carbon dioxide uptake by concrete through early-age...
TRANSCRIPT
CARBON DIOXIDE UPTAKE BY CONCRETE
THROUGH EARLY-AGE CURING
By
Gang Ye
Department of Civil Engineering and Applied Mechanics McGill University Montreal, Canada
A thesis submitted to the Faculty of Graduate Studies and Research in partially fulfillment of the requirement for the degree of Master of Civil Engineering
September 2003
® Gang Ye, 2003
All Rights Reserved
Abstract
Due to the anthropogenic activities, the increasing carbon dioxide concentration
in the atmosphere is disturbing the natural equilibrium of the greenhouse gas, and causes
the global temperature rise. In 1990, the CO2 emission from fossil fuel fired power plants
contributed 30% of global emissions. In the same year, the cement industry contributed
about 5% of the total. According to Kyoto Protocol, a tremendous effort is required to
reduce the carbon dioxide emission.
One potential technology in CO2 mitigation responses is the use of concrete
products as carbon sink through the early age fast curing. The cement compounds C3S
and C2S are instantaneously carbonized into calcium carbonate and silica gel, once
cement is mixed with water and exposed to the carbon dioxide gas. If it works, concrete
production lines can be set next to the power plants or cement kilns to produce the
concrete products using the captured CO2 as curing agent.
This thesis reports a feasibility study based on a preliminary work. The purpose of
the research was to find a proper combination of a large number parameters to use
cement, slag or waste cement to sequester CO2 emitted from industrial point sources, and
at the same time to make high performance concrete products. In order to understand the
carbonation curing, this study was directed towards the mix designs, carbonation
conditions and the mechanical properties of carbonated products. More than 40 batches
of carbonated concrete specimens were prepared with the following variables in their
preparation: chemical additive, CO2 concentration, carbonation time, carbonation
pressure, thickness of specimen, and CO2 supply method. The performance of the
carbonated specimens was assessed through the mass gain, the compressive strength, the
bending strength, the pressure drop, the temperature rise in the curing chamber, the
carbonation depth and the microstructure characteristics. Two-hour carbonated concrete
products can have a strength equivalent to 2-month air curing, and take up 8% carbon
dioxide by weight without special treatment.
Resume
En raison des activites anthropiques, la concentration croissante en dioxyde de
carbone (CO2) dans l'atmosphere destabilise l'equilibre normal de l'effet de serre et
suscite l'elevation de la temperature moyenne globale. En 1990, les centrales alimentees
en energie par des combustibles fossiles ont emis 30% des ejmissions globales de
dioxyde de carbone (CO2). En cette meme annee, l'industrie du ciment a contribue 5%
aux emissions globales de dioxyde de carbone (CO2). Selon le protocole de Kyoto, un
effort considerable est necessaire affin de reduire les emissions globales de dioxyde de
carbone (CO2) dans l'atmosphere.
Une technologie potentielle qui reduirait ces emissions consisterait a utiliser le
dioxyde de carbone (CO2) emis par les usines comme un agent de murissement du beton.
Lorsque le ciment est melange a l'eau et est expose au dioxyde de carbone(C02), les
principaux constituants (C3S et C2S) sont instantanement carbonises en une pate de
silicates de calcium hydrates. Si cela fonctionne, des chaines de production de produits
en beton pourraient etre placees a cote des centrales ou des fours a ciment affin d'utiliser
le dioxyde de carbone emis par ces sources en tant qu'adjuvant de cure.
Cette these est une etude de faisabilite basee sur un travail preliminaire. Le but de
cette etude est de proposer un arrangement de parametres qui maximiserait l'usage du
dioxyde de carbone (CO2) dans la production de produits en beton de qualite superieure.
Affin de comprendre plus amplement ce processus de carbonisation qui accelere la
periode de cure du beton, diverses recettes de beton sous diverses conditions de
carbonisation ainsi que diverses proprietes mecaniques seront etudiees. Divers
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parametres seront analyses dans plus de 40 melanges differents de beton tels que : Fusage
d'adjuvants, la concentration en dioxyde de carbone(C02), la duree de carbonisation, la
pression de carbonisation, l'epaisseur des specimens et la methode d'approvisionnement
en dioxyde de carbone (CO2). Les parametres qui permettront d'evaluer la performance
des specimens carbonises seront les changements en masse, la resistance en compression,
la resistance en flexion, les changements en pression, les changements en temperature, la
profondeur de carbonisation et la microstructure du beton. Un beton carbonise pendant 2
heures developpe une resistance equivalente a celle d'un beton non traite apres 2 mois.
Dans un tel cas, le beton traite absorberait 8% par masse de gaz carbonique (CO2) sans
avoir besoin de recourir a aucun traitement special additionnel.
IV
Acknowledgment
The author wishes to thank Dr. Y. Shao for his invaluable guidance,
knowledgeable advice, time and effort throughout this project.
The carbon dioxide carbonation system was installed by John Bartczak. The
author would like to thank John Bartczak and Ronald Sheppard for their help with the
tests.
Table of Content
Abstract i
Resume iii
Acknowledgement v
List of Figures ix
List of Tables xi
Charpter 1 Introduction 1
1.1 Greenhouse gas effect 1
1.2 Sources of carbon dioxide 2
1.3 Mitigation responses to global warming 4
1.4 Concrete as CO2 sink through early age curing 5
Chapter 2 Literature Review 9
2.1 Compacted calcium silicate mortars and powers on exposure to
C02 9
2.2 Rapid carbon dioxide curing for Wood-cement composite 12
2.3 Carbon dioxide curing of waste concrete 14
Chapter 3 Research Obj ectives 16
3.1 Mixture properties and CO2 curing 16
3.2 Experiments for performance evaluation 17
Chapter4 Experimental Program 19
4.1 Carbon dioxide curing setup 19
vi
4.1.1 Carbon dioxide gas tank 19
4.1.2 Air gas tank 19
4.1.3 Pressure curing vessel 20
4.1.4 Pressure gauge 20
4.1.5 Vacuum pump 20
4.1.6 Regulators 21
4.1.7 Heater 21
4.1.8 Thermocouple 21
4.1.9 Data acquisition system 22
4.1.10 Method of C02 injection 22
4.2 Materials 22
4.2.1 Cementitious binders and sand 23
4.2.2 Additives 24
4.2.3 Fibers 25
4.3 Mix design and specimen preparation 26
4.3.1 Cylinder mix design 26
4.3.2 Plate mix design. 27
4.4 Experiments for performance assessment 30
4.4.1 Mass gain after carbonation 30
4.4.2 Carbonation pressure drop 30
4.4.3 Carbonation temperature rise 31
4.4.4 Measurement of carbonation depth 31
4.4.5 Compression test for cylinder specimens 32
4.4.6 Compression test for plate specimens 32
Vll
4.4.7 Three-point bending test for plate specimens 32
4.5 Sample preparation 33
Chapter 5 Results and Discussion 40
5.1 Preliminary study with 12mm-diameter cylinder sample 40
5.1.1 Effect of additives 40
5.1.2 Effect of carbonation time 42
5.1.3 Effect of cementitious binder 42
5.2 Plate specimen test results 44
5.2.1 Effect of additives in plate specimens 45
5.2.2 Effect of CO2 concentration 47
5.2.3 Effect of carbonation time 49
5.2.4 Effect of carbonation pressure 50
5.2.5 Effect of specimen thickness 51
5.2.6 Effect of cellulose fibers 53
5.2.7 Effect of binder 53
5.2.8 Continuous supply with 100% CO2 concentration 55
5.2.9 Continuous supply with 25% CO2 concentration 56
5.2.10 Carbonation at continuous CO2 supply with different mix 57
5.2.11 Compressive strength of carbonated plates 60
Chapter 6 Conclusion 80
References 84
Appendix A Tables for Cylinder Test 86
Appendix B Tables for Plate Test 90
Vll l
List of Figures
Figure 4.1 schematic of setup 37
Figure 4.2 Picture of Setup 37
Figure 4.3 Carbonation depth definition 37
Figure 4.4 Compression test for plate specimens 38
Figure 4.5 Three-point bending test for plate specimens 38
Figure 4.6 Pressure vessel with carbonated sample 39
Figure 5.1 Effect of additives in cylinder specimens 62
Figure 5.2 Effect of sodium hydroxide to cement ratio 62
Figure 5.3 Effect of carbonation time in cylinder specimens 63
Figure 5.4 Effect of cementitious binder in cylinder specimens 63
Figure 5.5 Effect of cementitious binder in cylinder specimens 64
Figure 5.6. Effect of additives in plate specimens 65
Figure 5.7 SEM of batch Bl andB2 65
Figure 5.8 Cross section of Bl, B2, B3 and B8 66
Figure 5.9 Effect of CO2 Concentration 66
Figure 5.10 Pressure drop during carbonation curing 67
Figure 5.11 Effect of CO2 concentration on carbonation temperature 67
Figure 5.12 SEM of batch B20 and B21 68
Figure 5.13 Effect of carbonation time 69
Figure 5.14 Cross section of Bl, B12 and B17 69
Figure 5.15 Effect of carbonation pressure 70
IX
Figure 5.16 Cross section of BIO and Bl l 70
Figure 5.17 Effect of specimen thickness 71
Figure 5.18 Cross section ofBl and B14 71
Figure 5.19 Effect of cellulose fibers 72
Figure 5.20 Effect of binder 73
Figure 5.21 Cross section of B21, B4 and Bl8 73
Figure 5.22 SEM of batch B4 and Bl 74
Figure 5.23 Carbonation at 100% CO2 concentration 75
Figure 5.24 Cross section of B17, B21 and B22 75
Figure 5.25 Carbonation at 25% CO2 concentration 76
Figure 5.26 Cross section of B9, B16 andB20 76
Figure 5.27 Carbonation at continuous CO2 supply of two hours 77
Figure 5.28 Comparison of 2 -hours carbonation curing with 7-day air curing 78
Figure 5.29 Cross section of Mortar B26 andB27 78
Figure 5.30 Compressive strength of carbonated concrete plates 79
List of Tables
Table 1.1 Contribution of the major radiative gases affecting the
earth-atmosphere energy 2
Table 1.2 Global Energy Consumption by Energy Source and
Equivalent CO2 Emissions (1990) 3
Table 1.3 Distribution of world C02 emission (1990) 3
Table 4.1 Chemical compositions of candidate materials for C02 absorption 23
Table 4.2 Chemical component of calcium hydroxide 24
Table 4.3 Chemical component of sodium hydroxide 25
Table 4.4 Chemical component of calcium oxide 25
Table 4.5 Mix design of cylinder 27
Table 4.6 Mix design of plate specimen 29
XI
Chapter 1
Introduction
1.1 Greenhouse gas effect
Earth's atmosphere acts like a blanket to absorb the sun's solar radiation, which
heats the earth's surface and keeps it warm. The average temperature of the earth's
surface is about 15°C; without the atmosphere the surface temperature of the earth can be
calculated to be -19°C, like the moon's surface. There is no any atmosphere to help the
moon keep its surface warm. Even though it receives the same amount of solar radiation
as the earth, the mean surface temperature of the moon is about -23°C. This atmospheric
phenomenon that causes a +34°C warming of the earth is called the greenhouse gas effect.
The greenhouse gas effect maintains a viable and comfortable condition for the life on the
earth.
Due to human and anthropogenic activities, the increasing carbon dioxide gas
concentration in the atmosphere is currently disturbing the natural composition of the CO2
greenhouse gases. Furthermore, some argue that the atmospheric CO2 increase is causing
a global temperature increase. As the temperature increases, more water vapor, which is
also a greenhouse gas is released into the atmosphere. Most scientists agree that the earth
is warming at a faster rate than at any time in the last 10,000 years, and that this warming
is caused by increasing amounts of carbon dioxide and other greenhouse gases in the
earth's atmosphere. There are many potential effects and consequences expected to result
from a rise in global temperature. The ocean water level is expected to rise and threaten
many coast cities with floods due to melting glaciers, melting Antarctic ice caps, and the
thermal expansion of the ocean water. In the tropic zone, the desertification is expected to
be a prevalent trend. The impact of global warming on people and nature is severe, and
will disturb the viable and comfortable environment.
