2+ ions from the

solution, and it may explain why very small amount of nesquehonite forms in calcium bearing

mixes. Moreover, because of uneven solubility of brucite and portlandite the ratio of Mg2+ and

Ca2+ ions in the precipitated and also because of nucleation and separation of both ion's

enriched zones, solid material is heterogeneous. This in its turn affects the impact of bulk

magnesium calcite on the physical properties of a carbonated material (De Silva et al, 2009, Mo

and Panesar, 2012).

While the carbonation process is varying under many different factors, it itself affects physical

properties of the material where it occurs. As discussed in 2.1, the subject has been well

covered in a literature for ordinary PC materials. On the other hand, several studies has assessed

the changes in microstructure due to carbonation in reactive magnesia containing materials (Mo

and Panesar, 2012, Vandeperre and Al-Tabbaa, 2007) . This effect of carbonation on

microstructure can at first be summarized by reviewing the change in molar volume of solid

phases that occur due to carbonation. If the sole formation of nesquehonite is considered, there

is a change of 24.3 g/l-1

to around 75 g/l-1

which results in ~400% expasion. This is a

significant increase compared to ~3% expansion of 33 g/l-1

to 36.9 g/l-1

when calcite forms from

portlandite. When Mg-PC blends are considered, the amount of nesquehonite product is very

variable, and depends highly on the mix composition and curing conditions. Instead a formation

of magnesium calcite was reported to be prevalent. In its turn magnesium calcite has a different

molar volume due to varying Ca:Mg ratio in the forming product, that depends on several

factors (Mo and Panesar, 2013). Thus, the effect of carbonation cannot be estimated accurately

15

by empirical means. Studies of porosity using different techniques, such as MIP and SEM have

been conducted, and the decrease of porosity was found to be generally consistent with

presence of carbonated phases(Mo and Panesar, 2012, Shi and Wu, 2009, Liska and Al-Tabba,

2009 and others). It has not been entirely clear though what input each particular carbonate

phase has on such densification on microstructure.

Only small fraction of reactive magnesia studies have made an effort to cover a durability

aspects and properties. In general, an increase in early shrinkage resistance has been reported

for magnesia containing products, while the freeze-thaw resistance was found to be decreasing

due to %MgO (Choi et al 2014, Cwirzen and Habermehl-Cwirzen, 2013 and Choi et al, 2014).

2.3 Competition between hydration, carbonation and influence of environmental and

material conditions

Irrespective of if Portland cement is used alone or in the presence of reactive MgO or other

cement replacement materials, there are several competing processes involved when the

cement-based material is being subjected to carbonation. First and the foremost, the cement

hydrates, yielding more and more hydrated products that quickly fill the gaps in the initial pore

structure. Second, a parallel process is a carbonation of a material, first hydrated and then non-

hydrated if conditions allow them to be included in the process. Carbonation in its turn

densifies the pore structure of a material, when molecules of carbonate compounds precipitate

at walls inside the pores. Both reactions are exothermic, and both increase the speed at which

cement hydrates and further fills the pores. Considering these factors, as the path for

becomes more and more obstructed so the absorption slows, and so carbonation gradually

decrease its intensity (Shi and Wu, 2009). Furthemore, the obstruction of pore network due to

both carbonation and hydration leads to another phenomena. As more and more [Mg2+

] and

[Ca2+

] dissolves into the pore solution, and more carbonates precipitate on the sides, the flow

of each ion is becoming more and more restricted, and thus several zones start to occur where

the ratio of each ion is more prevalent. In its turn this leads to formation of nesquehonite, and

high-Mg magnesium calcite, both of which have different morphology and impact on the

material properties (De Silva et al, 2009, Mo and Panesar, 2012).

16

Moreover, there is an increased water demand for both hydration and carbonation reactions to

occur, but even more so in case of magnesium containing mixes (He et al, 2003). Although CH

releases water, and magnesia compounds consume them, there is still need for the water

solution to be present in order for the reaction to occur. As the relative humidity changes, so

does the amount of water in the solution. However if there is too much water, it will block the

pore system thus only allowing the much slower diffusion to take place of absorption .

All these factors indicate that the single most important period for the efficient carbonation is

early age. The governing condition in this case would be initial porosity - a porosity at the

moment of a beginning of accelerated carbonation curing. The properties such as density of the

material, degree of compaction, amount of free water in the pore network, and finally speed of

early hydration and carbonation - would all affect early age carbonation considerably (Mo and

Panesar, 2013).

The optimal ranges for relative humidity needed for an effective accelerated carbonation of

conventional concretes were extensively studied. Some reports indicate that optimal conditions

lie in the range of 60 to 75% (Shi, and Wu, 2009, Sulapha et al, 2003), while others reported

broader ranges from 40 to 80% (Sisomphon and Lutz, 2007). In some researches, ranges of 65

to 70% were successfully used to carbonate specimens based on plain Portland cements.

However, the more complicated chemistry of brucite carbonation means that these values might

not be sufficiently accurate for cements containing larger amount of r-MgO.

Since the penetration of a carbonating material by CO2 is primarily governed by permeability, it

is justified to mention the effect of pores other than the capillary. It is known that there is a

negative effect of entrapped air pores on durability that may occur during the mixing process if

inadequate compaction has been achieved (Lomboy and Wang, 2009). These entrapped air

voids that have a size greater than 1000µm are frequently penetrated by a capillary system thus

increasing the interconnectivity of pores and consequently permeability of the concrete

(Scherer, 2008; Kim et al, 2007, Balaguru and Ramakrishan, 1989). Thus, when high water

demand of magnesia mixes comes into play, it is possible that the amount of these voids that

occur during the mixing will be relatively greater with increasing %MgO.

17

As mentioned before, both hydration and carbonation processes in these concrete materials

have considerably larger water demand. Therefore while lower humidity values may allow a

better ingress of CO2 deeper inside the material's structure, higher amount of water in pores

may facilitate the onset of a reaction. CO2 concentration is also known to have a considerable

effect on efficiency of carbonation curing. Typically the concentrations of 20-30% and higher

are to be used for the carbonation curing to be accelerated (Liska and Al-Tabba, 2009,

Vandeperre and Al-Tabbaa, 2007). However in many studies concentrations have been close to

maximum - 90-99% (Mo and Panesar, 2012, Monkman and Shao, 2006). It is, however, not

entirely clear, what overall effect do intermediate concentration ranges have on the subject. In

addition to that, no study has been conducted to examine possible interplay between different

humidity and CO2 concentration, which is as pointed out even more important with the r-MgO

case.

The effect of cement composition is also known to have a significant influence on the rate

mechanism and also on the effect of carbonation. The reactivity of magnesia or conventional

cement, SCM replacement effect and other have been studied extensively (Mo and Panesar,

2013, Borges et al, 2012). It was indicated in several studies that chemical composition is

affected by ratio between PC and r-MgO. For instance nesquehonite formation was found to

increase dramatically when this ratio is around 1:1 and higher (Vandeperre and Al-Tabbaa,

2007). The possibility of formation of other compounds such as landsfordite was also been

found to relate to the cement composition (De Silva et al, 2009).

Therefore it is evident that more extensive approach towards the examination of effect of these

material and environmental variables. The replacement ratios up to 60% of MgO were chosen

in this research to further see its role on the carbonation and also its dependency over different

environmental conditions that must be replicated to cover the gaps in the existing research.

Early age must be more carefully examined, and this means the effect of the variables in this

period must be related to time needed to fully carbonate the concrete specimen. In this way it

might be possible to narrow the choice of both optimal conditions and more effective r-MgO

replacement ratios for the most effective CO2 sequestration without compromising physical

properties and microstructure.

18

3.Experimental program

3.1 Materials and Mix Design

For this study, general use (GU) Portland cement was supplied by Holcim Canada. Reactive

magnesia cement from Liyang Special Materials Company, China, that was prepared by

calcining of magnesite under the temperature of 800°C. The chemical composition of

cementitious materials can be viewed in Table 3.1

Table 3.1: Chemical composition of cementitious materials

Oxide General Use (GU) Reactive

Composition Portland Cement Magnesia

MgO (%) 2.33 89.67

CaO (%) 61.48 1.65

SiO2 (%) 19.19 0.36

Al2O3 (%) 5.35 0.23

Fe2O3 (%) 2.38 0.34

Na2O (%) 0.23 0.23

K2O (%) 1.14 0.06

SO3 (%) 4.07 –

LOI (%) 2.46 7.15

3.2 Sample preparation and curing

Four mortar mixes were tested with the same water-to-binder (w/b) of 0.57. Sand-to-cement

ratio of 2 was also identical in all mortar mixes. These mixtures were containing 0% MgO, 20%

MgO, 40% MgO, and 60% MgO, as a Portland Cement replacement in a mixture, and

designated as M-0, M-20, M-40 and M-60. For each mix a set of twelve 50mm cubes was

prepared, and it was cured in an environment with relative humidity of 90% and temperature of

23±2 °C for two days. After that they were demoulded and immediately placed into an

19

environmental chamber with a variable relative humidity (RH) and CO2 concentration and a

temperature of 23±2 °C and atmospheric pressure. Additionally, six paste 50 mm cubes were

casted for each magnesia replacement level with the same w/b, designated similarly to mortar

(M-0p, M-20p, M-40p and M-60p).

CO2 concentration was monitored using DCS inc. M400 CO2 sensor and controlled manually.

By adjusting CO2 concentration each day (or more often at early curing ages, when CO2

consumption is more rapid) specific concentration of 50, 75 or 99% has been achieved. Relative

humidity and temperature was monitored using two different hygrometers. In order to maintain

uniform CO2 concentration in the chamber, a ventilating fan was used to create air circulation,

and therefore maintain uniformity in CO2 distribution. Salt solutions were used keep the

humidity inside the chamber constant. More specifically, NaCl was used for ~75% RH, and a

combination of CaCO3 and LiCl2 were used to establish ~50% RH.

Table 3.2: Mortar and Paste Mixture Design Proportions

Mixture designation General Use (GU) Reactive Fine Water

Portland Cement (g) Magnesia (g) Aggregate (g) (g)

Mortar

M-0 1700 0 3400 969

M-20 1574 426 3400 969

M-40 1174 826 3400 969

M-60 761 1239 3400 969

Paste

M-0p 2000 0 0 1140

M-20p 1852 502 0 1140

M-40p 1381 972 0 1140

M-60p 895 1458 0 1140

20

Table 3.3: Carbonation Curing Scenarios

Environmental Condition CO2 Concentration (%) Relative Humidity (%) Temperature (°C)

50%RH, 50%CO2 50±5 50±5 23±3

50%RH, 75%CO2 75±5 50±5 23±3

50%RH, 99%CO2 95-99 50±5 23±3

50%RH, 50%CO2 50±5 75±5 23±3

50%RH, 75%CO2 75±5 75±5 23±3

50%RH, 99%CO2 95-99 75±5 23±3

3.3 Testing procedure

The 3, 7 and 28 day testing was conducted on both mortar and paste specimens. Pore structure

analysis on mortar and paste specimens was done using Mercury Intrusion Porosimetry. Paste

specimens were used for X-ray diffraction (XRD) and DTA testing for the purpose of chemical

composition analysis. In addition, the carbonation front was examined using 1%

phenolphthalein alcohol solution pH-indicator on a fresh-split specimen surface. Additionally,

compressive strength tests were conducted only on mortar specimens.

3.3.1 Sample preparation and drying procedures

Following the splitting procedure material from the specimens was prepared for the chemical

and porosimetry testing. First, material was taken from a carbonated area, or the area being as

close as possible to it. It was then crushed to a size of between 1.50 to 1.25 mm. A material was

then immersed in isopropyl alcohol for 24 hours, then vacuum dried for 24 hours, then put in a

desiccators with 20% RH for an additional period of 24 hours. The half of the material was used

for MIP testing, the other half was further grounded to powder with size of no larger than 35

microns and used in DT\TGA and XRD tests.