Carbon dioxide (CO2) is the dominant greenhouse gas (GHG) resulting from
anthropogenic activities, contributing to 63.9% of the enhanced greenhouse effect. Other
greenhouse gases include methane (CH4), nitrous oxide (N2O), ozone (O3),
chlorofluorocarbons (CFCs), and fluorocarbons (CFs), which are also the results from
anthropogenic activities. The relative contribution of these gases to the climate change is
shown in Table 1.1.
Table 1.1 Contribution of the major radiative gases affecting the earth-atmosphere energy (in 1992 Values) [Halmann,1999]
Gas Radiative
forcing (W/m2) % Effect
C02
1.56
63.6
CH4
0.47
19.2
N20
0.14
5.7
CFC and others
0.28
11.5
Total
2.45
100 Source: IPCC 1995.p. 17
1.2 Sources of carbon dioxide
Starting from the end of 19th century, the predominant energy supply included
coal, fossil fuel, biomass (agriculture and wood), solar energy, and nuclear energy. The
global (annual) energy consumption by energy source for 1990 is shown in Table 1.2.
75% of energy came from fossil fuels, and the total equivalent C02 emissions were 5.6
Giga ton carbon per year. Even though the energy consumption from the use of oil was
over 40%) higher than that from coal, they produced almost the same CO2 emission
amount. This is because the combusting efficiency of oil is higher than that of coal.
2-
Table 1.2 Global energy consumption by energy source and equivalent CO2 emissions (1990) [Halmann,1999] Energy source Coal Oil Gas Nuclear Hydro Biomass Total
EJ/yra
91 128 71 19 21 55 385
%
23.7 33.2 18.4 4.9 5.5 14.3 100.0
GT (C)/ yr" 2.3 2.4 0.9
5.6
%
40.4 42.7 16.9
100.0 a EJ= 1018joiiles b GT (C)/yr = 106 tone of C as C02 per year Source: IPCC 1996. p.83
The distribution of CO2 emissions from different sectors of the world economy is
shown in Table 1.3. In 1990, the CO2 emission from fossil fuel fired power plants
amounted to 30% of global emissions, equivalent to 1.8 billion tons of carbon. In the
industrial sector, the cement industry creates a relatively high concentrated CO2 emission.
It is estimated that the cement industry contributed about 5% to global CO2 emissions,
equivalent to 1100 million tons of CO2 and 300 million tons of carbon [Worrell, E., 2001].
The carbon dioxide emission from the transportation sector was hard to collect and to use.
Table 1.3 Distribution of world C02 emission (1990) [Halmann,1999] Energy-Consuming sector Power and heat generation from industry Transportation Commercial and residential CO2 emission
% of world CO2 emission 47 22 31
5.6GT/yr
In 1996, Canada was the ninth largest country producer of CO2 emissions from fossil
fuels. This amount of carbon dioxide gas from fossil fuel was about 120 million tons. The
cement production in Canada in 1994 was 10 million tons and the corresponding CO2
emission was 8 million tons, an equivalent of 2.2 million tons of carbon. Canada signed
3-
the Kyoto Protocol and committed the country to work toward the ratification of a
binding target in 2002. The target was to reduce annual GHG emission to a level of minus
6% relative to the 1990 level in the 2008-2012 time frame, which was estimated to have
been the equivalent of 601 Mt of C02 [Mourits, 2001]. According to the standards set by
the Kyoto Protocol, there is a lot of carbon dioxide to be removed, recovered and
disposed of in Canada in the following 10 years.
1.3 Mitigation responses to global warming
Carbon dioxide is the most influential greenhouse gas, so the current research of
mitigating technologies was focused on the CO2 emissions from the use of fossil fuel.
These mitigating efforts included the following [Halmann, 1999]:
• Improve energy efficiency. Improve the conversion efficiencies from fossil
fuel energy to electrical and thermal energy and improve the utilization
efficiencies of the electrical and thermal energy to reduce the CO2 emission.
• Fuel switch. Use natural gas and oil instead of coal to reduce the CO2
emissions.
• Removal, recovery, and disposal of COi. The CO2 emitted from the fossil-
fuel-burning power and engines can be recovered. The gas can also be
disposed into the ocean, land aquifers, depleted gas and oil wells, salt domes,
and natural minerals.
Utilization of COi. The recovered CO2 and collected waste gas from thermal
power plant could be converted into construction materials, or used for
beverage production and industrial chemical manufacturing.
Use non-fossil energy source. Nuclear energy, hydroelectric power plant,
geothermal energy and wind power energy do not release any carbon dioxide.
Solar energy has minimal carbon dioxide emissions.
1.4 Concrete as CO2 sink through early age curing
Studies on capturing and disposing CO2 in oceans, depleted gas and oil wells are
currently ongoing. Furthermore, utilization of CO2 recovered from stack gases has been
explored for urea production [Halmann, 1999] and enhanced oil recovery [Mourits, 2001].
Finding beneficial uses of as-captured or recovered CO2 is challenging and critical to
greenhouse mitigation. One potential technology is to use the captured or recovered CO2
as a curing agent in production of carbonated concrete products. The process is called
curing carbonation.
The curing carbonation process is different from weathering carbonation that
naturally occurs in hardened concrete. Weathering carbonation is well known and has
been extensively investigated. In weathering carbonation, hydration takes place first when
cement is mixed with water and is followed by natural carbonation, a reaction between
the hydration products and the atmospheric carbon dioxide. The weathering reactions of
major hydration products (calcium hydroxide and calcium-silicate-hydrates) are:
Ca(OH)2+2C02 -* CaC03+ H20 ( 1 )
3CaO«2Si02«3H20 +3C02+ -* 3CaC03«2Si02«3H20 ( 2 )
Weathering carbonation of concrete is a slow process, and becomes a concern in steel
reinforced concrete structure since the carbonation decreases concrete pH, which helps
initial corrosion of reinforcing steel.
The underlying principal is that the cement compounds C3S and C2S are
instantaneously carbonized into calcium carbonate and silica gel once cement is mixed
with water and exposed to the carbon dioxide gas. Curing carbonation is an accelerated
curing process that injects CO2 gas into the curing vessel at room temperature, diffuses
the carbon dioxide into the fresh concrete under low pressure, and transforms the gaseous
CO2 into solid calcium carbonates (CaCOs).
3CaO«Si02+3C02+MH20 -* Si02«MH20+3CaC03 ( 3 )
2CaO«Si02+2C02+MH20^ Si02«AtH20+2CaC03 ( 4 )
Equations (3) (4) are the summation of various reactions, e.g. the dissolution of C02(g) to
C02(aq); the reaction of C02(aq) with H2O resulting in the production of H+ and HCO3"
ions, and subsequent reaction of the H+ ions with the 3CaO«Si02 and 2CaO«Si02 to
release Ca2+(aq) and the subsequent reaction of Ca2+ and HCO3" to produce CaC03(s),
which forms the basis for the CO2 sequestration [Bukowski, 1978]. Since curing
carbonation is a highly exothermic reaction, concrete is solidified at a much faster rate
than by steam curing at 75°C. The carbonation products are primarily calcium carbonates
and silica gel (Eqs. 3 & 4). For applications without reinforcing steel, the carbonated
concrete products can increase performance with respect to achieving strength, durability
and stable dimensions, due to the near-complete depletion of calcium hydroxide. It is
most suitable for concrete products, such as blocks and cement boards.
Accelerated C02 curing for cement is not a new concept. Hardening of alkaline
earth hydroxide cements and mortars using their reaction with atmospheric C02 has been
practiced for thousands of years. However, strength development was very slow. In
1970's, a systematic study was carried out at the University of Illinois on the reactivity
and strength development of hydraulic and non-hydraulic calcium silicates activated by
CO2 [Young, 1974]. This technique was introduced to cement-bonded particleboard
production to reduce the press time due to the fast setting in a CO2 rich environment
[Simatupang, 1995]. The world's first CO2 curing production line for cement flake board
was established in Hungary in 1985. However, the high cost of the CO2 gas production
prevented this development from having a wider commercial application.
As concern for the greenhouse gas effects and global climate changes grows, the
interest in using concrete as a carbon sink has been renewed. This thesis will explore the
possibility of implementing accelerated C02 curing of concrete using as-captured and
recovered CO2 from cement plants. The gas from cement kilns (kiln gas) has a high CO2
concentration and could be an excellent curing accelerator for concrete. CO2 curing thus
can make concrete production environmental friendly through the following reaction:
Limestone (CaCOs) -» CaO + CO2 -» Cement -» captured CO2 -» cement +
aggregate + H2O + recovered CO2 -» concrete products with limestone structure (CaC03)
The production line can be set next to cement plants, using the cement produced in the
plant and the CO2 captured or recovered from the same kiln. The technology is also
applicable to the other major control sources such as thermal power plants. Similar
production lines can be established close to the power plant, using fly ash or bottom ash
generated from coal-burning process and CO2 recovered from the fuel gas to make the
carbonated concrete products.
Chapter 2
Literature Review
Carbonation cementitious system is not a new process; its origins can be traced
back thousands of years. Humans have used alkaline earth hydroxide cement and mortar
as a binder to build structures, which harden due to their reaction with the carbon dioxide
in the atmosphere. Because of the low concentration of CO2 in atmosphere and low
pressure of CO2, the diffusion of CO2 into mortar is very slow. This results in a slow
strength development of the mortar. Since 1970's, researches were conducted in an
attempt to understand the carbonation mechanisms and their applications in fast curing of
cement and concrete products.
2.1 Compacted calcium silicate mortars and powders on exposure to CO2
Bukowski and Berger [1978] of University of Illinois used the C2S, CS and
Portland cement as binder to research carbon dioxide gas curing. The ratio of binder to
sand was one to one by weight, and the ratio of water to binder was by weight was 0.202,
0.206 and 0.191 for C2S, CS and Portland cement, respectively. The mortar was mixed by
hand for approximately 3 minutes and then compacted at 26MPa pressure into 15.9mm in
diameter cylinders approximately 20mm in height. After compaction, the cylinder was
kept in a vessel with 95% relative humidity for 2 hours before carbonation. They also
made calcium silicate powders for carbonation with the same water to cement ratio as the
compact mortars.
- 9 -
In their research, two kinds of carbonation methods were developed: a dynamic
flow system in which carbon dioxide gas was at atmospheric pressure and flowed through
the reaction vessel at a rate of 1.4 L/min, and a static system in which the gas pressure
was used and ranged from 0.1 to 5.62 MPa over a constant time of 15 minutes.
Immediately after carbon dioxide curing, all these cylinders were tested for their
compressive strengths. Afterwards the broken cylinders were stored for taking scanning
electron microscopy and other further analysis.
For the carbonated cylinders at atmospheric CO2 pressure in a dynamic system,
they found that the percentage of reacted non-hydraulic calcium silicate mortars and
powders increased with time of carbonation over the range of 5 to 1440 minutes. The C2S
had a much higher initial reaction rate than CS. The degree of carbonation for powder
samples was lower than that of compact mortars. This might have been the result of the
gas flow catching the water from the powder during carbonation.
In the static system carbonation method, it was observed that the degree of
carbonation was proportional to the increase of gas pressure. For 15 minutes of
carbonation at atmospheric pressure, the static system looked like it was inhibiting the
carbonation reaction. The compressive strength of the static system cylinders was about
30MPa and 30% lower than that of equivalent test in the dynamic system under the same
gas pressure. The compressive strength of the C2S and portland mortar with 5 minutes of
carbonation was approximately equal to that of Portland cement hydrated under normal
conditions for one day. Bukowski and Berger [1978] also found that the compressive
strength was a function of the carbon dioxide gas pressure, and for portland cement at
0.31 MPa pressure, the mortar reached its maximum compressive strength, while a
10-
pressure increase from 0.31 to 5.62MPa did not produce a significant increase in
compressive strength.