21

3.3.2 Carbonation front

After the designated accelerated carbonation curing time, specimens were taken out of the

curing chamber. Phenolphthalein solution in alcohol of 1% was then applied to fresh split

surface of the corresponding mortar and paste cube specimens.

3.3.3 Compressive strength

The mortar cube specimens were tested on compressive strength on the corresponding testing

dates. For each testing day and mix tested a set of 5 cubes was used. Average compressive

strength values and coefficients of variation were then calculated.

3.3.4 Chemical composition tests

X-ray analysis was performed using Analytical X-Ray Powder Diffractometer with Cu Kα

radiation (λ=1.5418 Å), 2θ range of 5°–80°, and a step size of 0.02° to investigate crystalline

phases in the powdered samples. Same powder was used in DTA\TG test using NETZSCH

STA 409, with a temperature range of 25 to 1050°C, a heating rate of 10°C/minute and nitrogen

flow rate of 50 ml/minute.

3.3.5. Mercury Intrusion Porosimetry (MIP)

Autoscan 60 mercury intrusion porosimeter was used to perform a test. Specimens to be tested

were crushed with particle size being in a range from 1.5 to 2 mm. The average mass of a tested

sample was 2.5 g. Pore-size distributions, apparent densities and threshold values were

collected for each sample tested.

22

Results and Discussion

4.1 Chemical Analysis – XRD, DT and TGA

4.1.1 Carbonation front

Tables 4.1 and 4.2 show the results of carbonation front tests for the conditions with 75% and

50% relative humidity respectively, both mortar and paste. The visible difference in the rate of

carbonation of each mixture can be observed.

The only mixture to consistently carbonate completely is M-60. This mixture is regularly

penetrated almost completely as early as at 7d. In contrast, mix M-0 is consistently the least

carbonated at all ages. Other mixes, irrespective of CO2 concentration show the increase in zone

carbonated with time. CO2 concentration was found to improve the rate of carbonation in many

cases, however on the contrary the rate was reduced for 75% RH cured mixtures M-20 and M-

40. The reasons for this are unclear.

In general, most of the mixtures have carbonated more readily in the environments with 50%

RH compared to those cured in 75%RH. As CO2 concentration increased, all the specimens

carbonated more, same as with the case of 75% RH environments. The difference in

carbonation due to CO2 concentration was smaller, mostly because of much higher carbonation

rates of [50%RH, 50%CO2] compared to that of [75%RH, 50%CO2]. Mix M-0 has achieved

markedly quick and high carbonation rate in 50% RH conditions. Even though its complete

carbonation achieved in [50%RH, 50%CO2] has not been repeated at elevated CO2

concentrations, it has still carbonated more than both M-20 and M-40, in contrast to 75% RH

environments where it was consistently the least carbonated mix.

23

Table 4.1: Carbonated Area of Specimens cured in 75%RH

[75%RH, 50%CO2]

Mortar carbonated area, % Paste carbonated area, %

%MgO 3d 7d 28d 3d 7d 28d

0 8% 12% 26% 4% 23% 28%

20 8% 13% 42% 51% 80% 87%

40 15% 51% 84% 10% 54% 93%

60 50% 84% 100% 57% 71% 100%

[75%RH, 75%CO2]


%MgO 3d 7d 28d 3d 7d 28d

0 21% 25% 31% 25% 30% 77%

20 4% 9% 29% 23% 33% 51%

40 14% 42% 65% 26% 42% 52%

60 98% 100% 100% 51% 64% 100%

[75%RH, 99%CO2]


%MgO 3d 7d 28d 3d 7d 28d

0 24% 30% 66% 24% 50% 69%

20 6% 19% 26% 14% 28% 63%

40 5% 17% 46% 42% 59% 80%

60 74% 96% 100% 66% 94% 96%

24

Table 4.2: Carbonated Area of Specimens cured in 50%RH

[50%RH, 50%CO2]


%MgO 3d 7d 28d 3d 7d 28d

0 29% 51% 100% 21% 26% 66%

20 14% 19% 28% 15% 42% 50%

40 6% 15% 58% 32% 39% 66%

60 74% 88% 100% 36% 74% 94%

[50%RH, 75%CO2]


%MgO 3d 7d 28d 3d 7d 28d

0 24% 30% 66% 25% 30% 77%

20 6% 19% 26% 23% 33% 51%

40 5% 17% 46% 26% 42% 52%

60 74% 96% 100% 51% 64% 100%

[50%RH, 99%CO2]


%MgO 3d 7d 28d 3d 7d 28d

0 23% 33% 75% 34% 39% 88%

20 13% 22% 42% 23% 32% 47%

40 11% 24% 77% 39% 48% 71%

60 73% 90% 100% 66% 79% 86%

25

4.1.2 X-ray Diffraction results

Figures 4.1, 4.2, 4.3 and 4.4 show the example of XRD patterns of a carbonated paste material

after 3d and 28d of accelerated carbonation curing. For the curing condition of 75%RH and

50% CO2, Figure 4.1 and 4.2 shows the influence of percentage of MgO on the XRD pattern of

carbonated mortar and paste, respectively. The key compounds have been identified based on

matching their characteristic peaks as shown in Table 4.3. Although XRD is a proven

technique since it provides accurate phase identification, interpreting the results of cement-

based constituents does pose some challenges. Peak overlaps and a masking effect generated

by the presence of both coarse and fine aggregate has been reported by various researchers

(Sarkar and Cheng 1994; Beaudoin and Ramachandran 2001; Glasser and Sagoe-Crentsil,

1989). Even if some aggregates are removed during the sample prepartation stage (a technique

used by some researchers), some finer rock fragments tend to remain together with the fine

aggregates such as quartz and feldspar. This creates a permanent unavoidable masking effect in

the diffraction pattern. These observations also apply to the results reported in this study where

both mortars and pastes were examined. Consequently, the reminder of the curing condition

scenarios beyond 75%RH and 50% CO2, all XRD and chemical analysis was conducted on

pastes alone. In all mixes, irrespective of age and amount of MgO, calcite was the main

carbonate compound. It should be noted though that it was expected that magnesian calcite

would form through [Mg 2+

] precipitation in the pore solution. Calcite and magnesian calcite

have similar 2θ values, and thus are hard to be distinguished from each other in XRD patterns.

In addition to Calcite, peaks of uncarbonated calcium silicate hydrate, portlandite and Brucite

were found, along with ettringite and unreacted cement - calcium silicate and MgO.

Nesquehonite has been also found in mixes with 40% and 60% of r-MgO content.

26

Table 4.3: X-ray Diffraction Main Peaks Data (Catti et al. 1995)

Chemical compound X-Ray Diffraction Pattern Identity (2θ)

Calcite 29.285 47.142 39.413 43.215

Portlandite 34.089 18.089 47.124 54.337

Magnesia 42.867 62.233 78.532

Ettringite 9.091 15.784 22.944 25.614

Calcium Silicate 31.995 32.607 41.226 37.329

Brucite 37.976 18.586 58.564 68.214

CSH 30.750 37.353 22.011

Nesquehonite 13.612 23.022 34.331 47.306 50.978

Quartzite 26.624 20.848 50.109 36.527

Figure 4.2, 4.3, and 4.4 present the XRD patterns for the paste mix designs for the curing

conditions of [75%RH and 50%CO2], [75%RH and 75%CO2 ], and [75%RH and 99%CO2],

respectively. There are several influencing variables such as age (3d and 28d), percentage of

reactive MgO (0 to 60%) and the CO2 concentration which ranges from 50% to 99%. Some

general observations for all three curing scenarios are that, irrespective of age and amount of

MgO, calcite was the main carbonate compound In addition to calcite, peaks of uncarbonated

calcium silicate hydrate, portlandite and brucite were found, along with ettringite and unreacted

cement - calcium silicate and MgO. Nesquehonite has been also found in mixes with 40% and

60% of r-MgO content . Although it is recognized that there are some similarities in the

predominant compounds that are present in all mixtures, the chemistry of the carbonated

material is influenced by age (3 d to 28d), the percentage of reactive MgO which ranges from

20 to 60%, and the differences in curing condition (CO2 increases from 50 to 99%). In order to

understand the effect of all these variables on the formation of the various compounds, namely

calciute, portlandite, magnesia, ettringite, calcium silicate, brucite, calcium silicate hydrate, and

nesquehonite,

Table 4.4 summarizes the general trends observed for each variable (age, %MgO, CO2

concentration and relative humidity) in XRD results conducted in this study, and the effect of

age, r-MgO replacement and CO2 concentration on peaks intensity.

27

Table 4.4: Influence of Age, Percentage of MgO and CO2 Concentration on X-ray Diffraction Results

Chemical Compound Age % MgO CO2 Concentration Relative Humidity

3d to 28d 20 to 60% 50 to 99% 50 to 75%

Calcite Increases Similar Similar Decreases

Portlandite Decreases Decreases Similar Increases

Magnesia Decreases Increases Similar Decreases

Ettringite Decreases Decreases Similar Increases

Calcium Silicate Decreases Decreases Similar Increases

Brucite Decreases Increases Similar Increases

CSH Decreases Decreases Similar Increases

Nesquehonite* Increases Increases* Similar Increases

* only apparent in mixtures with 40 or 60% MgO and relative humidities of 75%

All peaks that correspond to the non-carbonated material in the carbonation zone show a visible

decrease in their intensity with age. The substantial reduction of CSH, portlandite and brucite,

along with simultaneous increase for carbonates may indicate the continuing carbonation

reaction, even though the material has already been penetrated by CO2. As the amount of

reactive magnesia in the mix increases, the reduction of Calcium bearing phases has been

observed. This can be attributed to overall reduction in Portland Cement content as it is

replaced by magnesia. Magnesia bearing phases, such as unreacted r-MgO, brucite and

nesquehonite in turn increase their intensity. Finally, no apparent effect of CO2 concentration

has been observed, as all phases remain relatively the same irrespective of changes in CO2

concentration. The indepth analysis of each particular phase is described below.

There is marked difference in chemical composition of the material when relative humidity

decreases from 75% to 50%. First of all, the intensities of calcite/magnesium calcite peaks

increase. In addition to this, an overall decrease in all hydration products, such as portlandite

and brucite is also visible. The intensity of reactive MgO increases significantly, while calcium

silicate peaks show small decrease. Finally, nesquehonite has not been found in any sample

28

which may indicate that it has not formed or only formed in the negligent quantities. The

detailed discussion on each group of phases is below.

Calcite/Magnesium Calcite

Table 4.4 indicates that the calcite peaks intensity increase through 3d to 28d for all

mixtures irrespective of the curing condition and magnesia content. It was expected that

more hydrated material in a zone where CO2 has penetrated the pore network will

carbonate. It is observed that the intensity of calcite peak is similar irrespective of if the

percentage of MgO increases even though it is expected that as reactive MgO

percentage increases, calcite would decrease due to lesser amount of Ca-bearing

reactants. The calcite peak appears to be unaffected by the concentration of CO2. One of

the reasons for that might be a negligible effect of CO2 concentration on chemical

structure of the carbonation zone (in contrast to its influence over the size of this zone in

a bulk material). As discussed earlier in the section, magnesium calcite has 2θ values

very similar to calcite, and these two phases cannot be distinguished from each other on

XRD pattern.

Calcite\magnesium calcite are also the main phase in 50%RH conditions. Same as in

conditions with 75%RH, even though there is the increase in intensity of this phase, it

cannot be reliably correlated with CO2 concentration rise cause of the small

significance. On the other hand there is a marked, visible increase in calcite's intensity

related to change in humidity (from 75% to 50%).

Portlandite

With age the intensity of the Portlandite peak decreases which is expected because as a

result of hydration processes portlandite or CH is consumed during hydration and

carbonation reactions. Similarly, as the percentage of reactive MgO increases in the

29

mixtures, the portlandite peak decreases because the cement content becomes diluted

which reduces the formation of portlandite. The portlandite appears to be unaffected by

the concentration of CO2 in a similar manner to calcite.

In specimens cured in 50% RH environments the intensities of portlandite have declined

greatly. For instance, almost no portlandite has been found in mix M-0 under the

conditions [50%RH,75% CO2] and [50%RH,99% CO2].

Magnesia

From 3d to 28d the peaks associated with magnesia reduce as a result of ongoing

hydration reaction. As the reactive MgO content increases in the mixtures, the magnesia

peaks increase owing to the increasing presence of unreacted material. The formation of

zones in the material with a higher than normal MgO concentration during the mixing,

which was observed to increase consistently with % of MgO may also be attributed to

this effect. Similar to the other compounds discussed the intensity of the magnesia peaks

are not affected by the CO2 concentration.

Unreacted magnesia is also present in low-humidity environments. The intensity of this

phase is severely greater under these conditions. While it also undergoes the decrease

over time, and increase over %MgO it still remains a major phase even at later ages.

Ettringite

With age from 3d to 28d ettringite intensity decreases as it is consumed in hydration

reactions. In addition to that, with increasing reactive MgO content ettringite also

decreases because less [Ca2+

] -bearing cementitious materials are present for its

formation to occur.

In contrast to 75% RH environments, ettringite has not been found in those with

50%RH. This can be related to both quicker hydration and reduced free water content in

pore network.

30

Calcium Silicate

The change in intensities of calcium silicate phases were observed to be decrease with

age, similarly to another unreacted cementitious compound, magnesia. Under the

accelerated carbonation conditions, the decrease is owing to hydration reactions that

occurred during that time. Although unaffected by the CO2 concentration, the increasing

presenece of reactive MgO tends to decrease the intensity of calcium silicate as a result

of cement content being dilluted by reactive magnesia. Almost no visible change in

calcium silicate intensity has occurred due to decrease in relative humidity.

Brucite

Brucite's intensity was found to decrease with age, which can be explained by the fact

that this compound is used in carbonation reaction. Less dramatic (compared to that of

portlandite) decrease is because of lower solubility of brucite compared to portlandite

(Mo and Panesar, 2012). As reactive magnesia content increases, brucite was found to

also increase. Brucite being the main product of magnesia hydration is a reason for such

increase. Under lower relative humidity brucite's intensity has declined, same as that of

other non-carbonated phases.

Calcium Silicate Hydrate

Calcium silicate hydrate is another hydration production of calcium silicate (with the

other being portlandite). Small wide peaks of amorphous CSH can be observed on most

patterns. Sharp peaks of crystalline material have also been identified. CSH intensities

decreased with age, and it may mean that the compound was consumed in carbonation

reaction. It also decreases with %MgO increase, due to the same reasons as with other

compounds described above. CSH has also decreased in lower humidity, however this

decrease is less significant than that of brucite.

31

Nesquehonite

Nesquehonite , also known as magnesium hydrocarbonate (MgCO3*3H2O), is the only

pure magnesium carbonate that has been found in the carbonated material. The amount

of this carbonated phase was markedly low compared to other carbonates -

calcite/magnesian calcite. Generally nesquehonite only occurred in mixes with high r-

MgO% (40 and 60%) and only at later ages (28d). While there has not been a visible

variability in nesquehonite intensities based on CO2 concentration, main peaks of this

compound were different and it is hard to accurately estimate the difference in

nesquehonite content based on %CO2. Another probable influencing factor were zones

in a material with higher than normal reactive magnesia concentration, that occur

regularly during the mixing procedure in high r-MgO containing mixtures and that occur

as more often as more magnesia there is in a mix. As discussed in section 2.3.7,

nesquehonite regularly forms in places where precipitating carbonates have blocked the

outflow of Mg2+ ions, and so the nucleation of this compound occurs in high-Mg2+

zones.Nesquehonite has not been found in any mixture cured under 50%RH. This is

attributed to the insufficient water content for the formation to occur. Based on this it

can be concluded that the formation of this particular phase is indeed extremely

dependent on water supply.

Quartzite

As already mentioned in this section, quartzite peaks have been present in XRD patterns

of mortar samples tested only for condition [75%RH,50% CO2]. These peaks occured in

the range of other important phases, such as nesquehonite, which can be seen on Figure

4.1. This way these compounds are effectively masked by quartzite presence, and make

it harder to establish a more accurate picture of chemical composition of a carbonated

material. Thus the testing of all other mixes was conducted on paste only.

32

a) Degrees (2)

Degrees (2)

b)

Figure 4.1: Condition [75%RH, 50%CO2] XRD Pattern of Mortar a) 3d, b) 28d

33

Degrees (2)

a)

b) Degrees (2)

Figure 4.2: Condition [75%RH, 50%CO2] XRD Pattern of Paste a) 3d, b) 28d

34

Degrees (2)

a)

b) Degrees (2)


35

Degrees (2)

a)

Degrees (2)

b)


36

4.1.3 DTA/TG results

Figures 4.5 and 4.6 presents respectively 3d and 28d DTA analysis of carbonated paste material

of [75% RH, 99% CO2] respectively. Three clear peaks are observed in mix M-0, first peak -

which can be related to decomposition of brucite (440º C), second peak - portlandite (~ 480º C)

and third peak - crystalline calcite (~840º C). First two peaks diminish over time and are only

slightly visible at 28d. All reactive magnesia mixes incorporate another clear peak around 450-

460º C, that can be attributed to both brucite and magnesian calcite decomposition. As r-MgO

content increases all peaks except that of calcite merge together making the difference between

them indistinguishable.

Closer look at DTA results show that there is a clear difference in peak shape, intensity and

mass loss over these peaks has been observed based on age and reactive magnesia replacement.

First of all, as magnesia content increases, second peak (at ~480º C ) increases significantly and

merges with first peak (440º C) starting from 40% of MgO, overshadowing it. Third peak

(~840º C) indicates decrease in intensity instead as magnesia content increases. This change

can be explained by increase of magnesian calcite compared to calcite in magnesia mixes. In

addition to this, as CO2 concentration decreases the intensity of first peak is increasing, that

may be attributed to greater amount of uncarbonated brucite. Finally, first and second peaks

reduced with age, which is consistent with portlandite and brucite decline reported in XRD

results. On the other hand almost no change has been observed in third peak intensity over age,

compared to calcite increase from XRD results.

Figures 4.7 and 4.8 show TG results for carbonated paste material at 3d and 28d respectively.

Generally, the results are consistent with DTA observations. On figures the small mass loss in

the area of second peak can be observed, along with the increasing mass loss between the area

that corresponds to second and third peak.

Table 4.5, presents the data of mass loss results from TG analysis over each particular peak. It

can be clearly seen from this table that mass loss in the area of each peak correlates with change

in DTA intensities. For instance third peak that relates to calcite decomposition has a decrease

of intensity as reactive magnesia content increases. This correlates with decrease in this peak

37

mass loss as can be inferred from the table. In addition to these changes, a rather consistent

shift of first peak intensity to the higher temperature (~405 to 440º C) and third peak to the

lower temperature (820 to 780º C). The shift in the first peak may be related to higher relevance

of magnesium calcite that occurs with increase in reactive MgO in the mix. Third peak's shift

on the other hand may be related to the decomposition of some particular CaCO3 phases

(Thiery et al, 2007).

In addition to those clear peaks, the clear weight loss is observed between 500º C and 800º C.

This is attributed to the decarbonation of both amorphous calcite and magnesium calcite.

Nesquehonite decomposition is believed to be also included in this range, as it first dehydrates

around 440º C to MgCO3 and then further decarbonates around 550ºC (Lanas et al, 2004). Due

to overlapping peaks it is not possible to distinguish each phase.

Another set of tests was conducted on specimens cured in 50%RH environments. Similarly to

75%RH scenario the position, intensities and the shift of main peaks related to decomposition

of various phases as well as the shape of mass loss curves were all practically identical to all

other conditions. Thus the effect of both % CO2 and %RH on phases' intensities gathered in

DTA/TG was found to be insignificant

38

Table 4.5: DTA/TG data for separate peaks, condition [75%RH, 99%CO2]

1st Peak 1st Peak

3d T (°C) Mass Loss (%) DTA(uV/mg) 28d T (°C) Mass Loss (%) DTA(uV/mg)

M-0 M-0

M-20 404.8 -3.97 0.112 M-20 405.6 -4.68 0.143

M-40 429.8 -9.3 0.084 M-40 431.8 -10.79 0.065

M-60 440.5 -14.64 -0.105 M-60 439.3 -14.72 -0.012

2nd Peak 2nd Peak

T (°C) Mass Loss (%) DTA(uV/mg) T (°C) Mass Loss (%) DTA(uV/mg)

M-0 461.5 -1.55 0.165 M-0 508.9 -1.77 0.316

M-20 470.2 -1.3 0.167 M-20 445 -2.13 0.205

M-40 M-40

M-60 M-60

3rd Peak 3rd Peak

T (°C) Mass Loss (%) DTA(uV/mg) T (°C) Mass Loss (%) DTA(uV/mg)

M-0 818.5 -9.95 0.133 M-0 825.6 -12.34 0.164

M-20 804.6 -7.05 0.151 M-20 801.6 -7.3 0.183

M-40 798.8 -6.24 0.17 M-40 797.3 -6.74 0.152

M-60 783.7 -4.47 0.128 M-60 783.9 -4.44 0.221

RH=75%- CO2 = 99%

Temperature (degrees C)

0 200 400 600 800 1000 1200

DT

A (

V/m

g)

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0%MgO

20%MgO

40%MgO

60%MgO

Figure 4.5: Condition: [75%RH, 99%CO2] Influence of MgO on DT at Day 3

39

RH=75%- CO2 = 99%


0 200 400 600 800 1000 1200

DT

A (

V/m

g)

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0%MgO

20%MgO

40%MgO

60%MgO

Figure 4.6: Condition: [75%RH, 99%CO2] Influence of MgO on DT at Day 28

RH=75%- CO2 = 99%


0 200 400 600 800 1000 1200

Mas

s L

oss

(%

)

60

70

80

90

100 0%MgO

20%MgO

40%MgO

60%MgO

Figure 4.7: Condition: [75%RH, 99%CO2] Influence of MgO on TGA at Day 3

40

Figure 4.8: Condition: [75%RH, 99%CO2] Influence of MgO on TGA at Day 28

RH=75%- CO2 = 99%


0 200 400 600 800 1000 1200

Mas

s L

oss

(%

)

60

70

80

90

100 0%MgO

20%MgO

40%MgO

60%MgO

41

4.2 Mechanical Properties

4.2.1 Effect of Reactive MgO on Compressive Strength

Figures 4.9-4.14 present the effect of incorporating up to 60% MgO on the compressive

strength of mortar cubes at 3, 7 and 28d. Tables 4.6 and 4.7 present a summary of compressive

strength data as well as coefficients of variation for each particular strength value.

In general, the results irrespective of the increasing CO2 concentration from 50% to 99%

indicate that the compressive strength increases with age from 3d to 28d. This observation

holds for mixtures without MgO and with 20, 40 and 60% MgO. Closer examination of the

effect of MgO on the compressive strength development reveals that compared to the 0% MgO

mixture, the mixtures with 20% MgO exhibit an early age (3d, 7d) and later age (28d)

reduction in compressive strength as presented in Table 4.8. At day 3 and 7, the percentage

change in compressive strength from 0% MgO mixtures to 20% MgO varies markedly from -

31% to +4% depending on the CO2 concentration. It should be noted that the largest reductions

in compressive strength occur at day 3 and 7 for the condition with the lowest concentration of

CO2, 50%. But in general, it should be noted that there is no clear correlation between changes

in compressive strength and the concentration of CO2 at early ages (3d and 7d). In contrast, by

28 days, irrespective of the curing conditions (50-99% CO2) at a 75% RH, the reduction of

compressive strength for the 20% MgO mixtures (compared to the 0% MgO mixtures) fall

within a narrow range of 10-13%.