Another study was carried out by Young, Berger, and Breese [1974]. In their
experiments, the C3S and /3-C2S was mixed with silica sand in a 1:1 weight ratio. Then
water was added to give a water to cement ratio of 0.125, and the mortar was then
compacted at a pressure of 6500Psi (44MPa) into cylinders 3/8 inch in diameter and 3/4
inch in height. To prevent moisture evaporation, their cylinders were kept at 100%
relative humidity for 30 minutes before they were exposed to carbon dioxide gas.
These cylinders were cured with CO2 for 3 to 81 minutes in both a dynamic and
static system, which is similar to that used by Bukowski and Berger [1978]. However, in
the dynamic flow curing, CO2 gas was passed through a saturated Mg(N03)2«6H20
solution to condition the gas to a nominal 50% relative humidity before CO2 was allowed
to slowly flow across the vessel. In the static method, the cylinders were put into a bag
that was then injected with carbon dioxide gas. For long durations of carbonation, the bag
was periodically re-inflated with CO2 to avoid CO2 starvation.
The split-cylinder test was done to determine tensile strengths, and compressive
strengths test was done directly on the uncapped cylinders. They also performed scanning
electron microscope tests to analyze the carbonation.
For the first 10 minutes, the carbonation reaction was rapid and considerable heat
was released. It was clear that significant amounts of water evaporated from the sample in
the static system. The strength was developed quickly in the compact mortars of both C3S
and 18-C2S. The compressive strength of both samples was over 7000Psi (50MPa) after
81 minutes carbonation. They found that the water content was the most important
parameter for the carbonation process, and the distribution of the reaction was not
-11 -
uniform through the specimen. Most of the reaction took place at the cylinder's surface
while little occurred at the center.
Wagh and Singh [1995] of Argonne National Laboratory also studied carbonation
curing using commercial Type I Portland cement. They mixed the cement with silicate
sand, with the proportion of cement ranging from 100 to 30 %. The mixture paste was
compressed into a cylindrical mold at 2000 Psi (14 MPa). The curing system used was a
desiccator with an inlet and an outlet. Some water was put in the bottom of the desiccator
to maintain the humidity of the C02 gas. After carbon dioxide curing for 30 minutes, the
cylinders were dried at 75°C. They also made reference cylinders cured in air at the same
time, but these cylinders did not set. The compressive strength of the cylinders containing
only cement was 4780 Psi (33.46 MPa), and that of cement concrete (50 % sand) was
2612 Psi (18.28 MPa). The carbonated cement sample almost reached the strength of
regular 28-day-set Portland cement.
2.2 Rapid carbon dioxide curing for wood-cement composite
Simatupang and his research group [1995] developed a manufacturing process for
cement particleboards in order to reduce the press time. First the wood particles were
soaked with water, then added to Portland cement and mixed until it became a
homogeneous mixture. A special stainless steel apparatus was used to do rapid CO2
cement curing. It included three parts: the lower part with a perforated disc and a three-
way valve to apply either vacuum or carbon dioxide pressure, the press sleeve to take up
the moist wood/cement mixture and the piston to compress the mortar. The mixture was
put into the sleeve and compacted slightly by piston. The compaction pressure was 4 MPa.
- 1 2 -
After the vacuum of 0.1 Bar, carbon dioxide was injected into the specimen. A special
press plated was designed for the C02 injection during compression. This press plate was
installed on both sides of the specimen, so the top and bottom surfaces of each specimen
could be carbonated. The water to cement ratio was varied from 0.1 to 0.6, which took
into account the water absorption of wood. The diameter of specimen was 50mm and the
thickness was about 12.4mm.
The bending strength of the cement particleboards cured under a carbon dioxide
gas pressure of 5 bar was 13.8 MPa, which showed a slightly higher bending strength
than those made at 7 or 9 bar. The bending strength of 28-day conventional steam cured
spruce board was about 14.4 MPa.
They found that higher gas pressure creates a higher maximum hydration
temperature and shorter setting time. But the increase from 7 Bar to 9 Bar did not
apparently increase the carbonation. The water to cement ratio and the porosity were also
important parameters for the carbon dioxide curing. The water to cement ratio should be
high enough to provide sufficient water, but if the ratio was too high, the gas permeability
was hindered to achieve good penetration and carbonation [Simatupang, 1995].
A group from Kyoto University, Japan, also conducted research on cement-
bonded particleboard by using carbon dioxide curing [Hermawan, 1998]. They used three
types of curing methods: C02 gas curing, supercritical CO2 curing, and conventional
curing. The samples were fabricated at a cement/wood particle/water ratio of 2.2:1.0:1.1
by weight. The concentration of CO2 gas was close to 100 %; while the supercritical CO2
concentration was 10, 20, 50, and 100 %. The dimension of the sample was 50 x 210 x 12
mm. After CO2 curing, samples were put in an oven at 80°C for 10 hours, then they were
kept at room temperature prior to property evaluation. The samples were evaluated by
- 1 3 -
following three tests: three point bending test, internal bond strength, and water
absorption test.
Compared to the conventional curing method, the gaseous and supercritical
carbon dioxide curing significantly increased the mechanical properties and dimensional
stability of the cement-bonded particleboard. The mechanical properties of the particle
boards made by gaseous CO2 were similar to those by supercritical carbon dioxide curing;
although the supercritical C02 curing accelerated the strength development faster than the
gaseous CO2 curing.
2.3 Carbon dioxide curing of waste concrete
Teramura and Isu [2000] interested in the use of waste autoclaved lightweight
concrete (ALC) as binder in carbonation process. The waste ALC were crushed and
sieved to under 1.8 mm and then pulverized by a ball-mill for 60 minutes. The water to
solid ratio was in the range of 25-65% by weight. The wet waste ALC was compacted in
the mold under lOMPa pressure to form the plate 100 x lOOx 12 mm. The carbonation
process used 100% concentration CO2 and gas pressure from atmospheric to 0.4 MPa.
They also experimented atmospheric carbonation by using 3% CO2 concentration and
atmospheric pressure. The carbonated samples were dried in an oven at 60°C for 24 hours
after carbonation. These plates were tested by a three point bending test, at a cross-head
rate of 0.2mm/min.
For the waste concrete, Teramura found that 50% was the optimum water to
cement ratio for carbonation. The highest bending strength was 4.8 MPa. The bending
strength of samples made of waste concrete increased linearly with the carbonation
- 14-
degree. Also, the strength of fine powder carbonated samples was higher than that of
coarse powder samples at the same carbonation degree [Teramura, 2000].
-15
Chapter 3
Research Objectives
The purpose of this research was to explore the possibility of using as-captured
CO2 or recovered CO2 from point source such as cement kiln in the manufacture of
carbonated concrete products. A preliminary study is reported in this thesis. The goal was
different from the previous work in that the present study in directed towards not only the
early strength development by carbonation, but also the increase of mass gain due to the
carbonation by the cementitious binder. In order to understand the mechanism of the
carbon dioxide absorption by concrete materials, this research was focused on the effect
of the mix design, the carbonation conditions and the properties of carbonated products.
3.1 Mixture properties and CO2 curing
The objective of developing various mixture properties was to examine their
effect on CO2 absorption and early age strength. Chemical additive method was employed
to enhance CO2 absorption. The additives include calcium oxide, sodium hydroxide,
calcium hydroxide. Type 10 portland cement, ladle slag, and waste cement were used as
the binder and the CO2 absorbent in carbonated samples. Cellulose fibers were added in
some batches for better toughness and absorption. And fine sand as filler for mortar batch.
Water to cement ratio of 0.15 was kept as a constant and samples are press formed under
8 MPa pressure.
- 16
To find the best conditions for the carbonation process, samples were treated by a
variety of combinations of process parameters:
• Concentration of C02: 25%, 50%, and 100%;
• Carbonation time: 5 minutes, 10 minutes, 15 minutes 120 minutes and 180 minutes;
• Carbonation pressure: 2 Bar, 4 Bar, 5 Bar;
• Method of CO2 supply: one-time supply and continuous supply.
25% and 50% concentration were used to simulated the as-received and partially
recovered exhaust gas from thermal power plant or cement plant. The use of 100% CO2
was to mimic the fully recovered gas. This was designed to understand how the
concentration of CO2, the carbonation time, the carbonation pressure and the supply
methods could affect the properties of carbonated samples.
3.2 Experiments for performance evaluation
The following experiments were performed to assess the behavior of carbonate
products:
• The mass gain: The mass of samples was measured before and right after carbonation.
Disregarded the water evaporation, the difference of the mass was the mass gain of
the sample. The actual mass gain could be higher because of the evaporation.
• The pressure drop: The carbon dioxide was sequestered by the cement samples while
the carbonation reaction took place. The pressure drop in the vessel was monitored as
an indicator of the reaction.
17-
• The temperature rise: The temperature rise of the plate was another indication of
carbonation, which was recorded using thermocouple during the carbonation.
• The carbonation depth: Solution of 1% phenolphthalein was used to determine the
depth of carbonation in comparison with the direct measurement of solid carbonated
skin.
• The flexural and compressive strength: The three-point bending test was done for
determination of flexural strengths of carbonated plates, and the compressive tests for
compressive strength right after carbonation.
• SEM observation: Scanning electron microscope was employed to analyze
microstructure of the carbonation products.
Chapter4
Experimental Program
4.1 Carbon dioxide curing setup
The test setup for carbonation of concrete included carbon dioxide gas tank, air
gas tank, pressure curing chamber, pressure gauge, vacuum pump, heater, thermocouple,
and date acquisition. Figure 4.1 is the schematic of the setup. Carbonation system had
both air and CO2 gas tanks so that CO2 could be diluted to simulate the exhaust gas from
the thermal power plant or the cement plant. The system is shown in Figure 4.2.
4.1.1 Carbon dioxide gas tank
Carbon dioxide gas used in this project was a manufactured gas, compressed in
liquid state and stored in high-pressure tank (Megs Inc.). The purity of carbon dioxide
was 99.8%o. The CO2 in the cylinder of a size 1A weights 27.22 Kg. The cost was
approximately $1.36/Kg.
4.1.2 Air gas tank
Compressed air was stored in high-pressure tank and was used to dilute the CO2 to
achieve low concentration. The air cylinder was supplied by Praxair Canada Inc (product
NO. MSDS# E-6778-D).
- 19-
4.1.3 Pressure curing vessel
The Model 1500 15 bar pressure plate extractor was the product of Soil moisture
Equipment Corp; this model Pressure Plate Extractor was used as a pressure curing
chamber in a pressure range from -1 Bar to 15 Bar. The pressure vessel was 10 cm (4
inch) deep and had an inside diameter of 30 cm (12 inch). Up to 6 cement plate samples
(127 x 76.2 x 12.7 mm) could be accommodated at one time. The model 1500 consisted
of a pressure vessel and lid, clamping bolts, O-ting seals, and outflow tube assemblies.
4.1.4 Pressure gauge
Pressure gauge was the product of Duro United Instruments. The pressure of the
gauge ranges from -1 Bar (-14 Psi) to 14 Bar (200 Psi) and the precision of this gauge
was 0.2 Bar. The pressure gauge measured the volume of the gas injected into the vessel
and monitored the possible pressure drop during carbonation in one-time supply.
4.1.5 Vacuum pump
This pump was used to generate a vacuum in the curing vessel. This Vacuum
pump was the product by Central Scientific Company. (Catalog No 91308, Serial No.
146). The vacuum was needed when 100% C02 gas was used in carbonation.
20-
4.1.6 Regulators
Regulators were used in gas delivery system to reduce pressure from high-
pressure source to a safe working pressure for use. There were two regulators in the CO2
curing system. Matheson Inc made both of the regulators for C02 and for Air cylinder.
Each regulator had an inlet gauge and an outlet gauge, the inlet gauge was used to
monitor the source gas pressure and the outlet gauge was used to monitor the outlet gas.
Both of these two inlet gauges had the same pressure range, from 0 to 210 Bar; and had
the same precision 7 Bar. Both of the outlet gauges had the same pressure range from 0 to
14 Bar (0 to 200 Psi), and the same precision 0.35 Bar. It was very useful when a
continuous supply of CO2 or air for the curing chamber was required at a designed
pressure value.
4.1.7 Heater
Because the carbon dioxide was stored in a gas tank in liquid state, it emitted from
the high-pressure tank while absorbing a lot of heat. The temperature of the gas was much
lower than the room temperature. This heater was necessary to warm the Carbon dioxide
from the tank and made sure the gas reaches a room temperature when it passed over the
heater. This heater used power and was also a product of Matheson.
4.1.8 Thermocouple
21
Thermocouple was employed to measure the temperature of the carbonation
reaction between the cement and the CO2 gas. One Type T (copper-constant Nist 1990)
thermocouple with a precision of 0.1 °C was used with data acquisition system.
4.1.9 Data acquisition system
The data acquisition system is the product of Measurement Groups Inc (System
5100). There was 40-s total conversion time per reading. Input channels in each single
scanner were scanned sequentially at 0.04-ms intervals and stored in random access
memory within a 1 -ms window.
The data acquisition operation software was Strain smart Version 2.21.
StrainSmart is a ready-to-use, Window based software system for acquiring, reducing,
presenting, and storing measurement data from strain gages, strain-gage-based
transducers, thermocouples, temperature sensors, LVDT's, potentiometers piezoelectric
sensors, and other commonly used transducers.
4.1.10 Method of C02 injection
Two methods were used to supply gas into the vessel: one-time supply method
and continuous supply method. For one-time supply method, the valve was switched off
when the pressure reached designed value; for continuous supply condition, the valve was
left on when the pressure reached designed value.
4.2 Materials
- 2 2 -
4.2.1 Cementitious binders and sand
A CSA type 10 Portland cement (PC) was used as the binder in this carbonation
research. Because the content of the tricalcium and dicalcium silicate in type 10 cement
was high, it should have the ability of taking up a significant amount of carbon dioxide.
Actually it is the calcium oxide component in Portland cement that reacts with Carbonate
dioxide in the presence of water. The type 10 cement contains 62.9% CaO and a fineness
of350m2/Kg (Table 4.1).
Table 4.1: Chemical compositions of candidate materials for COi absorption
Portland cement
Ladle slag
Waste cement
Class C fly ash
CaO
62.9
57.0
62.9
24.8
Si02
20.7
26.8
20.7
39.5
A1203
3.7
5.2
3.7
16.9
Fe203
3.0
1.6
3.0
6.4
MgO
4.2
3.3
4.2
6.3
Na20
0.1
-
-
1.4
K20
0.6
-
-
0.53
S03
2.6
1.7
2.6
2.0
The ladle slag was a waste product from steel production. Its chemical
composition is shown in Table 4.1. Since ladle slag's CaO content was very high,
reaching 57% and just a bit less than cement, it was studied for its ability of absorbing
carbon dioxide. Ladle slag was used in some batches to completely replace the cement as
binder.
The fly ash showed a low content of CaO, and was used in hybrid system with
Portland cement in the carbonation test. Fly ash is an effective additive to makes concrete,
more durable and easier to work with.
- 2 3 -
As shown in Table 4.1, ground waste cement has 62.9% CaO by weight, and
therefore could have ability to sequester the carbon dioxide. The ground waste cement
was obtained through crushing and grinding the used fiber-cement board to a size that is
close to cement.
The fine river sand of a fineness modulus of 2.3 was also used as filler to make
mortar samples.
4.2.2 Additives
Calcium Hydroxide [Ca(OH)2]. Calcium hydroxide can react with CO2 by itself to form
CaC03 and was used as adhesive filler between bricks and stones in the masonry
buildings. Ca(OH)2 reacted with atmospheric carbon dioxide form a durable binder.
Calcium hydroxide was added in the cement paste in the hope to absorb more carbon
dioxide, and help gain higher early strength.
Table 4.2 Chemical component of calcium hydroxide component
Weight(%)
Ca(OH)2
99.6
Chloride
(CI)
0.05
Heavy
Metals
(as Pb)
0.002
Iron
(Fe)
0.01
Magnesium
and Alkali
Salt
0.3
Sulfur
Compounds
(as S04)
0.09
Certified CAS 1305 -62-0 F. W.74.09
Sodium Hydroxide [NaOH]. Sodium hydroxide is an alkaline material. The pH value of
the NaOH aqueous solution is almost 14. The carbon dioxide needs to be dissolved into
water to form HCO3" or C03"2 then react with CaO. The solubility of carbon dioxide in
-24-
water is very low. However the solubility of carbon dioxide in alkaline solution is much
higher than that in water. So sodium hydroxide (NaOH) was used as an additive to help
carbon dioxide gas to dissolve into water to form CO3"2.
Table 4.3 Chemical component of sodium hydroxide
Component
Weight(%)
NaOH
97.9
Sodium
Carbonate
0.3
Amnonium
Hydroxide ppt
<0.02
Chloride
(CI)
0.001
Potassium
(K)
0.002
Others
<1.78
Certified A.C.S S318 -500 Solid Unl823
-2 Calcium Oxide [CaO]. Calcium oxide can react with CO3" rapidly. In fact it is the main
component in cement that reacts with carbon dioxide. For the same reason as calcium
hydroxide, calcium oxide was added into the cement paste to enhance the CO2 absorption
and promote higher early strength.
Table 4.4 Chemical component of calcium oxide Component
Weight(%)
CaO
97.9
Fluoride (F)
0.3
Loss on ignition
O.02
Certified F.W.56.08
4.2.3 Fibers
Cellulose fibers were supplied by Weyerhaeuser CO. It was hoped the cellulose
fibers would increase the bending strength and ductility of carbonated cement plates and
•25
also improve the CO2 absorption. The length and density of fibers was 2.3 mm and 4 g/cc
respectively.
4.3 Mix design and specimen preparation
Two specimen geometries were adopted in this study: the cylinder (12 mm in
diameter and 25 mm in height) and the plate (76 mm wide, 127 mm long and 12.7mm
thick). All the specimens were press-formed under constant pressure of 8 MPa.
4.3.1 Cylinder mix design
14 batches cylinder specimens were prepared and their mix design is shown in
Table 4.5. The batch CI was designed as control batch without additive. Because the
solubility of carbon dioxide is higher in sodium hydroxide than that in water, sodium
hydroxide was tried as additive. Batches C2, C3, C4, and C5 were made with different
sodium hydroxide content from 0.1 % to 5.0 %. Compared with batch C2, water to
cement ratio of batch C6 was reduced from 20% to 15% and additive to cement ratio was
increased from 0.2% to 2.4%. Batch C9 was added calcium oxide as additive, in fact the
CaO content was improved in the mixture. Batch C7, C8, CIO, Cll were used ground
waste cement, slag and fly ash to partially or fully replace the cement as binder. The
batches CI2, CI3, and C14 were designed as the same batch C6, but had different
carbonation time or only air curing. All of these cylinders (CI-CI3) were carbonated
under the 100% concentration carbon dioxide gas, 5 Bar (73Psi) pressure, and one-time
-26-
supply condition. Since specimens were small, it was difficult to measure the temperature
rise.
Table 4.5 Mix design of cylinder
CI C2
C3 C4
C5 C6 C7 C8 C9 CIO Cl l C12
C13 C14
Binder
(g) PC,(50)
PC,(50) PC,(50)
PC,(50) PC,(50) PC,(50) WC,(50) Slag,(50) PC,(50)
PC,(35)+FA,(15) PC,(35)+WC,(15)
PC,(50) PC,(50) PC,(50)
Water
(g) 10 10 10 10
10 7.5 7.5 7.5 10 7.5 7.5 7.5
7.5 7.5
Additive
(g) 0
NaOH (0.1) NaOH(l)
NaOH (1.8) NaOH (2.5) NaOH (1.2) NaOH (1.2) NaOH (1.2) CaO (1.8)
NaOH (1.2) NaOH (1.2) NaOH (1.2) NaOH (1.2)
NaOH (1.2)
Pressure (bar)
5 5 5 5
5 5 5 5 5 5 5 5 5 5
Time (minute)
15 15
15 15 15 15 15 15 15 15 15 30 120
24hours*
*: curing in the air, PC = Portland cement, WC = waste cement, Slag = Ladle slag, FA = Fly ash
4.3.2 Plate mix design
Based on the preliminary cylinder results, the relative large plate specimens with
12.7 mm thick by 76 mm wide by 127 mm long were also tested. The mix designs of 27
batches are shown in Table 4.6 using the combination of the following varied parameters:
additives, binders, carbon dioxide gas concentration, carbonation time, carbonation
pressure, thickness of plate, and gas supply method.
Batch Bl was the control batch: cement plus water with water to cement ratio of
0.15; the compaction pressure was 8 MPa; 15 minutes carbonation at 5 bar pressure with
one-time supply and using 100% CO2 gas.
• 2 7 -
Batches B2, B3 and B8 were experimented with additives: Batch B2 was added
sodium hydroxide, which had a high performance in cylinder test. Calcium hydroxide and
calcium oxide were added to batches B3 and B8 respectively. The additive to
cementitious ratio by weight in B2 was 2.4%, the same as that used in cylinder
experiment. But the ratio in B3 was 7%, which was the maximum among the three
batches; the ratio in B8 was 1.2%. Because too much calcium oxide in the paste resulted
in fast temperature rise, which could generate cracks during carbonation. Therefore the
ratio in batch B8 was limited to 1.2%.
Batches B4, B7 and B15 used ladle slag as the binder to fully replace Portland
cement. The thickness of B15 was only 6.4mm, half of the thickness of batches B4 and
B7, to study the thickness effect. Cellulose fiber was added into the batch B7, to compare
with batch B4.
Batches B5 and B9 had the same mix design as batch Bl. But these two batches
were carbonated under the diluted carbon dioxide gas. The CO2 concentration was 50% in
batch B5 and 25% in B9. The other carbonation conditions were exactly the same as
Batch Bl.
To obtain the relationship between carbonation degree and time, effect of
carbonation time was investigated. Batches B12, B13 and B17 had the same mix design
as batch Bl, but different carbonation time. They were carbonated by 10 min, 5 min and
120min respectively, with the comparison with 15min in Bl. Carbonation pressures were
changed in batches BIO and Bl l . They were 4 bar and 2 bar respectively to again
compare with Bl of 5 Bar.
28-
Two kinds of thickness of plate were used in the experiments. Most plates were
about 12.7 mm thick, except batches B14 and B15. The thickness of specimens in these
two batches was about 6.4 mm to study the full carbonation, if possible.
Table 4.6 Mix
Bl B2 B3 B4 B5 B6 B7 B8 B9 BIO Bll B12 B13 B14 B15 B16 B17 B18 B19 B20 B21 B22 B23
B24 B25 B26 B27
Binder (g)
PC,(240) PC,(240) PC,(224) Slag,(240) PC,(240) PC,(220) Slag,(220) PC,(237) PC,(240) PC,(240) PC,(240) PC,(240) PC,(240) PC,(120) Slag,(120) PC,(240) PC,(240) WC, 240 PC,(240) PC,(240) PC,(240) PC,(240) WC,(165)
PC,(240) PC,(240) PC,(120) PC,(120)
design Water
(g) 36 36 36 36 36 33 33 36 36 36 36 36 36 18 18 36 36 71 36 36 36 36 50
36 36 20 20
of plate specimen Additive
(g) 0
NaOH,(5.8) Ca(OH)2,(16)
0 0
Fiber, 1.7wt% Fiber, 1.7wt%
CaO, (3) 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0
Sand,(120) Sand,(120)
Pressure (bar)
5 5 5 5 5 5 5 5 5 4 2 5 5 5 5 5 5 5
3.5 5 5 5 5
5 0 5 0
Time (minute)
15 15 15 15 15 15 15 15 15 15 15 10 5 15 15 120 120 15
120 120 120 180 120
120 7d, air
120 7d, air
Concentration
100% 100% 100% 100% 50% 100% 100% 100% 25% 100% 100%, 100% 100% 100% 100% 25% 100% 100% 0 (air) 25% 100% 100% 100% Moist, 100%
0 100%,
0
Supply
one one one one one one one one one one one one one one one one one one
-
Conti. Conti. Conti. Conti.