Table 4.8 compares the influence of mortars containing 40% MgO on the compressive strength

development to the 0% MgO mortars. The addition of 40% MgO largely reduces the

compressive strength by 50-91% at day 3, 41-78% at day 7, and by 34 to 42% by day 28 for all

of the three CO2 conditions (50, 75 and 99%). Based on the ranges in reductions it is apparent,

that age tends to improve the properties of the 40% MgO mixtures. From Table 4.8 it should

also be noted that by far the largest reductions, 91 and 78% occurred at early ages 3 and 7d,

respectively for the scenario where the specimens were cured at the lowest CO2 concentration

42

(50%). At later ages, however, this reduction is much smaller, which is specifically evident in

environmental conditions with 50% RH.

Table 4.8 also presents the change in compressive strength between the 0 and 60% MgO

mortars. In general, some similar observations as reported for the 20 and 40% MgO mixes are

also true for the 60% MgO case. Firstly, the early age (3 and 7d) drop in compressive strength

compared to the 0% MgO mix, is greater than that at 28d, (except the 75% CO2 curing

condition). Secondly, at early ages (3 and 7d), mixtures exposed to the lowest CO2

concentration (50%), exhibited the greatest reductions in compressive strength compared to the

75 and 99% CO2 curing conditions. By 28 days, in contrast to the mixtures containing 20 and

40% MgO, the 60% MgO mortar exhibited a markedly less severe reduction in compressive

strength and infact had a 5% compressive strength increase compared to the 0% MgO mix. By

far the highest increase compared to M-0, mixture M-60 has achieved in condition with 50%

RH and 50% CO2. This is not to be confused with the highest strength achieved, as M-60

continued to improve with CO2 concentration increasing, however M-0 also increase its gain

over the same period, which leads to a smaller gap between the two.

43

Table 4.6: Mean Compressive Strength (f'c) and Coefficient of Variation (COV) for 75%RH

Condition Age 0% MgO 20% MgO 40% MgO 60% MgO

RH (%) [CO2] (days)

75% 50% 3d f'c (MPa) 49.6 38.4 26.0 32.9

COV (%) 2.7% 1.4% 4.9% 6.9%

7d f'c (MPa) 56.4 43.2 31.6 45.2

COV (%) 3.2% 6.5% 4.9% 7.8%

28d f'c (MPa) 68.0 60.0 48.0 66.0

COV (%) 2.7% 2.5% 6.4% 4.3%

75% 75% 3d f'c (MPa) 51.5 53.1 34.4 45.0

COV (%) 3.9% 5.1% 3.6% 4.8%

7d f'c (MPa) 60.2 59.0 42.7 55.8

COV (%) 3.6% 1.8% 4.5% 5.9%

28d f'c (MPa) 75.4 67.7 56.2 65.8

COV (%) 3.7% 4.1% 5.9% 6.9%

75% 99% 3d f'c (MPa) 48.6 50.9 32.5 45.0

COV (%) 6.2% 1.3% 2.4% 4.8%

7d f'c (MPa) 64.3 56.0 42.2 63.2

COV (%) 6.7% 4.7% 3.9% 5.0%

28d f'c (MPa) 73.5 67.0 52.0 77.5

COV (%) 4.7% 3.7% 5.0% 7.0%

Table 4.7: Mean Compressive Strength (f'c) and Coefficient of Variation (COV) for 50%RH


RH (%) [CO2] (days)

50% 50% 3d f'c (MPa) 42.9 43.1 37.4 51.5

COV (%) 2.8% 2.7% 4.6% 2.1%

7d f'c (MPa) 50.4 54.1 48.0 61.3

COV (%) 2.2% 4.8% 2.4% 2.0%

28d f'c (MPa) 66.2 66.9 70.6 84.2

COV (%) 2.0% 4.1% 1.9% 5.4%

50% 75% 3d f'c (MPa) 47.7 48.1 33.6 52.4

COV (%) 3.8% 1.3% 3.5% 2.6%

7d f'c (MPa) 59.8 59.1 42.2 60.9

COV (%) 2.0% 3.6% 8.7% 1.5%

28d f'c (MPa) 78.3 78.2 71.0 85.12

COV (%) 2.6% 1.4% 6.6% 5.0%

50% 99% 3d f'c (MPa) 50.72 42 39.68 49.44

COV (%) 2.9% 3.8% 3.3% 3.5%

7d f'c (MPa) 64.72 53.84 52.32 63.12

COV (%) 1.8% 4.4% 1.3% 1.0%

28d f'c (MPa) 79.0 72.3 83.7 84.2

COV (%) 6.6% 5.1% 4.7% 2.8%

44

Table 4.8: Percentage Change in Compressive Strength Due to MgO used as Cement Replacement

Condition Age % Change in Compressive Strength Due to % Change in MgO

RH (%) [CO2] (days) 0 to 20% MgO 0 to 40% MgO 0 to 60% MgO

75% 50% 3d -29% -91% -51%

75% 50% 7d -31% -78% -25%

75% 50% 28d -13% -42% -3%

75% 75% 3d 3% -50% -14%

75% 75% 7d -2% -41% -8%

75% 75% 28d -11% -34% -15%

75% 99% 3d 4% -50% -8%

75% 99% 7d -15% -52% -2%

75% 99% 28d -10% -41% 5%

50% 50% 3d 1% -15% 17%

50% 50% 7d 7% -5% 18%

50% 50% 28d 1% 6% 21%

50% 75% 3d 1% -42% 9%

50% 75% 7d -1% -42% 2%

50% 75% 28d 0% -10% 8%

50% 99% 3d -21% -28% -3%

50% 99% 7d -20% -24% -3%

50% 99% 28d -9% 6% 6%

45

75RH- 50%CO2

% MgO

0 20 40 60

Com

pre

ssiv

e S

tren

gth

(M

Pa)

0

20

40

60

80

100

3d

7d

28d

Figure 4.9: Condition: [75%RH, 50%CO2] Effect of MgO on 3, 7, and 28d compressive strength

75%RH-75%CO2

% MgO

0 20 40 60

Com

pre

ssiv

e S

tren

gth

(M

Pa)

0

20

40

60

80

100

3d

7d

28d


46

75%RH-99%CO2

% MgO

0 20 40 60

Com

pre

ssiv

e S

tren

gth

(M

Pa)

0

20

40

60

80

100

3d

7d

28d


50%RH- 50%CO2

% MgO

0 20 40 60

Com

pre

ssiv

e S

tren

gth

(M

Pa)

0

20

40

60

80

100

3d

7d

28d


47

50%RH-75%CO2

% MgO

0 20 40 60

Com

pre

ssiv

e S

tren

gth

(M

Pa)

0

20

40

60

80

100

3d

7d

28d


50%RH-99%CO2

% MgO

0 20 40 60

Com

pre

ssiv

e S

tren

gth

(M

Pa)

0

20

40

60

80

100

3d

7d

28d


48

4.2.2 Strength-Porosity Correlation

For conventional concrete and cement based mortars, there is a general consensus that the

compressive strength increases as the porosity decreases. The change in porosity is influenced

by physical microstructure and chemical hydration processes and in turn can affect the

compressive strength may be controlled by the water to binder ratio, incorporation of SCMs,

use of air entraining agents, age and degree of hydration (Yusuf et al, 2014; Cook and Hover,

1999).

Figures 2.1-2.4 show the work by several authors which generally shows a relationship

whereby increasing porosity is correlated to decreasing compressive strength. Furthermore,

research related to carbonation studies of conventional concrete reveals a similar relationship.

Non-carbonated concrete exhibits a relatively higher compressive strength and relatively lower

porosity as shown in Figure 2.1.

When the concrete is subjected to higher CO2 conditions, it becomes carbonated and exhibits a

lower porosity and higher compressive strength in comparison to the uncarbonated concrete.

The reason for this is a densification of a microstructure, that occurs when precipitated reaction

products have higher molar volume compared to the solid reagents.

Few studies have reported this correlation for mortars containing reactive MgO. The results

from this study are shown in Figure 4.19, 4.20, 4.21 and 4.22 show the effect of 0, 20, 40 and

60% MgO on porosity under six carbonation conditions. As it can be seen, the degree of such

densification is relatively larger depending on %MgO. The reason for that is primarily a greater

volume expansion and densification of a pore structure due to formation of magnesian calcite

and especially nesquehonite (De Silva et al, 2007).

49

Compressive Strength (MPa)

20 30 40 50 60 70 80

Tota

l P

oro

sity

(%

)

0

2

4

6

8

10

3d

7d

28d

Mix = 0%MgO and Condition RH=75%, CO2=50,75,99%

Figure 4.19: Correlation between compressive strength and total porosity for 0%MgO mortar


20 30 40 50 60 70 80

Tota

l P

oro

sity

(%

)

0

2

4

6

8

10

3d

7d

28d



50


20 30 40 50 60 70 80

Tota

l P

oro

sity

(%

)

0

2

4

6

8

10

3d

7d

28d




20 30 40 50 60 70 80

Tota

l P

oro

sity

(%

)

0

2

4

6

8

10

3d

7d

28d



51

4.2.3 Effect of CO2 on Compressive Strength

Figures 4.23-4.26 present the effect of increasing the CO2 concentration from 50%, 75%, and

99% for curing conditions where the relative humidity is constant at 50% (a) and 75% (b) for

mixtures made with 0, 20, 40, and 60% MgO.

Figure 4.23 reveals that at early ages (3d and 7d), there is no statistically significant difference

in compressive strength in context with the various curing conditions. By 28 days, the mixtures

cured at 75 and 99% have statistically similar compressive strengths that are statistically

significantly greater than the specimens cured at 50% CO2.

For mixtures containing 20 and 40% MgO shown in Figure 4.24 and 4.25, the results indicate

that curing the specimens at 50% CO2 is statistically significantly lower to a 95% confidence

level, then those cured at 75 and 99% CO2 for all ages (3, 7, and 28d) with the exception of

40% MgO at 28 d (and 60% MgO) at 28d (Figure 4.26). In contrast, for mixes cured at lowered

50% relative humidity (Figures 4.23-4.26 (b) ) such difference due to CO2 concentration is less

stronger.

52

Mix Design = 0% MgO and Condition RH=75%

Age (days)

0 5 10 15 20 25 30

Co

mpre

ssiv

e S

tren

gth

(M

Pa)

0

20

40

60

80

100

50% CO2

75% CO2

99% CO2

a)


Age (days)

0 5 10 15 20 25 30

Com

pre

ssiv

e S

tren

gth

(M

Pa)

0

20

40

60

80

100

50% CO2

75% CO2

99% CO2

b)

Figure 4.23: Effect of CO2 concentration on compressive strength development of 0%MgO

mortar, a) Relative humidity - 75%, b) Relative humidity - 50%

53

a)


Age (days)

0 5 10 15 20 25 30

Co

mpre

ssiv

e S

tren

gth

(M

Pa)

0

20

40

60

80

100

50% CO2

75% CO2

99% CO2

b)


Age (days)

0 5 10 15 20 25 30

Com

pre

ssiv

e S

tren

gth

(M

Pa)

0

20

40

60

80

100

50% CO2

75% CO2

99% CO2



54


Age (days)

0 5 10 15 20 25 30

Co

mpre

ssiv

e S

tren

gth

(M

Pa)

0

20

40

60

80

100

50% CO2

75% CO2

99% CO2

a)


Age (days)

0 5 10 15 20 25 30

Com

pre

ssiv

e S

tren

gth

(M

Pa)

0

20

40

60

80

100

50% CO2

75% CO2

99% CO2

b)



55

a)


Age (days)

0 5 10 15 20 25 30

Com

pre

ssiv

e S

tren

gth

(M

Pa)

0

20

40

60

80

100

50% CO2

75% CO2

99% CO2

b)




Age (days)

0 5 10 15 20 25 30

Com

pre

ssiv

e S

tren

gth

(M

Pa)

0

20

40

60

80

100

50% CO2

75% CO2

99% CO2

56

4.2.4 Effect of Relative Humidity on Compressive Strength

Data from Table 4.7 also shows the comparison of compressive strength results of samples

cured under constant CO2 concentrations but varying relative humidities. The effect of relative

humidity can be also seen on Figures 4.9-4.14. Several important trends can be underlined.