Conti. 0
Conti. 0
Thickness (mm) 12.7 12.7 13.5 12.2 13.0 12.2 11.5 12.8 12.7 12.7 12.6 12.8 12.7 6.4 6.4 12.7 12.7 13.6 12.7 12.7 12.7 12.6 12.4
12.7 12.7 12.1 12.0
PC = Portland cement, Slag = One = one-time supply, Conti
Ladle Slag, Fiber = cellulose fiber, = continuous supply
-29-
4.4 Experiments for performance assessment
The following tests and measurements were performed:
4.4.1 Mass gain after carbonation
Mass gain is an important indicator for CO2 uptake. It was calculated based on the
mass measured before CO2 curing and the mass measured right after CO2 curing. The
ratio of the mass increase to the mass of binder is defined as mass gain by the following
Equation:
(Mass) aftC02-(Mass) abfC01
Massgain = ( 5 ) (Mass)binder
The balance with the precision of 0.005g was used to measure the mass of the
cylinder specimen before and after carbonation, and the balance with precision of 0.1 g
was used to measure the mass of plates before and after carbonation.
4.4.2 Carbonation pressure drop
The carbon dioxide is sequestered by the samples while the carbonation reaction
takes place. So the pressure of the gas inside the vessel is a good indicator for the reaction.
The higher the pressure drop, the more likely the carbon dioxide is sequestered. During
the carbonation curing, the pressure of the gas was recorded at 0 minute, 5 minutes, 10
minutes and 15minutes, and more. This pressure drop was measured only for the plates,
- 3 0 -
because the cylinder was too small to absorb enough gas that could be shown by the
pressure gage.
4.4.3 Carbonation temperature rise
The temperature of the plate was recorded with the thermocouple and data
acquisition system during carbonation. The carbonation reaction is an exothermic process
that emits considerable heat. The curve of temperature vs. time was again an indicator to
show the reaction. A small hole was drilled at comer of the specimen and the
thermocouple was placed inside the hole to monitor the reaction inside specimen.
4.4.4 Measurement of carbonation depth
Carbonation of concrete could be visualized by using phenolphthalein solution.
The phenolphthalein turned the non-carbonated concrete red and remained colorless in
carbonated concrete. A solution of 1 % phenolphthalein in 70% ethyl alcohol was suitable
for determining the depth of carbonation [RIELM, 1994]. The measurement was done on
the breaking cross section right after three-point bending test. The carbonation depth dk is
defined in Figure 4.3.
The carbonation depth was also measured directly from inspection right after CO2
curing. If only the surface was carbonated, the surface skin was solid and the core
remained soft powder. By scraping the soft core, it was easy to determine the thickness of
solid skin. The reported carbonation depth was measured by this scraping method and
confirmed with phenolphthalein solution.
-31 -
4.4.5 Compression test for cylinder specimens
The compression strengths of carbonated cylinders were evaluated with MTS
testing machine and the load rate was constantly 0.5mm/min till failure.
4.4.6 Compression test for plate specimens
The compression tests for plate were preformed for batches Bl, B17, B20, B21,
B25, B26 and B27. The plate was tested in compression with a compression surface area
is about 127 x 12.7 mm. Compression test was done by MTS machine and the load rate
was 0.5mm/m till failure. The setup is shown in Figure 4.4.
4.4.7 Three-point bending test for plate specimens
Three-point bending tests were conducted to determine the bending strength of the
plate specimen. Here the plates were evaluated as beams on a span of 101.6 mm, with a
beam thickness of 12.7 mm and a beam width of 76.2 mm. Three-point bending tests were
done on MTS machine and load rate was 0.5mm/min till failure. The test setup is shown in
Figure 4.5.
The flexural strength equation is given as follows:
o =My/I=3PL/2bh2 ( 6 )
32-
where: o = the flexural strength
M= the bending moment
y = the distance from the neutral axis to the bottom of the beam
I = the second moment of area about the neutral axis.
P = the breaking load of the beam
L = the span of the beam
b = the width of the beam
h = the height(thickness) of the beam.
4.5 Sample preparation
Calcium oxide (CaO) and calcium hydroxide (CaOH) were added to the cement in
advance and were mixed with cement in dry condition for one minute. Fine sand and
cement cementitious were initially mixed in dry condition for one minute. Fibers were
then mixed with cementitious while they were added. Sodium hydroxide (NaOH) was
dissolved into water before mix. Water was then added to mix for at least four minutes till
each powder was wetted. This procedure was followed for the cylinder and plate batches.
For the cylinder samples after mixing, the mixture was filled into the special
cylinder mould, use a aluminum bar with a diameter 0.5inch to push and compact the
paste into the small hole by hand till the paste reach the surface of the mould. Then put
the compaction bolt above the paste, in which there six bolts were fixed on a plate. The
fixed compact bolt could distribute the compact load to each sample. MTS test machine
was used to compact samples, which was easy to control the load. The maximum load
-33
was 6 kN, in another words, for each sample the compaction pressure is 8 MPa. The
maximum load was 6kN for all except batches CI and C2, which were pressed under 1
MPa and 20 MPa compact force. After compaction the height of cylinder was a little bit
smaller than 25.4 mm. After compaction, the mould was supported by two blocks and the
samples was push out from the mould by MST machine.
For the plate samples after mixing, the mixture was measured to keep each sample
the same mass to get the same thickness after compaction. The paste was poured into
mould and was flatted by a soft brush, which was tried to make the up surface and bottom
surface parallel. Then the mould with paste was compacted by MTS machine. The
maximum load was 78kN, corresponding to about 8 MPa. At same time the thickness of
plate reached approximately 12.7mm. After compaction, the plate sample was demolded
and was kept in a Ziploc until the other two plates were finished. In each batch three
samples were tested for average.
After compaction, the mass of each specimen was recorded as mass before
carbonation. For the plate, one sample in each batch was drilled a small 2 mm deep hole
on the edge before mass measured for thermocouple.
As many as 30 cylinders could be carbonated in the pressure curing vessel at one
time. After the vessel was sealed, use the vacuum pump to pull all the air form the vessel.
The pressure gage reading reached -20" Hg. Then the carbon dioxide was injected into
the vessel. After designed time, the pressure carbon dioxide was released, then the mass
of the samples were measured immediately. The mass is called mass after carbonation.
The pressure vessel and three samples are shown in Figure 4.6. The thermocouple
was inserted in one specimen to monitor the temperature. The heater was switched on at
least for 5 minutes before injecting carbon dioxide gas.
- 3 4 -
Two kinds of carbonation environments were adopted for plates: one was the
100% manufactured carbon dioxide (purity 99.5%); and the other was diluted. For the
100% carbon dioxide, the vacuum pump was used to drive the air out off the vessel, and
then inject appropriate carbon dioxide to reach the designed pressure. For the diluted
carbon dioxide, after the vessel was sealed, appropriate air gas was first injected, followed
by an appropriate carbon dioxide gas. For 25 % diluted carbon dioxide, the air gas was
injected into the vessel to reach a pressure of 3.5 Bar, and then carbon dioxide gas was
added to increase the final pressure to reach 5 Bar. The result was a mixed gas mixed gas
with 25% C02 concentration at a pressure of 5 Bar. For the 50% diluted carbon dioxide
gas, the same procedure was followed by injecting 2 Bar air first and them 3 Bar CO2 to
add up to 5 Bar in total. This procedure followed the basic principle: the ratio of gas by
volume at the same pressure equal to the ratio of gas by pressure at the same volume. The
equation can be expressed as:
V , x P , = V 2 x P 2 ( 7 )
Under the one-time supply method, the valve was switched off when the pressure
reached 5 Bar; for continuous supply condition, the valve was left on when the pressure
reached 5 Bar. The reading of pressure gage was recorded at designed time, such as 0-
minute, 5 minutes, 10 minutes, 15 minutes and 120 minutes. At the same time, the data
acquisition system recorded the temperature rise data through thermocouple automatically.
After designed carbonation time, the pressure gas was released and the mass of samples
35
was measured immediately to get the mass after carbonation. Samples were tested on the
MTS machine right after mass reading.
-36
Thermocouple
Vacuum Pump
Outlet J
II i
To water
Pressure Vessel
sample |n|et
. . . . ' . . . . . . . J . . . . . . . . . . .. \ = ^ Pressure Gauge
p=tx3 Regulator
Heater
Air
CO?
Figure 4.1 schematic of setup
Figure 4.2 Picture of Setup
form a
/ / / / .
§P • / / / / / * > / / / / / / / ,
^ ^ ^ ^
form b ///A
YS^/, Y777>7C/// W/ WMMMMi
/ /
//7A
Figure 4.3 Carbonation depth definition
-37 .
Figure 4.4 Compression test for plate specimens
Figure 4.5 Three-point bending test for plate specimens
- 3 8 -
Figure 4.6 Pressure vessel with carbonated sample
39-
Chapter 5
Results and Discussion
5.1 Preliminary study with 12mm-diameter cylinder sample
The cylinder strength and mass gain were shown in Appendix A. All of these
samples were made in the same condition at 5 Bar pressure 100%, CO2 concentration, and
15 minutes carbonation time except batches CI2, CI3, and CI4.
5.1.1 Effect of additives
The effect of additive on carbonated cylinders is shown in Figure 5.1. The
compressive strengths and mass gain of batch CI, the control, were 1.3 MPa and 1.63%
respectively. The strengths of batch C4 and C9 are 6.0 and 14.7 MPa, and the mass gain
3.72% and 4.28%. It was apparent that the additives were effective in improving the
carbonation degree and the strength. The % increase in strength by NaOH and CaO was
361% and 1031%, if compared to that of control. And the mass gain increased 128% and
163% respectively.
After the compression test, the core of the samples was examined. The core of
batch CI was still soft, only the surface of CI was hardened; however for the batch C4
and C9, not only the surface was solid, but the core of some batches (C4) was hardened as
well. The depth of carbonation in batches with additives was much deeper than that of
control batch, suggesting that the additives were promoting the carbonation.
- 4 0 -
Figure 5.2 shows batches with different sodium hydroxide content. Four different
sodium hydroxide contents, with additive to cement ratio of 0.2%, 2%, 3.6% and 5%,
were tested and compared with the control (CI). With the additive to cement ratio
increased from 0% to 3.6%, the mass gain increased from 1.6% to 3.7%, however the
mass gain was decreased with the increase of the NaOH from 3.6% to 5%.
The similar trend was observed in the cylinder compressive strength. From 0% to
3.6%, of sodium hydroxide to cement ratio, the compressive strength and mass gain
increased accordingly with the sodium content increase. It was noticed that the increase in
mass gain was not proportional to that in compressive strength. For the batches CI, C2,
C3 and C4, the carbonation depth increased with the increase of mass gain and
compressive strength, but the strength of C4 had a sudden a jump from batch C3. This
was because of the different carbonation depth of cylinder. The cylinder looked like
sandwich, outside surface was hardened but the core was still soft. Even the mass gain of
C3 reached 3.0%, which was larger than that of CI, Batch C3 was still a sandwich. So the
compression strength increased very slowly. For the batch C4, even it was not fully
carbonated, it obtained enough hardened skin to take more the compressive load.
For the batch C5 the mass gain and strength were 3.0% and 4.3 MPa respectively,
both were lower than those of batch C4. For the high NaOH ratio, it was not obvious.
Because of the high content of sodium hydroxide, the mixture reacted more with carbon
dioxide in a short time, leading to more evaporation of water during the carbonation in
batch C5 and a mass gain reduction from 3.7% to 3.0%. At the same time, because of the
water evaporation from the cylinder, there was more sodium hydroxide crystallized in the
cylinder, which caused the reduction of cylinder strength.
41
5.1.2 Effect of carbonation time
Three different carbonation times were tried; 15 minutes, 30 minutes and 120
minutes. They were compared in Figure 5.3 with 24 hours air curing. All of these batches
had same mix design, 2.4% sodium hydroxide to cement ratio, 0.15 water to cement ratio,
which is shown in Table 4.6. The maximum strength of 26.6MPa was achieved in batch
CI3, the minimum strength was 7.4 MPa in batch C6 that was almost the same as the
batch C14. The strength of 30 minutes carbonation (C12) was 11.6 MPa, which was
higher than that of batch CI4. It was shown that the strength of 15 minutes carbonated
samples was close to that of 24 hours air cured samples, The strength of 30-minute
carbonated samples was stronger than the 24 hours air cured specimens.
The maximum mass gain was 11.2% of batch C13 and the minimum mass gain
was 5.1% of batch C6 in the carbonated samples. Actually mass loss of 0.5% was
measured in batch CI4, owing to the water evaporation in the air curing. With the
carbonation time increased, the compressive strength and mass gain also increased. The
compressive strength increase was likely accompanied by an increase in the mass gain.
5.1.3 Effect of cementitious binder
In Figure 5.4, carbonated ladle slag and waste cement were compared with
carbonated Portland cement. Batch C7 was all waste cement and batch C8 was all ladle
slag. It was designed with the same water to cement ratio and the same additive to
cementitious ratio as used in batch C6.
•42-
While carbonated slag and cement had similar compressive strength of about
7.6MPa, the compression strength of ground waste cement (C7) was 6.3MPa. However
the mass gain of batch C7 was 9.1%, which is much larger than that of batch C6 and C8.
The mass gain of batch C6 and C8 were around 5.1% and 4.8% respectively. Because the
fineness of waste cement was lager than that of ladle slag and cement, the waste cement
cylinder structure was more porous than ladle slag and cement cylinder. This was the
reason that the mass gain of batch C7 was lager than that of batch C6 and C8. It also
explained why the compressive strength of batch C7 was lower than that of Batch C6 and
C8.
Ladle slag performed similarly as Portland cement. Therefore it had high potential
to be used as carbonation binder and carbon dioxide absorber.
Figure 5.5 shows the hybrid cementitious systems, one was 70% cement plus 30%
fly ash (batch CIO), the other one was 70% cement plus 30% waste cement (batch Cll) .
Batch CIO and CI 1 was designed with the same water to cementitious ratio and additive
to cementitious ratio as in batch C6.
Compared with batch C6, which was 100% cement, batch C10 had low
compressive strength and carbon dioxide absorption. The content of Calcium oxide in the
fly ash was much lower than that in cement (Table 4.1). This was possibly the reason why
hybrid batch (C10) with Fly ash was weaker than C6 with all cement.
However, the hybrid mixture of cement with waste cement had a superior
performance. The mass gain of batch CI 1 was 6.7%, which was larger than batch C6 and
was smaller than batch C7 (Figure 5.4) that used all waste cement as binder. But it had a
very high compressive strength 9.3MPa, larger than that of batch C6 (all cement) and C7
(all waste cement). The larger particle size of waste cement increased the porosity of the
- 4 3 -
paste, which helped the sample to absorb more carbon dioxide. So appropriate hybrid of
waste cement with Portland cement, will increase the ability of carbon dioxide absorption,
and enhance the compressive strength of the carbonated product.
5.2 Plate specimen test results
In the second phase of this project, rectangular specimens with large dimension
were employed. A thickness of 12.7 mm (0.5 inch) was selected to study if total
carbonation along the thickness was possible. This was also intended to simulate the
thickness of face shell in concrete block and the thickness of the mesh-reinforced cement
boards, the two candidate concrete produced that could be made by carbonation curing.
All batches had same specimen dimensions of around 127 x 76.2 x 12.7 mm,
except that batches B14 and B15 were made with a thickness around 6.4 mm. Each
sample was compacted under 8MPa pressure with MTS machine. Water to cement ratio
was kept 0.15 for most batches, except batches B18 and B23. The water to cement ratio
of these two batches was 0.3 because of the coarse particle size and the high water
absorption capacity of the ground waste cement. Twenty seven batches were investigated
with the combination of the following parameters: additive, binder, CO2 concentration,
carbonation time, carbonation pressure, fiber, and thickness. The summary of test results
was shown in the Appendix B.
The mass gain, flexural modulus, MOR and carbonation depth were compared.
The pressure drop and the temperature rise during carbonation were also recorded in
some batches. Scanning electron microscope (SEM) was used to analyze the
.44.
microstructure of different carbonation produces. Discussions will be focused on the
effect of different carbonation parameters.
5.2.1 Effect of additives in plate specimens
Based on the preliminary cylinder study, Sodium hydroxide, Calcium hydroxide
and Calcium oxide were added as additives into plate batches B2, B3 and B8 respectively
for better CO2 uptake and strength. The results were compared with control (Bl) in the
Figure 5.6. Batch Bl the control for all plate batches with no additive 100% CO2
concentration, at 5 Bar pressure, 15 minute carbonation under one-time supply condition.
As is shown in Figure 5.6, the control batch (Bl) without additives, had the
maximum bending strength of 4.6 MPa after only 15 minutes. However, in Figure 5.1, for
cylinder specimen, the compressive strength of control batch was lower than that of
batches with additive. It seemed that additives did not make the same contributions to the
bending strength as it did to the compressive strength.
The mass gain of B3 with Ca(OH)2 was high, while the other three were about the
same. The additives did not seem to promote more CO2 absorption in plate specimens. In
the cylinder tests (Figure 5.1), the mass gain was higher in batches with additive. This
was possibly caused by size effect and also due to the inaccurate definition of mass gain,
which disregarded the water evaporation.
The carbonation depth was about 60% in batches Bl, B3 and B8, and only 40% in
the batch B2 with NaOH. Sodium hydroxide did not function in plate specimen as well as
it did in the cylinder. This is shown by the SEM pictures taken near the surface of the two
specimens in Figure 5.7. The small white grains in batch Bl were tobermorite, the
-45 -
carbonation product that provided the interparticle binding enhanced the strength of plate.
They were not seen in batch B2. Obviously the carbonated plates of batch Bl and B2 had
different carbonation quality. The cross sections of these samples coated by
Phenolphthalein solution are shown in Figure 5.8. The dark color was not carbonated,
while the light color showed carbonation. The 15 minutes carbonation made the 12.7 mm
thick specimen a sandwich structure with a soft core.
The flexural modulus of batch Bl was larger than that of batches with additives,
following the same trend as the strength. Because the concrete is a brittle material, the
bending strength of the plate was determined by the bottom surface in tension. But the
compression strength was determined not only by the quality of carbonated surface, but
also by the carbonation depth. The un-carbonated core significantly reduced the
compression strength. This explained why the bending strength in control was higher than
that with additive.
A high CaO to cement ratio of 1.8 % was also investigated. However after 15
minutes carbonation, plates were cracked at a temperature over 60°C on the plates surface.
A higher reaction rate did not allow for the dissipation of heat and introduced the
laminate cracking. This phenomenon needs future study. Considerable heat was generated
rapidly in carbonation due to the addition of CaO, however the mass gain was not
significant. Fast reaction did not produce more solid carbonation.
Since additives did not show significant improvement on strength, flexural
modulus and mass gain during carbonation curing, it was decided not to use the additive
approach in the other plate batches.
• 4 6 -
5.2.2 Effect of CO2 concentration
The exhaust gas from thermal-power plant or the cement had a CO2 concentration
about 20-25%. The exhaust gas was simulated by a 25% concentration CO2 in this project
to explore the possible application of the exhaust gas in carbonation curing. 50% and
100% concentration were also studied for the purpose to mimic the partially or fully
recovered C02. These batches were carbonated under the conditions of 5 Bar pressure,
one-time CO2 supply and 15 minutes carbonation time.
Figure 5.9, compares batches Bl, B5 and B9 with 100%, 50% and 25% C02
concentration respectively. It was clear that the higher the CO2 concentration, the more
the carbonation, leading to higher strength and modulus, more mass gain and deeper
carbonation depth. For the 25% CO2 gas, the carbonation depth was only about 1.5mm.
The calcium carbonate layer was too thin to bear the bending strength.
The pressure drop was also an indication of the carbonation reaction. Figure 5.10
was the plot of pressure vs. time during carbonation curving. In 15 minutes, the pressure
of batch Bl dropped from 5 Bar to 2.2 Bar; and the drop in batch B9 happened from 5
Bar to 4 Bar. There was much more carbon dioxide gas sequestered in batch Bl than in
B9; and the pressure curve also showed the carbonate rate of batch Bl was high than B9.
This confirmed that the higher CO2 concentration promoted more carbonation reaction.
The pressure curves of batch B16 and B17 are also shown in Figure 5.10. These
two batches were carbonated for 120 minutes; the concentration of batch B16 was 25%
and that of B17 was 100%. After 120 minutes, the CO2 pressure inside the vessel of batch
B16 with 25%, CO2 concentration dropped to 3.8 Bar. It was corresponding to 80 % C02
absorbed by the cement plate during this period. For the batch B16, 100 % concentration
-47-
batch, the pressure was reduced to 0.2 Bar at the end of 120 minutes curing. This meant
80 % carbon dioxide gas was absorbed during 120 minute curing time. The mass gain of
B16 was 1.2% and that of B17 was 6.5%. Based on these observations, mass gain of the
plate was proportional to the pressure drop. The pressure drop was an indication of the
carbonation.
The tangent of batch B1 pressure curve was around 0.1 Bar/min, and that of B9
was about 0.02 Bar/min. Trends of B16 and B17 curves were both around 0.002 Bar/min
and 0.008 bar/min, the rate of carbonation was very slow after 120 minute because of the
decreasing of concentration of CO2 in this one-time supply method.
Figure 5.11 shows carbonation temperature curve of batch B16, B17, B20 and
B21. The CO2 concentration in batches B16 and B20 was 25%, and that of the B17 and
B21 was 100%. It was very clear that the maximum carbonation temperature was related
to the CO2 concentration. B17 and B21 had almost the same peak temperature, which was
over 50°C. It was much higher than the peak temperature of 35°C generated in B16 and
B20. For batch B16 and B20, the temperature drop was slow and at 120 minutes the
temperature was still over room temperature. This meant after temperature peak, the
carbonation reaction between CO2 and cement continued. This was why the temperature
curve of carbonation reaction was unlike that of hydration reaction, of which temperature
dropped quickly after peak. This was in accord with the CO2 pressure drop. The same
trend was found in batch B17 and B21, but the temperature of 100%, CO2 batches was
totally over that of 25%, CO2 batches at any time. The temperature curve was another
indication of the carbonation reaction.
•48-
Typical SEM pictures near surface in batches B20 and B21 are displayed in
Figure 5.12. The white grains are the tobermorite, the carbonation product. A large
number of white crystal grains were dispersed within the sample in batch B21. However,
only a few white grains tobermorite were observed inside of batch B20 with 25%
concentration.
As is noticed, the concentration of the carbon dioxide is an important parameter
for the CO2 curing. The increasing of CO2 concentration results in higher carbonation
degree of cement, more mass gain, and a stiffer and stronger material.