Decrease of RH has affected all mixes differently. As the magnesia content in the mixture

increased, the change due to lower %RH is observed to be larger. This is especially evident for

mix M-40, which had the lowest strength results for all 75% RH conditions, and has increased

substantially in 50% RH environment. Second, the existing strength trend observed in 75%RH

conditions has changed, and this change is the most clear at later ages, where almost linear

increase in strength due to %MgO can be observed. Third, there is significant increase of

compressive strength for MgO mixes, with mix M-60 reaching mean values of more than 80

MPa. On the other hand almost no change of compressive strength has been observed in M-0

which contains more magnesia. These trends indicate that the impact of relative humidity, and

the subsequent impact on the strength development is much more important for magnesia

mixes. In general, mixes cured in 50%RH environments have all achieved higher compressive

strengths, especially at later ages.

4.3 Porosity

Tables 4.9 and 4.10 show the total porosity from MIP tests for all environmental conditions for

mortar and paste specimens respectively. While there is a relatively large degree of a variability

in a data that hampers the ability to accurately distinguish the differences between the samples,

there are few patterns that repeat consistently through the whole bulk of data, and that allow to

make certain conclusions on pore development of the materials.

The first distinct pattern is a change in porosity of paste samples, based on %MgO. Mixes with

higher MgO content have considerably higher early porosity, reaching as high as 8% for M-60.

In contrast, late age porosity is reduced with increase of %MgO, with M-60 now reaching the

lowest porosities up to 3%.

57

Second trend shows that while the porosities of mixture M-0 do not refine much with age

irrespective to the environmental condition, magnesia mixes experience a significant

densification, which directly shows that there is a continuing process going in mixes containing

MgO that refines the pore network structure.

In general, almost no difference due to %RH has been observed for mortar samples. In all

conditions the total porosity was almost exactly same for M-0, while it has decreased with

%MgO increase. On the other hand, paste samples have reached much higher early porosities,

especially for mixes M-0 and M-20. Still ,it is observed that there is the same trend of porosity

reduction being dependent on %MgO and that the lowest porosity was achieved by late age for

mixes with the highest magnesia content, as in the conditions of 75% RH.

Tables 4.11 and 4.12 show the changes in porosity due to MgO replacement for mortar and

paste respectively. Again, it is evident that there is a major densification for magnesia mixes,

especially cured in 75% RH environments. Mix M-60 has the biggest late age difference due to

reaching the lowest porosity consistently.

Table 4.9: Total Mortar Intruded Porosity


RH (%) [CO2] (days)

75% 50% 3d 4% 6% 5% 4%

7d 4% 5% 4% 3%

28d 4% 4% 4% 2%

75% 75% 3d 4% 4% 5% 3%

7d 4% 4% 3% 3%

28d 4% 3% 3% 2%

75% 99% 3d 3% 3% 3% 4%

7d 3% 3% 3% 2%

28d 3% 2% 2% 2%


RH (%) [CO2] (days)

50% 50% 3d 5% 4% 5% 4%

7d 5% 5% 4% 3%

28d 5% 4% 3% 2%

50% 75% 3d 4% 4% 5% 3%

7d 4% 4% 3% 3%

28d 4% 3% 3% 2%

50% 99% 3d 4% 3% 3% 3%

7d 4% 3% 3% 2%

28d 4% 3% 2% 2%

58

Table 4.10: Total Paste Intruded Porosity


RH (%) [CO2] (days)

75% 50% 3d 4% 5% 6% 8%

7d 4% 5% 6% 6%

28d 4% 6% 6% 5%

75% 75% 3d 4% 5% 5% 6%

7d 4% 5% 5% 5%

28d 4% 5% 5% 4%

75% 99% 3d 4% 6% 4% 5%

7d 4% 6% 5% 4%

28d 4% 4% 5% 3%


RH (%) [CO2] (days)

50% 50% 3d 9% 9% 8% 7%

7d 9% 8% 6% 6%

28d 8% 8% 6% 5%

50% 75% 3d 10% 9% 8% 6%

7d 9% 7% 6% 4%

28d 8% 7% 5% 4%

50% 99% 3d 9% 8% 8% 6%

7d 9% 6% 6% 4%

28d 8% 6% 5% 3%

59

Table 4.11: Percentage Change in Total Mortar Porosity Due to MgO used as Cement Replacement

Condition Age % Change in Total Porosity Due to % Change in MgO


75% 50% 3d 26% 21% -1%

75% 50% 7d 10% 3% -41%

75% 50% 28d 7% -2% -131%

75% 75% 3d 4% 11% -42%

75% 75% 7d -18% -32% -40%

75% 75% 28d -34% -52% -102%

75% 99% 3d 5% 3% 12%

75% 99% 7d -18% -37% -89%

75% 99% 28d -67% -58% -87%

50% 50% 3d -16% 13% -27%

50% 50% 7d -11% -20% -85%

50% 50% 28d -32% -53% -136%

50% 75% 3d -10% -22% -55%

50% 75% 7d -3% -31% -73%

50% 75% 28d -33% -33% -100%

50% 99% 3d 99% -56% -68%

50% 99% 7d -39% -48% -79%

50% 99% 28d -33% -100% -100%

60

Table 4.12: Percentage Change in Total Paste Porosity Due to MgO used as Cement Replacement



75% 50% 3d 21% 33% 47%

75% 50% 7d 19% 26% 29%

75% 50% 28d 34% 31% 20%

75% 75% 3d 12% 19% 16%

75% 75% 7d 13% 9% 30%

75% 75% 28d 15% 12% 5%

75% 99% 3d 26% -3% 93%

75% 99% 7d 22% 19% -2%

75% 99% 28d 0% 8% -24%

50% 50% 3d 0% -6% -24%

50% 50% 7d -8% -40% -55%

50% 50% 28d 3% -67% -35%

50% 75% 3d -21% -1% -63%

50% 75% 7d -19% -47% -105%

50% 75% 28d -14% -60% -100%

50% 99% 3d -9% -16% -57%

50% 99% 7d -47% -62% -169%

50% 99% 28d -60% -60% -204%

61

5. Interplay between Chemical Analysis and Mechanical Properties

The mechanical properties are influenced by the reactive MgO content, age and CO2

concentration as discussed in Section 2.2.2. In addition, the chemical analysis based on XRD,

DTA and TGA also revealed that the formation of chemical compounds as a result of the

combined effect of hydration and carbonation yield differences in material composition

between the mixtures as discussed in Section 4.2.1.1. This discussion is aimed to highlight the

interplay and the implications of changes to the chemistry of the material and the effect on the

mechanical and microstructural properties.

5.1 Interplay between Chemical and Physical properties and the Effect of CO2

Concentration

5.1.1 Compressive strength.

As shown in earlier studies as magnesia hydrates it is producing a porous brucite solid, and it is

generally adversely affects strength development of a concrete material. XRD results show that

brucite increases significantly with MgO increase, and that the decrease in strength can be

correlated to this. However, mix with the highest magnesia content, M-60, has disproportionate

strength while the highest brucite amount in a carbonated zone. On the other hand this may be

overshadowed by the facts that M-60 carbonated more than any other mix, and that in any

carbonated material brucite quantity is low in comparison with the carbonated phases. It should

be noted however, that the actual carbonation depth was reported to be bigger than that shown

by phenolphthalein indicator, and in some cases being double of color change depth (Chang and

Chen, 2004). Such a discrepancy may explain some inconsistencies in carbonated area to

compressive strength relationships observed in this study.

The increase in strength for M-60 which is not consistent with linear decrease for all other

magnesia mixes (as can be seen on figures 4.9 to 4.14) may also be explained by nesquehonite

formation, observed in this particular mixture. While relatively little nesquehonite has formed

in general (compared to primary phase - calcite\magnesium calcite), much larger volume

expansion of its formation may affect strength development at later ages. Nesqehonite

62

formation in M-40 is much smaller than in M-60, and it has not affected the strength

development apparently. It has been reported that one of the most dominant factors in strength

development of carbonated materials is the morphology of the carbonation zone. As it was

shown in several studies, the impact of Mg incorporation in the calcite structure leads to the

more dense packing and aggregation of crystalline carbonated phases. Mixes that contain more

magnesia were found to have much higher magnesium calcite content, and thus this factor may

explain higher compressive strength of M-60.

Figures 6.1-6.4 show the relationship between compressive strength and carbonated area. It can

be noted, that for all mixtures there is relatively clear linear relationship between the two.

Furthermore, magnesia mixes, especially M-60 express somewhat bigger impact of the

carbonation on strength, which is consistent with all the factors discussed in this section.

Mix = 0%MgO and Condition RH=50,75%, CO2=50,75,99%


20 30 40 50 60 70 80 90

Car

bonat

ed A

rea

(%)

0

20

40

60

80

100 3d

7d

28d

Figure 5.1: Correlation between compressive strength and carbonated area for 0%MgO mortar

63


20 40 60 80

Car

bonat

ed A

rea

(%)

0

20

40

60

80

100 3d

7d

28d




20 30 40 50 60 70 80 90

Car

bo

nat

ed A

rea

(%)

0

20

40

60

80

100 3d

7d

28d




64


20 40 60 80

Car

bonat

ed A

rea

(%)

0

20

40

60

80

100

3d

7d

28d



5.1.2 Porosity

It is generally accepted that a carbonation of ordinary Portland cement concrete densifies the

microstructure. The porosity of such material should then decrease in comparison to

uncarbonated state (You et al. 2014). While MIP tests were conducted on a material that has

already carbonated, some decrease in porosity was observed for magnesia mixes, and this

decrease was largely dependent on MgO%. Larger initial porosities were also reported for

mixes with higher magnesia content. This is consistent with XRD data that presents larger

amount of brucite in magnesia mixes, and that decrease in brucite intensity with time matches

with porosity reduction. Such reduction may mean that there is further densification of a pore

structure with time as brucite continues to carbonate.

Mix M-0 has experienced almost no porosity change over time. If most of material in the

carbonated zone has reacted early on, or the pores sealed quickly by combined effect of

65

carbonation and accelerated hydration, the absence of such change can be attributed to that.

Nevertheless, XRD results were showing a distinct decrease in uncarbonated phases' intensities

that may mean that the reaction was still occurring after 3d, yet no such change has been

captured by MIP tests. The densification of a paste due to magnesium calcite formation also

refined the microstructure leading to more significant densifications for later age high magnesia

mixes.

5.1.3 Carbonation Front

As noted above in Section 4.1.2, nesquehonite has formed only in mixes with the higher

magnesia content, and only at latter ages. Mixture M-60 was the only one that carbonated

completely, and this means that there was a greater relative amount of nesquehonite in a bulk

material, or in other words, larger chance to find the compound in it. In addition to that, the

material has been penetrated by CO2 relatively rapidly in these mixes, thus giving more time for

nesquehonite to form before the material has dried out.

5.2 Interplay between Chemical and Physical Properties and the Effect of Relative

Humidity

5.2.1 Compressive Strength.

The highest compressive strengths, and early strength gain was observed for all mixes, cured in

50%RH environments. Furthermore, the highest compressive strength for all materials, 85MPa

for 28d of mix M-60, has been observed in [50%RH, 75% CO2]. In addition to that, a change in

strength development pattern has been identified. It has been noted in section 4.2.3, that

improvement in early age strength gain of magnesia mixes can be related to CO2 concentration,

as on higher concentrations early strength reduction due to %MgO became markedly lower. In

comparison, such improvement in early strength is becoming evident even in [50%RH, 50%

CO2]. This change means that the effect of CO2 concentration is much higher in lower

humidity. Furthermore, even more distinct change is observed at later (28d) age, as all mixtures,

especially M-20 and M-40, increase their strength substantially. While M-40 still remains the

66

weakest mixture in most 50%RH conditions, there is much smaller incremental difference

between 20% and 40% of MgO in these environments.

The reduction in internal humidity of the specimens, and subsequently smaller water content in

pores is a main reason for this pattern change. As CO2 flow through the pores becomes less

obstructed, more material can be penetrated early on. This in its turn promotes quick

carbonation and the increase in strength. In other words, the effect of increased CO2

concentrations is inhibited at higher humidity. Therefore at lower humidity the effect of CO2

concentration can be observed and assessed more accurately.

The increase in strength is also supported by carbonation front data. All mortar specimens

showed an increase in zone carbonated, and this change correlates with strength improvement.

5.2.2 Carbonation and Chemistry of the Carbonation zone

As noted in section 4.1.1, mortar specimens carbonated more in 50% RH. On the other hand the

effect on paste is not that clear, and in some cases (mixture M-40) the reduction has been

observed. Otherwise, total area carbonated at early age is almost identical for both 50% and

75% RH.

The process of a carbonation reaction in magnesia, and the nature of the forming carbonate

phases has been adequately explained and assessed before. It is described in detail in sections

2.1, 2.2.2 and 2.3. One of the marked effect of reduced humidity was an absence of

nesquehonite in the carbonation zone. Under 75% RH condition this compound has formed in

mixes containing at least 40% of reactive MgO, and only at 28d in M-60 the amount of

nesquehonite became comparatively significant in relation to other secondary phases. No

nesquehonite has been found in specimens cured under 50% RH. This can be attributed to the

high water demand of the reaction. Since at lower humidity the amount of pore water is

considerably smaller and due to the fact that increased carbonation rate blocks the access of

water to high-magnesia areas more quickly the precipitating Mg ions are used in the formation

of magnesium calcite instead. On the other hand no apparent effect of humidity on the quantity

67

of magnesium calcite has been found, as mass loss of crystalline calcite that can be related to

the overall calcite\magnesium calcite does not change significantly.

The improvement in CO2 diffusion due to lower humidity also affected the overall amount of

material carbonated. Even though nesquehonite was not present at lower humidity, there was

more calcite\magnesium calcite relatively to non-carbonated phases. The sudden increase in

unreacted magnesia intensity can be also explained by the lack of water available for the

continuing reaction.

5.2.3 Carbonation and Porosity

While most mortar specimens have shown porosities being close to that cured in 75% RH

environments, paste specimens had much higher porosities especially early on. Such high

porosities can be explained by the higher pore water content in the specimens cured under 75%

RH, and thus the drying being less efficient for these specimens. On the other hand, at later ages

most mixes have achieved low porosities consistent to those of specimens cured at 75%. This

means that at least for later age paste porosity, and also the porosity of mortar specimens is

more or less an accurate estimate. In addition to that since the overall reduction of late age

porosity was not very significant, it cannot be fully related to a supposedly better degree of

carbonation achieved under 50% RH.

Mix M-0 has shown markedly high carbonation, which correlates with much more major

change in porosities observed. This may mean that the drying that occurred in 50% RH

environment eased the access of CO2 inside the material, and that the actual early porosity

observed at early ages in 75%RH environment might not be accurate due to excess water

content in pores.

It has been mentioned that the occurrence of entrapped air during the mixing procedure is not

uncommon, especially when reaching a certain degree of compaction is a challenging task.

Since the fresh properties of the mixture were expected and found to be very variable

depending much on the ratio of reactive magnesia in the mixture, it is possible that those

mixture that have larger %MgO may have their entrapped air content increased. The effect of

entrapped air has been already discussed in section 2.1. While no air content tests has been

conducted in this study, the highest and evidently greater degree of carbonation of M-60 can be

68

explained by this factor. It is possible that due to drastic loss of workability at the point of

60%MgO some air remains entrapped even with the most thorough compaction and mixing

procedure. On the other hand, this effect would then be expected to be more evident for M-40

as well at lower humidity, but as the degree of carbonation for this mixture was not markedly

different it may mean that either the effect of air content is not very significant or that it can

only be associated with more water demanding M-60.

69

6. Conclusions

The efficiency of the reactive magnesia based concrete materials, their sustainability prospects

along with the effect of this new binder type on physical and chemical properties have been

evaluated in this study. It has been done from the perspective of accelerated carbonation curing,

as the main measure to sequestrate CO2 quickly and efficiently, and to gain improved

compressive strength, strength gain and porosity. Based on the conclusions of the study optimal

and practical environmental conditions for accelerated carbonation curing can be established. In

addition, the most effective MgO replacement level can also be chosen relying on this study.

Optimal material replacement range

o Mix M-60, containing 60% of reactive magnesia has usually achieved the

highest compressive strength (up to 85MPa), early strength gain and the lowest

porosity of all magnesia mixes, very often exceeding control mix M-0,

especially at later ages. Since it has achieved the highest carbonation level it

sequestrated more CO2 than any other material

o Mix M-20 has achieved properties similar to mix M-0, while in some cases

slightly overcoming it. In contrast mix M-40 has the lowest compressive strength

in almost all environmental conditions, that was related to the combination of

smaller degree of carbonation and greater ratio of brucite in uncarbonated bulk

material.

o Even without any reactive magnesia, mix M-0 has achieved very high results

that are compared to M-60, showing the potential of an accelerated carbonation

curing to improve early age properties of concrete materials.

Optimal CO2 concentrations:

o The greatest results were achieved at the highest, 99% CO2 concentration. All

mixes have achieved their highest compressive strengths and CO2 sequestrations

at this level.

o On the other hand, the difference in efficiency between 75% and 99% CO2 has

been far less significant than that between 50% and 75%.

70

Optimal relative humidity:

o Specimens cured in environments with 50% relative humidity have achieved the

greatest improvements in desirable properties. By far, the effect of relative

humidity was found to be much stronger than that of CO2 concentration.

o Nevertheless, the reduced humidity has influenced the chemistry of carbonated

material, and if higher MgO replacement will be used than those used in this

study, this factor might influence the properties of the material.

The effect of environmental conditions on chemical properties of a carbonated zone:

o Calcite\magnesium calcite has been identified as the dominant phase in all tested

mixtures. While it was difficult to accurately distinguish the two, it was

estimated that up to 50% of the carbonated material may include magnesium

calcite, although the actual Ca:Mg ratio of the resulting compound is not known.

o Nesquehonite has only formed in mixtures with 40% and 60% of MgO, and only

at later ages. It has also only occurred in the environments with elevated (75%)

relative humidity, and thus was found to be very dependent on the available

water for the reaction to occur.

o The densification of a microstructure was found to correlate with the increase in

Mg-bearing carbonate phases, and the densification of up to 40% in comparison

to control mixture along with a denser packing of a crystalline structure has been

attributed to a significant (up to 20%) compressive strength improvement.

Optimal element size for carbonation

o For the materials with 60% MgO replacement level the whole material was

effectively penetrated by CO2 at 7d in most conditions. This means that concrete

elements with sizes of >50mm can be used to maximize the sequestration of

CO2

o For other magnesia materials, namely mixtures M-20 and M-40, the carbonation

depth rarely exceeded 30mm, and thus they are less optimal for the sequestration

71

o In 75%RH environments mixture M-0 has achieved very low carbonation depth

of about 10-20mm. Greater depth of 30mm and more has been achieved in

50%RH environments, however this material remains the least effective in terms

of carbonation depth achieved.

o Porous blocks with wall size smaller than 50mm are the most practical for

mixtures containing 40% of MgO and less, however greater width can be used

for materials containing 60% of MgO.

72

7. Recommendation for Future Research

While reactive magnesia, accelerated carbonation and the related topics has been started to gain

much needed attention in the concrete research sphere, there are still many blind-spots that are

in the need of a special study. These questions and topics include:

- While the effect of varying CO2 concentration on MgO based material properties has been

studied under different relative humidity, there is no clear picture on higher levels of humidity

(95% and more). The formation of several important carbonate phases has been found to be

highly dependent on water content, and thus the chemistry of carbonation for magnesia

materials in these conditions remains unknown.

-Several different studies have dealt with magnesia replacement ranges higher than 60%. There

has not been though a consistent all-sided study of chemical and physical properties of such

materials, yet alone cured in different environments.

-The importance of nesquehonite, magnesium calcite and its Ca:Mg ratio has been already

highlighted, and studied before. On the other hand it was not very often been directly related to

the physical properties of the materials, especially when these phases are clearly identified to be

significant enough to influence compressive strength and other.

-This study have shown a feature of magnesia cements, where there is a drastic difference

between mixtures containing 40% and 60% of MgO. While largely assessed to be relying on

available Mg2+ content during the carbonation , it has not been entirely clear why such clear

threshold exists, and at what exact percentage it becomes the most clear.

-It is established that the increase in entrapped air content affects permeability. The degree with

which this factor affects carbonation of the concrete is, however, not entirely clear. It is also not

known how the entrapped air content is affected by %MgO content and the consequent

workability issues at specific water contents in the mixture.

-Optimization of curing conditions in this study was limited by the actual environmental

conditions in the curing chamber itself. Preconditioning techniques remain being vaguely

covered in the literature, and are an important next step to understand the most efficient

practical use of the magnesia material.

73

-It is known that carbonation is mostly a harmful process to the concrete reinforcement. Albeit

the effect of long term carbonation has been covered extensively, almost no study has actually

studied the severity of this process if it occurs as a part of the accelerated curing. In addition to

that, it has been claimed that magnesia materials induce much smaller pH drop during the

carbonation, and this claim is still to be studied and assessed, and its effect on concrete

reinforcement understood.

Reactive magnesia in general remains being only very lightly covered in the field of concrete

research. Its importance as a highly potent and a very sustainable alternative to conventional

materials is a central reason why more extensive research efforts must be relocated to study this

truly interesting cementitious material.