5.2.3 Effect of carbonation time
Four different carbonation times were experimented in batches Bl, B12, B13 and
B17 with the time of 15 minutes, 10 minutes, 5 minutes and 120 minutes for each
respectively. The other conditions were kept the same for comparison: 5 Bar CO2
pressure, one-time supply and 100% CO2 concentration.
As is shown in Figure 5.13, 120 minutes curing produced a carbonated product
with more than doubled mass gain and full carbonation along the thickness. 15 minutes
carbonation generated a strength close to that by 120 minutes, but mass gain was only
one-half and carbonation depth of 62%. There was a significant increase in bending
strength between 10 minutes and 15 minutes. With increase of the carbonation time, the
mass gain of plate also increased.
The relationship between the carbonation depth and carbonation time was similar
to that of mass gain. The longer the carbonation time, the deeper the carbonation depth.
The carbonation depth of B17 was total thickness of the plate. The Figure 5.14 shows the
-49 -
cross section of the B12, and B17 together with control Bl. The dark color was indicative
of no carbonation, and the light color implied carbonated product. Although 120 minutes
carbonation produced all solid sample, there were dark spots on the surface of B17 that
were not carbonated during CO2 curing process. It was clear that the carbon dioxide gas
took time to penetrate the skin to harden the core and 120 minutes was adequate for full
carbonation through the entire 12.7 mm thickness under the 5 Bar pressure.
The flexural modulus of the four batches were all around 2.0GPa. With the
reference to Figure 5.9, the carbon dioxide concentration had more direct effect on the
flexural modulus. This was why the bending strengths of Bl and B17 were close to each
other. Because the carbonation depth in batches B12 and B13 was not enough to bear
high bending load, even it had a considerable flexural modulus, the bending strength of
B12 and B13 was relatively low.
5.2.4 Effect of carbonation pressure
The carbonation pressure is an important parameter of the carbonation curing
procedure. Three batches, Bl, BIO and Bl l , were experimented for the different CO2
pressure at 5 Bar, 4 Bar and 2 Bar, with other conditions kept the same: 15 minutes
carbonation, one-time gas supply and 100% CO2 concentration. The carbonation pressure
was intentionally kept low to make the process practically feasible.
The results are presented in Figure 5.15. The bending strength of Bl was twice as
high as that of batches BIO and Bl l . The bending strengths of BIO and Bl l were only
around 2.2 MPa. However the flexural modulus of batch BIO was close to Bl, and higher
than that of Bl 1. The mass gain was increased with the increase of carbon dioxide gas
-50 -
pressure. The mass gain of batch Bl, BIO and Bl l was 3.8%, 3.4% and 2.4%
respectively. For the carbonation depth of plate, the higher the carbonation pressure, the
deeper the carbonation depth. The pictures of carbonation depth for BIO and Bll are
shown in Figure 5.16, the dark color of the cross section was hydration, and the light
color was carbonated cement.
In Figure 5.15, for the flexural modulus, there was not apparent difference
between 4 Bar and 5 Bar pressure. But for the strength, mass gain and carbonation depth,
the 5 Bar pressure produced better results than the 4 Bar pressure. It indicated that 4Bar
gas pressure had the similar ability as 5Bar pressure to carbonate the cement. It also
showed an increase in the degree of carbonation as the carbon dioxide pressure was
increased.
It is suggested that properly increasing the CO2 pressure in the curing vessel could
be an alternative method to enhance the carbonation if the other methods turned out to be
ineffective.
5.2.5 Effect of specimen thickness
The major concern of carbonation for commercial applications is the non-uniform
carbonation through the thickness. This also limits many potential concrete products to be
processed by carbonation. In this study, 12.7 mm and 6.4 mm thick specimens were
selected. 12.7 mm sample was to simulate the block face shell and the cement board,
while 6 mm to exam the condition for full carbonation. Batch Bl and B14 were cement
plate, batch B4 and B15 were ladle slag plate. The thickness in Bl and B14 was 12.7mm,
and the thickness in batch B14 and B15 was 6.4mm. The other conditions were the same:
-51 -
15 minutes carbonation, 5 Bar pressure, one-time gas supply and 100% CO2
concentration. The results are shown in Figure 5.17.
The bending strength of Bl was 4.6MPa, close to that of B14. However the
flexural modulus of the thin plate was much higher than that of thick plate. The modulus
of B14 was almost twice as high as that of Bl. Not only the modulus of thin plate was
higher, the percent mass gain was also enhanced in thin plate, so was the carbonation
depth. This was all probably owing to the full carbonation in thin plate, (see Figure 5.18).
Similar trend was observed in ladle slag samples. The bending strength of B4 was
2.1 MPa, and that of B15 was only 1.2MPa. The modulus of B15 was 3.1GPa which was
over two time of that of B4. The mass gains of batch B4 and B15 were almost the same.
Slag plates with 12.2 mm and 6.4 mm thickness were fully carbonated and all solid across
the entire section.
For the flexural modulus of both cement and ladle slag plates, the thin plate had
higher value than the thick plate. It seemed that more carbonation took place in thin plates
than that in thick plate. The quality of carbonates in thin plate was better than that of thick
plate. Figure 5.18 compares the fully carbonated thin plate with thick one.
The bending strength and the flexural modulus of cement specimens were high
than those of ladle slag plates. However ladle slag plate had higher mass gain and deeper
carbonation depth than those cement plat. Compared the cement plate and ladle plate, the
content of CaO in the ladle slag was relatively high. Therefore ladle slag could be used as
a candidate for hybrid binder system, for Carbon dioxide curing.
It is suggested that reducing the thickness would be a good method to make the
product fully carbonated. Reducing the thickness of samples results in a higher
carbonation degree and better mechanical properties.
- 5 2 -
5.2.6 Effect of cellulose fibers
Cellulose fibers are commonly used in cement fiberboard for strength and
toughness. In this study, cellulose fibers were employed also for CO2 uptake. The
cellulose fiber was tried with two binders, cement and ladle slag. The weight ratio of
cellulose fiber to cement or ladle was 1.7%. Batch B6 was the cement with cellulose fiber
and B7 was ladle slag with cellulose fiber. Batches Bl and B15 were control batches for
comparison. The other conditions were kept the same: 15 minutes carbonation, 5 bar
pressure, one-time gas supply and 100% CO2 concentration.
From Figure 5.19, it was obvious that small quantity of cellulose fiber did not
significantly improve the bending strength, flexural modulus, and carbonation depth of
the cement and ladle slag plate. A trend was noticed in mass gain. With the help of
cellulose fibers, CO2 uptake seemed to be a bit higher. This phenomenon is worthwhile
further study, since other studies pointed out cellulose material could be a good CO2
absorbent.
5.2.7 Effect of binder
Three binders were investigated in this study for their abilities to take up CO2 and
develop strength through carbonation. They were Portland cement, ground waste cement
and ladle slag. In Table 4.1, type 10 Portland cement has 62.9% of the Calcium Oxide the
effective chemical part for carbonation. The ground waste cement and ladle slag showed
- 5 3 -
57 % of Calcium Oxide, and were both waste materials. Batch B4 was designed for ladle
slag plate, and Batch B18 was for ground waste cement plate. The mix-design of B4 was
the same as that of Bl, They has the same water to cementitious ratio. However the water
to cementitious ratio in batch B18 was 0.3. This was because the coarse ground waste
cement had a higher water absorption than cement and ladle slag. The other conditions
were the same, which include 15 minutes carbonation, 5 bar pressure, one-time gas
supply and 100% CO2 concentration.
As shown in Figure 5.20, the cement plate had the highest bending strength and
flexural modulus. The ladle slag had the maximum mass gain and carbonation depth. The
bending strength of waste cement (B18) was only 1.3MPa, and strength of ladle slag (B4)
was 2.1 MPa. Compared with cement plate, the bending strength of ladle slag and ground
waste cement was really low. The flexural modulus of batch Bl, B4 and B18 was
2.4MPa, 1.4MPa and l.lMPa respectively. It followed the trend in bending strength. The
trend of mass gain between the Bl and B4 was inversely proportional to the bending
strength. The carbonation depth in the ladle slag plate was the whole thickness, the depth
of the other two was only 62% and 35%. The carbonated cross sections of Bl, B4 and
B18 are shown in Figure 5.21. The ladle slag plate was easy to be carbonated, but did not
have the same microstructure as the cement plate, as is shown in the Figure 5.22. There
were more white grains carbonation products (tobermorite) in the batch Bl near the
surface than in slag sample (B4). This explained why the bending strength and flexural
modulus of ladle slag were lower in slag plates than in cement plates, even slag absorbed
more CO2 and had fully carbonated cross section.
It is conclusive that, although mechanical properties of carbonated ladle slag were
not as good as the cement, the ladle slag had shown considerable ability to absorb the
-54 -
carbon dioxide gas. It maybe used as carbon dioxide sink for the environment. Ground
waste cement did not seem to be proper for serving as a structure material by itself.
5.2.8 Continuous supply with 100% C02 concentration
It was noticed in the experiments, that there existed CO2 starvation, indicated by
the pressure drop, in one-time CO2 injection method. To enhance the CO2 uptake,
continuous CO2 supply method was therefore adopted. Figure 5.23 compares the results
of 120 minutes one-time supply carbonation with that of 120 minutes as well as 180
minutes under continuous CO2 supply condition, tested under 120 minutes carbonation
with one-time gas supply. Batch B21 was 120 minutes carbonation with continuous gas
supply. Batch B22 was 180 minutes carbonation with continuous gas supply. The other
conditions were kept the same: 5 Bar pressure, 12.7 mm thickness and 100% CO2
concentration.
It was surprised that under the one-time supply curing condition, the bending
strength developed in 120 minutes carbonation (B17) was almost the same as that from 15
minutes curing (Bl), although the mass gain of B17 was much high than that of Bl, since
batch B17 was fully carbonated. Even the carbonation time of B17 was 7 times over that
of Bl, the bending strength and modulus of were close in both batches. The increase in
carbonation time did increase the carbonation depth and mass gain, but did not improve
the bending strength and modulus. Because the pressure of the gas decreased with the
time elapsed, the low carbonation was not efficient at all.
For the 120 minutes and 180 minutes continuous supply, batch B21 and B22 had
almost the same bending strength, flexural modulus, mass gain and full carbonation depth.
- 5 5 -
It was evident that 120 minutes would be enough to fully carbonate 12.7 mm cement plate.
The cross section of samples was shown in Figure 5.24. The quantity and size of the
uncarbonated dark spots were much larger in B17 than in B21 and B22. It was hard to
find uncarbonated spots in batch B21 and B22. The continuous supply method really
improved the quality and degree of carbonation.
The bending strength generated by continuous supply method (B21 and B22) was
higher than that by one-time supply (B17). The modulus of one-time supply batches was
only half of that by continuous supply. However the mass gain of batch B17 was close to
that of continuous supply produced samples. The three batches were all fully carbonated.
The scratched on specimen surface in Figure 5.24 were made right after carbonation and
were indicative of hardness of the carbonated solids down to the core. The continuous
supply method provided a constant gas pressure and a constant CO2 concentration
gradient in carbonation curing and promote CO2 uptake.
5.2.9 Continuous supply with 25% CO2 concentration
Because the CO2 concentration of exhaust gas from thermal power plant or
cement kiln was about 25%, the diluted CO2 was used to simulate to the exhaust gas.
Different carbonation time for diluted Carbon Dioxide gas was experimented as shown in
Figure 5.25. Batch B16 was 120 minutes carbonation and one-time gas supply; Batch B20
was carbonated 120 minutes in continuous gas supply; these two were compared with
batch B9, which was 15 minutes carbonation and one-time supply. The other conditions
were the same: 5Bar pressure, 12.7 mm thickness and 25%, CO2 concentration.
56-
The bending strength of batch B16 was 1.7MPa; the flexural modulus was l.OGpa;
the mass gain was 1.2% and the carbonation depth was 25%. All of these attributes were
higher than that of B9, because of the longer carbonation time. On the other hand, the
pressure and concentration of C02 decreased with the time elapsed, and the ability to
carbonate the binder was reduced (Figure 5.10). Due to the limit of the one-time supply
method, the contribution of 120 minutes carbonation was not significant.