74

Appendix A: Compressive strength

Table A.1: Percentage Change in Compressive Strength Due to MgO used as Cement Replacement Compared to 0%MgO Mixes)



50% 50% 3d 1% -15% 27%

50% 50% 7d 7% -13% 18%

50% 50% 28d 1% 6% 21%

50% 75% 3d 1% -42% 9%

50% 75% 7d 1% -42% 2%

50% 75% 28d 0% -10% 8%

50% 99% 3d -21% -28% -3%

50% 99% 7d -20% -24% -3%

50% 99% 28d -9% 6% 6%

75% 50% 3d -29% -91% -51%

75% 50% 7d -31% -78% -25%

75% 50% 28d -13% -42% -3%

75% 75% 3d 3% -50% 14%

75% 75% 7d -2% -41% -8%

75% 75% 28d -11% -34% -15%

75% 99% 3d 4% -50% -8%

75% 99% 7d -15% -52% 2%

75% 99% 28d -10% -41% 5%

75

Table A.2: Percentage Change in Compressive Strength Due to MgO used as Cement Replacement (Incremental)



50% 50% 3d 1% -15% 27%

50% 50% 7d 7% -13% 30%

50% 50% 28d 1% 5% 16%

50% 75% 3d 1% 43% 36%

50% 75% 7d -1% -40% 31%

50% 75% 28d 0% -10% 17%

50% 99% 3d -21% -6% 20%

50% 99% 7d -20% -3% 17%

50% 99% 28d -9% 14% 1%

75% 50% 3d -29% -48% 21%

75% 50% 7d -31% -37% 30%

75% 50% 28d -13% -25% 27%

75% 75% 3d 3% -54% 24%

75% 75% 7d -2% -38% 23%

75% 75% 28d -11% -21% 15%

75% 99% 3d 4% -57% 28%

75% 99% 7d -15% -33% 33%

75% 99% 28d -10% -29% 33%

76

Appendix B: Carbonation Area

75RH- 50%CO2

% MgO

0 20 40 60

Car

bonat

ed A

rea

(cm

2)

0

5

10

15

20

25

30

3d mortar

7d mortar

28d mortar

3d paste

7d paste

28d paste


75RH- 75%CO2

% MgO

0 20 40 60

Car

bonat

ed A

rea

(cm

2)

0

5

10

15

20

25

30

3d mortar

7d mortar

28d mortar

3d paste

7d paste

28d paste


77

75RH- 99%CO2

% MgO

0 20 40 60

Car

bonat

ed A

rea

(cm

2)

0

5

10

15

20

25

30

3d mortar

7d mortar

28d mortar

3d paste

7d paste

28d paste


50RH- 50%CO2

% MgO

0 20 40 60

Car

bonat

ed A

rea

(cm

2)

0

5

10

15

20

25

30

3d mortar

7d mortar

28d mortar

3d paste

7d paste

28d paste


78

50RH- 75%CO2

% MgO

0 20 40 60

Car

bonat

ed A

rea

(cm

2)

0

5

10

15

20

25

30

3d mortar

7d mortar

28d mortar

3d paste

7d paste

28d paste


50RH- 99%CO2

% MgO

0 20 40 60

Car

bo

nat

ed A

rea

(cm

2)

0

5

10

15

20

25

30

3d mortar

7d mortar

28d mortar

3d paste

7d paste

28d paste


79

Appendix C: Porosity

75RH- 50%CO2

% MgO

0 20 40 60

Tota

l P

oro

sity

(%

)

0

2

4

6

8

10

3d mortar

7d mortar

28d mortar

3d paste

7d paste

28d paste


75RH- 75%CO2

% MgO

0 20 40 60

Tota

l P

oro

sity

(%

)

0

2

4

6

8

10

3d mortar

7d mortar

28d mortar

3d paste

7d paste

28d paste


80

75RH- 99%CO2

% MgO

0 20 40 60

To

tal

Po

rosi

ty (

%)

0

2

4

6

8

10

3d mortar

7d mortar

28d mortar

3d paste

7d paste

28d paste


50RH- 50%CO2

% MgO

0 20 40 60

Tota

l P

oro

sity

(%

)

-4

-2

0

2

4

6

8

10

12

3d mortar

7d mortar

28d mortar

3d paste

7d paste

28d paste


81

50RH- 75%CO2

% MgO

0 20 40 60

Tota

l P

oro

sity

(%

)

-4

-2

0

2

4

6

8

10

12

3d mortar

7d mortar

28d mortar

3d paste

7d paste

28d paste


50RH- 99%CO2

% MgO

0 20 40 60

Tota

l P

oro

sity

(%

)

-4

-2

0

2

4

6

8

10

12

3d mortar

7d mortar

28d mortar

3d paste

7d paste

28d paste


82

Table C.1: Percentage Change in Total Mortar Porosity Due to MgO used as Cement Replacement (Incremental)



75% 50% 3d 26% -6% -29%

75% 50% 7d 10% -7% -45%

75% 50% 28d 7% -10% -126%

75% 75% 3d 4% 8% -59%

75% 75% 7d -18% -12% -6%

75% 75% 28d -34% -14% -33%

75% 99% 3d 5% -2% 9%

75% 99% 7d -18% -17% -38%

75% 99% 28d -67% 5% 18%

50% 50% 3d -16% 24% -46%

50% 50% 7d -11% -7% -55%

50% 50% 28d -32% -16% -55%

50% 75% 3d -10% -11% -28%

50% 75% 7d -3% -28% -32%

50% 75% 28d -33% 0% -50%

50% 99% 3d 33% -50% 20%

50% 99% 7d 33% -20% -25%

50% 99% 28d 25% -5% -63%

83

Table C.2: Percentage Change in Total Paste Porosity Due to MgO used as Cement Replacement (Incremental)



75% 50% 3d 21% 14% 22%

75% 50% 7d 19% 8% 4%

75% 50% 28d 34% -4% -16%

75% 75% 3d 12% 8% -3%

75% 75% 7d 13% -4% 24%

75% 75% 28d 15% -4% -8%

75% 99% 3d 26% -40% 93%

75% 99% 7d 22% -5% -26%

75% 99% 28d 0% 8% -34%

50% 50% 3d 0% -6% -17%

50% 50% 7d -8% -29% -11%

50% 50% 28d 3% -72% 20%

50% 75% 3d -21% 17% -61%

50% 75% 7d -19% -23% -40%

50% 75% 28d -14% -40% -25%

50% 99% 3d -13% 0% -33%

50% 99% 7d -50% 0% -50%

50% 99% 28d -20% -5% -50%

84

Appendix D: X-ray Diffraction and Thermogravimetric analysis

Degrees (2)

a)

b) Degrees (2)

Figure D.1: Condition [50%RH, 50%CO2] XRD Pattern of Paste a) 3d, b) 28d

85

Degrees (2)

a)

b) Degrees (2)


86

Degrees (2)

a)

b) Degrees (2)


87

RH=50%- CO2 = 50%


0 200 400 600 800 1000 1200

Mas

s L

oss

(%

)

60

70

80

90

100 0%MgO

20%MgO

40%MgO

60%MgO

a)

RH=50%- CO2 = 50%


0 200 400 600 800 1000 1200

Mas

s L

oss

(%

)

60

70

80

90

100 0%MgO

20%MgO

40%MgO

60%MgO

b)


88

RH=50%- CO2 = 75%


0 200 400 600 800 1000 1200

Mas

s L

oss

(%

)

60

70

80

90

100 0%MgO

20%MgO

40%MgO

60%MgO

a)


0 200 400 600 800 1000 1200

Mas

s L

oss

(%

)

60

70

80

90

100 0%MgO

20%MgO

40%MgO

60%MgO

b)


89


0 200 400 600 800 1000 1200

Mas

s L

oss

(%

)

60

70

80

90

100 0%MgO

20%MgO

40%MgO

60%MgO

a)


0 200 400 600 800 1000 1200

Mas

s L

oss

(%

)

60

70

80

90

100 0%MgO

20%MgO

40%MgO

60%MgO

b)


90

Appendix E: Relative Humidity and Compressive Strength Relationships

Mix Design = 0% MgO and Conditions СО2=50%

Age (days)

0 5 10 15 20 25 30

Co

mpre

ssiv

e S

tren

gth

(M

Pa)

0

20

40

60

80

100

50% RH

75% RH

a)


Age (days)

0 5 10 15 20 25 30

Co

mpre

ssiv

e S

tren

gth

(M

Pa)

0

20

40

60

80

100

50% RH

75% RH

b)

91


Age (days)

0 5 10 15 20 25 30

Co

mpre

ssiv

e S

tren

gth

(M

Pa)

0

20

40

60

80

100

50% RH

75% RH

c)

Figure E.1: Effect of Relative Humidity on Compressive Strength for Mixture with 0% of MgO a) 50%

CO2, b) 75% CO2, c) 99% CO2


Age (days)

0 5 10 15 20 25 30

Co

mpre

ssiv

e S

tren

gth

(M

Pa)

0

20

40

60

80

100

50% RH

75% RH

a)

92


Age (days)

0 5 10 15 20 25 30

Co

mpre

ssiv

e S

tren

gth

(M

Pa)

0

20

40

60

80

100

50% RH

75% RH

b)


Age (days)

0 5 10 15 20 25 30

Co

mpre

ssiv

e S

tren

gth

(M

Pa)

0

20

40

60

80

100

50% RH

75% RH

c)

Figure E.2: Effect of Relative Humidity on Compressive Strength for Mixture with 20% of MgO a)

50% CO2, b) 75% CO2, c) 99% CO2

93


Age (days)

0 5 10 15 20 25 30

Co

mpre

ssiv

e S

tren

gth

(M

Pa)

0

20

40

60

80

100

50% RH

75% RH

a)


Age (days)

0 5 10 15 20 25 30

Co

mpre

ssiv

e S

tren

gth

(M

Pa)

0

20

40

60

80

100

50% RH

75% RH

b)

94


Age (days)

0 5 10 15 20 25 30

Co

mpre

ssiv

e S

tren

gth

(M

Pa)

0

20

40

60

80

100

50% RH

75% RH

c)


50% CO2, b) 75% CO2, c) 99% CO2


Age (days)

0 5 10 15 20 25 30

Co

mpre

ssiv

e S

tren

gth

(M

Pa)

0

20

40

60

80

100

50% RH

75% RH

a)

95


Age (days)

0 5 10 15 20 25 30

Co

mpre

ssiv

e S

tren

gth

(M

Pa)

0

20

40

60

80

100

50% RH

75% RH

b)


Age (days)

0 5 10 15 20 25 30

Co

mpre

ssiv

e S

tren

gth

(M

Pa)

0

20

40

60

80

100

50% RH

75% RH

c)


50% CO2, b) 75% CO2, c) 99% CO2

96

References

Balaguru P., Ramakrishnan V. (1989) "Chloride Permeability and Air-Void Characteristics of

Concrete Containing High Range Water Reducing Admixture", Cement and Concrete Research,

V18 p.401-414

Beaudoin, J. J and Ramachandran, V. S., (2001) "Handbook of Analytical Techniques in

Concrete Science and Technology : Principles, Techniques and Applications", illustrated

edition, Building materials science series.

Blaha J. (1997) "Kinetics of Hydration of Magnesium Oxide in Aqueous Suspension" Ceramics

- Silikaty V41 pp.21-27, 121-123.

Caijun S., Yanzhong W. (2009) "CO2 Curing of Concrete Blocks" Concrete International:

p.39-43.

Catti, M., Ferraris G., Hull S., Pavese A., (1995) Phys.Chem. Miner, V22, p.200.

Chang C.F., Chen J.W.(2005) "Strength and Elastic Modulus of Carbonated Concrete" ACI

Materials Journal, V. 102, No. 5,102-M36.

Chang C.F., Chen J.W (2004) "The experimental investigation of concrete carbonation depth"

Cement and Concrete Research V36 p.1760– 1767.

Chindaprasirt P., Jaturapitakkul C., Sinsiri T.(2005) "Effect of fly ash fineness on compressive

strength and pore size of blended cement paste" Cement & Concrete Composites V27 p.425–

428.

Choi S., Jang B., and Kim J., Lee K., (2014) "Durability characteristics of fly ash concrete

containing lightly-burnt MgO" Construction and Building Materials, V8, p. 77 - 84.

Cook R.A., Hover K.C.(1999) "Mercury porosimetry of hardened cement pastes" Cement and

Concrete Research V29 p.933–943

Cwirzen A., Habermehl-Cwirzen K.(2013) " Effects of Reactive Magnesia on Microstructure

and Frost Durability of Portland Cement–Based Binders" Journal of Materials in Civil

Engineering V25 p.1941-1950

97

Das B. B., Singh D. N., Pandey S. P., (2012) "Rapid Chloride Ion Permeability of OPC- and

PPC-Based Carbonated Concrete" J. Mater. Civ. Eng. V24 p.606-611.

De Silva P., Bucea L., Sirivivatnanon V., (2009) "Chemical, microstructural and strength

development of calcium and magnesium carbonate binders", Cem. Concr. Res. 39 460–465

Filippou D., Katiforis N., Papassiopi N. Adam K. (1999) “On the Kinetics of Magnesia

Hydration in Magnesium Acetate Solutions,” J. Chem. Technol. Biotechnol., V74 p.,322–328

García-González C.A. , Hidalgo A. , Andrade C., Alonso M.C., Fraile J., López-Periago A.M.

,and Domingo C. (2006) " Modification of Composition and Microstructure of Portland

Cement Pastes as a Result of Natural and Supercritical Carbonation Procedures" Ind. Eng.

Chem. Res., 45 (14), pp 4985–4992

Guo L., Yingshi Y., Jianmin, D., Yongsheng J., (2013) " Determination of the Apparent

Activation Energy of Concrete Carbonation" Journal of Wuhan University of Technology.

Materials science edition, Volume 28, Issue 5, p. 944 - 949.

Ghosh S. and Kanjilal K., (2014) "Long-term equilibrium relationship between urbanization,

energy consumption and economic activity: Empirical evidence from India" Energy, ISSN

0360-5442, 03/2014, Volume 66, p. 324

Guneyisi E., Gesoglu M. and Mermerdas K.(2008) "Improving strength, drying shrinkage, and

pore structure of concrete using metakaolin" Materials and Structures V41 p.937–949

Glasser F. P., Sagoe-Crentsil K. K (1989) "Steel in concrete: Part II Electron microscopy

analysis" Magazine of Concrete Research, Volume 41, Issue 149, pp. 213 - 220.

Harrison A.J.W., (2003) "The case for and ramification of blending reactive magnesia with

Portland cement", Presented at the 28th Conference on Our World in Concrete Structures,

Singapore.

Harrison A.J.W (2010) “Empirical Evidence of The Importance of Using Low Temperature

Calcined Reactive Magnesia for Cements”

98

He Y., Ye G., Troczynski T., Oprea G., (2004) "Hydration Behaviour of Magnesia in Binder

Systems For Basic Castables", Canadian Metallurgical Quarterly, V43, No 2, p. 173-176.

Johannesson, B. and Utgenannt, P. (2001) “Microstructural Changes Caused by Carbonation of

Cement Mortar”, Cement and Concrete Research V. 31, p.925-931.

Johari M.A.A., Brooks J.J., Kabir S., Rivard P. (2011) "Influence of supplementary

cementitious materials on engineering properties of high strength concrete" Construction and

Building Materials, V25, p. 2639–2648.

Kim D.H., Kim K.J., Yun K.K. (2007) "Effect of micro air void system on the permeability of

latex-modified concretes with ordinary Portland and very early strength cements" Can. J. Civ.

Eng. V34 p.895-901.

Kumar R., Bhattacharjee B. (2003) "Porosity, pore size distribution and in situ strength of

concrete" Cement and Concrete Research V33 p.155–164.

Kuo J.S. Huang W.Y., Lin C.H., (2006) "Effects of organo-modified montmorillonite on

strengths and permeability of cement mortars" Cement and Concrete Research V36 p.886 –

895.

Kropp, J., Hilsdorf, H.K. (1995) Performance Criteria for Concrete Durability. Rilem Report

12, E&FN Spon. p. 327.

Lanas J., Alvarez J.I., (2004) "Dolomitic lime: thermal decomposition of nesquehonite"

Thermochim. V421 p.123–132.

Li Y.,Chen Y., Wei J.X., He, X.Y., Zhang W.S., Zhang H.T., (2006) "A study on the

relationship between porosity of the cement paste with mineral additives and compressive

strength of mortar based on this paste, Cement and Concrete Research, V36, Issue 9, p. 1740 -

1743.

Li G., Yuan Y., Du J., and Ji Y. (2013) " Determination of the apparent activation energy of

concrete carbonation" Journal of Wuhan University of Technology-Mater. Sci. Ed, ISSN 1000-

2413, Volume 28, Issue 5, pp. 944 - 949.

99

Liska M., Al-Tabbaa A. (2008) "Performance of magnesia cements in pressed masonry units

with natural aggregates: Production parameters optimisation" Construction and Building

Materials 22 (2008) 1789–1797.

Liska M., Al-Tabbaa A. (2009) “Ultra-green construction: reactive magnesia masonry

products” Waste and Resource Management V162 Issue WR4 p. 185-196.

Lomboy G., Wang K. (2009) "Effects of Strength, Permeability, and Air Void Parameters on

Freezing-Thawing Resistance of Concrete with and Without Air Entrainment" Journal of

ASTM International, Vol. 6, No. 10

Mehta, P.K., and Monteiro, P.J.M. (2006) Concrete- Microstructure, Properties, and Materials.

3rd

Edition.McGraw Hill. P.659.

Mo, L., Panesar, D.K. (2012) "Effects of accelerated carbonation on the microstructure of

Portland cement pastes containing reactive MgO" (2012) Cement and Concrete Research V42,

p 769–777.

Mo, L., Panesar, D.K. (2013) "Properties of binary and ternary reactive MgO mortar blends

subjected to CO2 curing", Cement & Concrete Composites.

Monkman S., Shao Y.(2006) " Assessing the Carbonation Behavior of Cementitious Materials"

Journal of Materials in Civil Engineering, ISSN 0899-1561, Volume 18, Issue 6, pp. 768 - 776.

Mukherjee A., Cass D. (2012) "Project Emissions Estimator Implementation of a Project-Based

Framework for Monitoring the Greenhouse Gas Emissions of Pavement Transportation

Research Record", Issue 2282, pp. 91 - 99.

Muntean A., Peter M.A.., Meier S.A. and Böhm M. (2008) "Competition of several

carbonation reactions in concrete: A parametric study" Cement and Concrete Research V 38

(Issue 12) p.1385-1393.

Nokken R. (2010) "Expansion of MgO in Cement Pastes Measured by Different Methods" ACI

Materials Journal, V107, Issue 1, p. 80.

100

Pera J., Soudee E. (2001) "Influence of magnesia surface on the setting time of magnesia–

phosphate cement" Cement and Concrete Research V32 p.153 – 157.

Poon C.S., Kou S.C., Lam L. (2006) "Compressive strength, chloride diffusivity and pore

structure of high performance metakaolin and silica fume concrete" Construction and Building

Materials V20 p.858–865.

Powers, T.C. (1958) Journal of American Ceramic Society, V.41, No.1, p.1-6.

Rocha S.D., Mansur M.B., Ciminelli V.S., (2004) "Kinetics and mechanistic analysis of caustic

magnesia hydration" Journal of Chemical Technology & Biotechnology, V79, Issue 8, p. 816 -

821.

Rossler M., Odler I. (1985) " Investigations on the Relationship Between Porosity, Structure

and Strength of Hydrated Portland Cement Pastes. I. Effect of Porosity" Cement and Concrete

Research. Vol. 15, p. 320-330.

Rostami V., Boyd A.J., Shao Y., (2012) "Carbonation Curing versus Steam Curing for Precast

Concrete Production" J. Mater. Civ. Eng. V24 p.1221-1229.

Salomao R., Bittencourt L.R.M., Pandolfelli V.C. (2007) "A novel approach for magnesia

hydration assessment in refractory castables", Ceramics International V33 p803–810.

Salomao R., Pandolfelli V.C. (2007) "Magnesia sinter hydration–dehydration behavior in

refractory castables" Ceramics International V34 p.1829–1834

Sarkar S. L., Cheng Q., (1994) "A study of rheological and mechanical properties of mixed

alkali activated slag pastes" Advanced Cement Based Materials, ISSN 1065-7355, 1994,

Volume 1, Issue 4, p. 178 - 184.

Scherer G.W. (2008) "Poromechanics analysis of a flow-through permeameter with entrapped

air", Cement and Concrete Research V38 p.368–378.

Scott C.D., Goodkin N.,Levine N., Doney S., and Wanninkhof R. (2001) "Impacts of temporal

CO2 and climate trends on the detection of ocean anthropogenic CO2 accumulation", Global

Biogeochemical Cycles V 25, No.3.

101

Shao Y., Rostami V., He Z., and Boyd A. (2014). ”Accelerated Carbonation of Portland

Limestone Cement.” J. Mater. Civ. Eng., 26(1), 117–124.

Shi C., Wu Y. (2009) "CO2 Curing of Concrete Blocks" Concrete International, ISSN 0162-

4075, V02, p. 39.

Sisomphon K., Franke L.(2007) "Carbonation rates of concretes containing high volume of

pozzolanic materials" Cement and Concrete Research, ISSN 0008-8846, 2007, Volume 37,

Issue 12, p. 1647 - 1653.

Souza T., Luz A.P., Braulio M., Pagliosa C., Pandolifelli V.C. (2014) "Acetic Acid Role on

Magnesia Hydration for Cement-Free Refractory Castables" J. Am. Ceram. Soc., v97 p.1233–

1241

Stark J. and Ludwig H-M. (1997) “Freeze-deicing Salt Scaling Resistance of Concretes

Containing Blast-Furnace Slag-Cement” Freeze-Thaw Durability of Concrete. Edited by J.

Marchand, M. Pigeon and M. Setzer. E&FN Spon, p.107-120.

Stocker T. (2013) "The Closing Door of Climate Targets", Science Vol. 339 no. 6117 pp. 280-

282.

Stumm W. (1992) "Chemistry of the Solid - Water Interface" New York, Wiley - Interscience

Suvorov S.A., Nazmiev M.I. (2007) "Refractories based on High-Purity Magnesia-Lime Raw

Material" Refractories and Industrial Ceramics Vol. 48 p.284-289

Tans P. P., Biraud S. C., and Torn M. S., Smith J. R., Sweeney C., Riley W. J. (2013) "A multi-

year record of airborne CO2 observations in the US Southern Great Plains" Atmospheric

Measurement Techniques, V6, pp. 751 - 763.

Taylor F. H. W. "Cement Chemistry" (1990), Academic Press, New York.

Thomas S.,Liu B., Ray S., Guerbois P. (2007) " ATG analysis of the effect of calcination

conditions on the properties of reactive magnesia", Journal of Thermal Analysis and

Calorimetry, Vol. 88, 145–149.

102

Tumidajski, P.J., and Chan, G.W., (1996) “Boltzmann-Matano Analysis of Chloride Diffusion

in to Blended Cement Concrete”, Journal of Materials in Civil Engineering Nov. p.195-200.

Thiery M., Villain G., Dangla P., Platret G., (2007) "Investigation of the carbonation front

shape on cementitious materials: effects of the chemical kinetics", Cem. Concr. Res. V37

p.1047–1058.

Unluer C., Al-Tabbaa A. (2014) "Enhancing the carbonation of MgO cement porous blocks

through improved curing conditions" Cement and Concrete Research, V59, p. 55 - 65.

Vandeperre L. J., Al-Tabbaa A. (2007) "Accelerated carbonation of reactive MgO cements"

Advances in Cement Research, 19, No. 1, p.1–13.

You K., Jeong H.and Hyung W.(2014) "Effects of Accelerated Carbonation on Physical

Properties of Mortar" Journal of Asian Architecture and Building Engineering vol.13 no.1

p.217-221.

Yusuf M. O., Johari M. A.,, Ahmad Z.A. and Maslehuddin M., (2014) " Strength and

microstructure of alkali-activated binary blended binder containing palm oil fuel ash and

ground blast-furnace slag" Construction and Building Materials, ISSN 0950-0618, Volume 52,

pp. 504 - 510.

Zhang T.., Cheeseman C.R., Vandeperre L.J. (2011) " Development of low pH cement systems

forming magnesium silicate hydrate (M-S-H)" Cement and Concrete Research V41(4) p.439-

442.

effect of relative humidity and co2 concentration on the ... · yaroslav bilan master of applied...

Documents