However, for the continuous supply method, the bending strength of B20 doubled
that of one-time supply (B16). The flexural modulus of B20 was more than three times
higher than that of B16 and B9. The similar trend was observed in the mass gain and
carbonation depth increase. Compared with one-time supply method, the continuous
supply method is efficient to make high quality carbonated product. But if compared with
B21, which was totally solid after 120 minutes carbonation, B20 was only carbonated
34%. The 25% CO2 concentration in B20 was too low to make the core exposed to CO2.
The gradient line of the carbon dioxide did not reach the core of the plate. The chemically
treated cross sections B9, B16 and B20 are shown in Figure 5.26. It was clear that the
continuous supply method did help carbon dioxide to penetrate the surface of plates and
improve the carbonation degree, even with 25% CO2 concentration.
5.2.10 Carbonation at continuous CO2 supply with different mix
Based on the previous discussion, the continuous supply method with 120 minutes
curing seemed to be an effective method for the cement plate carbonation. More batches
were studied under different conditions. Batch B21 was cement plate, carbonated under
dry CO2 condition as was used so far. Batch B24 was the same batch as batch B21 except
-57-
that moist C02 was used. The moist CO2 was obtained by using water to fill the vessel
under the specimens to increase the relative humidity for carbon dioxide gas. It was
hoped the moist CO2 could promote more carbonation reaction. Batch B26 was mortar
with sand at one cement to one sand ratio, and was carbonated under dry carbon dioxide
gas. Batch B23 was ground waste cement plate, and was carbonated under dry carbon
dioxide gas in 120 minutes under continuous CO2 supply. Batch B21, B24 and B26 had
the same water to cement ratio, and B23 required more water to keep consistency. The
other conditions were the same: 5 Bar pressure, 12.7 mm thickness, continuous CO2
supply method at 120 minutes and 100% CO2 concentration.
In Figure 5.27, batch B21 serves as reference for comparison. B24 with moist CO2
obtained only 5.1 MPa, lower than B21 with dry C02. However, B21 and B24 had almost
the same flexural modulus, the same mass gain and both were fully carbonated.
Carbonation could not take place in the absence of water, but the high relative humidity
environment did not enhance the reaction. Other methods should be explored to damp the
C02 gas.
The bending strength of mortar plate was 6.2 MPa, and the mass gain was high
that reached 7.9%. Mortar plates were totally carbonated. It was interesting to compare
batch B26 with B27, which was cured seven days in the air and had the same mortar mix-
design as B26. It is shown in Figure 5.28, the bending strength and flexural modulus of
the 120-minute carbonation paste (B21) were both higher than those of the same paste
cured 7 days; the bending strength of the 120-minute carbonation mortar (B26) reached
93% of the same mortar cured 7 days in the air, and the same flexural modulus of 2.4 GPa.
The mortar mix with sand was porous if compared to the cement paste mix. It was this
high porosity that allowed more CO2 penetration, leading to a high mass gain in B26. The
-58 -
cross section of B26 and B27 is shown in Figure 5.29. These cross sections were treated
by phenolphthalein solution, which changed the hydrated concrete to red. It was clear that
the carbon dioxide hardened the whole mortar plates in batch B26. The section of batch
B27 was almost fully covered by dark color, indicating non-carbonation.
If compared to paste and mortar (B21, B24) at 120-minute carbonation at
continuous supply (Figure 5.27), the ground waste cement in batch B23 was not well
carbonated. The strength was low and carbonation depth was only 45%. If compared with
B18 carbonated only 15 minutes at one-time supply, B23 was much better in bending
strength, flexural modulus, mass gain and CO2 absorption. The long time carbonation and
continuous CO2 supply helped the ground waste cement to gain more performance.
Batch B19 was another control to compare with carbonated plates. Specimens
were cured in the vessel at 3.5bar pressure with only air. No CO2 was injected. After 120-
minute in the vessel under pressure, specimen were tested. Batch B19 had a bending
strength of 0.4 MPa, a flexural modulus 0.4 GPa, and a mass gain of -0.3%. The slight
strength gain was obtained through 8MPa compaction. This test was to show that the
early strength development in carbonated plates was truly due to the reaction between
carbon dioxide gas and cementitous material.
Figure 5.11 exhibits the carbonation temperature curve. There was no big
difference between the continuous supply and the one-time supply methods. Zig-zag
waves in the curve of 25% and continuous supply test was observed. The local drop of the
temperature implied a pressure loss and carbonation reaction was slowed down; for a
considerable time, the system detected the pressure drop and automatically injected CO2
into the vessel; resulting in an increase in carbonation reaction and a temperature rise seen
-59 -
as the peaks. Even the temperature of B20 was not much higher than that of B16, the
continuous gas supply method did help cement plate to sequester more carbon dioxide gas.
5.2.11 Compressive strength of carbonated plates
Compressive strength of carbonated 12.7 mm plates was indicative for the
concrete block application. A few typical batches were chosen for the compressive tests.
They included Bl, B17, B20, B21 and B26.
Figure 5.30 compares compressive strengths at different age: one was right after
carbonation and the other was tested 2 months after carbonation for long term strength.
They were tested in a setup shown in Figure 4.5. Batch Bl was almost the same
compressive strength as B20. Batch Bl was carbonated under 15-minute with 100% CO2
concentration and under one-time supply condition. B20 was carbonated under the 120-
minute using 25% CO2 gas and continuous supply method. The bending strength of Bl
was 4.6 MPa, and that of B20 was 3.5 MPa (See Appendix B). Both compressive
strengths of Bl and B20 right after carbonation satisfied the Building Code for Masonry
blocks. Therefore it is possible to use carbonation method to produce blocks.
The compressive strength was very high in the batches B17 and B21, both of
which were over 50 MPa. The carbonated mortar was also tested. Its compressive
strength was 42.3 Mpa, with a sand to cement ratio of 1:1. All of the five batch
concretes showed sufficient strength for fast production of concrete blocks.
These five batches were tested again after two months of carbonation. Their
compressive strength are also shown in Figure 5.30. After two months, the compressive
strength of Bl was increased to 29.5 MPa. Since Bl cement plate only partially
-60-
carbonated; after two months, the uncarbonated core was hydrated to develop
additional strength. The batch B20 obtained a much higher strength gain. However
batches B17, B21, and B26 were quite different. Since B17, B21 and B26 were almost
fully carbonated and gain all solids across the section during CO2 curing, there was no
additional strength gained from hydration after the carbonation. In fact, the
compressive strengths of the plates tested right after carbonation were higher than that
after 2 months. This explained why the compressive strength in two months after
carbonation was not higher than that just carbonation. Even the strength was slightly
decreased, the compressive strengths of B17, b21 and B26 were still strong enough for
concrete blocks. On the other hand, the air cured paste (B25) and the cured mortar
(B27) demonstrated a two months compressive strength of 45 Mpa. It was conclusive
that 2-hours continuous supply curing could produce a concrete with properties
comparable to the same concrete air-cured in 2 months; the partially carbonated
concrete products, either at 15 minutes carbonation or with 25% CO2 concentration,
still could develop full strength during secondary curing.
61
10
5
1 4 7
0 no additive (C1)
SNaOH(3.6%)(C4)
• CaO ( 3.6%) (C9)
Cylinder (r=12.7mm,h=25.4mm), Compaction=8 MPa; C02 pressure=5bar
4 28 3 7 2
Strength(MPa) Mass Gain(%)
Figure 5.1 Effect of additives in cylinder specimens
2
0NaOH(O%)(C1) E3NaOH(0.2%)(C2) El Na0H(2%) (C3) 0NaOH(3.6%)(C4) 0 NaOH(5%) (C5)
6 0
4 3
Cylinder (r=12.7mm,h=25.4mm), Compaction=8 MPa; C02 pressure=5bar
Strength(MPa) Mass Gain(%)
Figure 5.2 Effect of Sodium hydroxide to cement ratio
-62
30
25
20
® 15
10
26.6
11.6
Cylinder (r=12.7mm,h=25.4mm), Compaction=8 MPa; C02 pressure=5bar
Q15 min Carbonation (C6) a 30 min Carbonation (C12) 0120 min Carbonation (C13) CI 24 hours Air curing (C14)
80
11.2
Strength(MPa) MassGain(%) -0-5
Figure 5.3 Effect of carbonation time in cylinder specimens
0) ~o rj E 5 o> ra
4
3
2
Cylinder (r=12.7mm,h=25.4mm), Compaction=8 MPa; C02 pressure=5bar
74 11
a PC(C6)
SWC(C7) BSIag(C8)
Strength(MPa) Mass Gain(%)
Figure 5.4 Effect of cementitious binder in cylinder specimens
63
12
10
CD "D 3
c 6
Cylinder (r=12.7mm.h=25.4mm), Compaction=8 MPa; C02 pressure=5bar
13 100% PC (C6)
• 30%FlyAsh(C10)
• 30%WC(C11)
5 1
s-Jl
6.7
Strength(MPa) Mass Gain(%)
Figure 5.5 Effect of cementitious binder in cylinder specimens
64-
c 3 Dl
4.6
C02 time=15min Pressure=5bar Concentration=100% One supply
3.0
J t
2.3 2.4 2.4
;
LJ
2.1
1.5 1.2
4.9
3.8 3.6
_M
^ N o additives (B1) DNaOH (B2) 0Ca(OH)2(B3) BCaQ(B8)
3.3
0.62 0.62n 6 ".40^-
MOR(MPa) Flexural Modulus(GPa)
Mass Gain(%) CaC03 Depth (x100%)
Figure 5.6. Effect of additives in plate specimens
Batch Bl Batch B2
Figure 5. 7 SEM of batch Bl and B2
65
Batch Bl
Batch B2
Batch B8 Figure 5.8 Cross section ofBl, B2, B3 and B8
0) •a
£3
4.6
Q100%CO2(B1)
Q50%CO2(B5)
Q25%C02(B9)
C02 time=15min Pressure=5bar
One supply
3.8
2.4
1.6
0.8
1.2
'\W\ 0.7
1.3
0.8 0.62
0.24 0.12
MOR(MPa) Flexural Modulus(GPa)
Mass Gain(%) CaC03 Depth (x100%)
Figure 5.9 Effect of CO2 concentration on properties
66
100% C02, 15min(B1) 25%C02, 15min(B9) 100% C02, 120min(B17) 25%C02, 120min(B16)
40 _ 60 80 Time, (minute)
120
Figure 5.10 Pressure drop during carbonation curing
60
50
O 40
30
20
10
B21. 100% dry C02, continuous supply
\ / B17, 100% dry C02, one-time supply
0 1000 2000 3000 4000 5000 6000 7000 8000 Elapse Time (second)
Figure 5.11 Effect ofC02 concentration on carbonation temperature
67
Batch B20 Batch B21
Figure 5.12 SEM of batch B20 and B21
68
6
3 4 =>
2
4.9 4.6
0120 min (B17)
S15
• 10
0 5
min (B1)
min(B12)
min (B13)
6.5
2.6
2.0
2.4
2.0
^ « - - - v .
2.0 1.7
3.8
Pressure=5bar Concentration=100%
One supply
3.5
1 1-.-.-J
2.6
Figure 5.13 Effect of carbonation time
I 6 % . 4 %
J , 39
MOR(MPa) Flexural Mass Gain(%) CaC03 Depth Modulus(GPa) (x100%)
Batch Bl
Batch B12
Batch B17
Figure 5.14 Cross section ofBl, B12 and B17
69
CD
C 3
0
4.6
S5Bar(B1) H4Bar(B10) 0 2Bar(B11)
2.4 2.4
C02 time=15min Concentration=100%
One supply
0.62 0.52 0.39
MOR(MPa) Flexural Modulus(GPa)
Mass Gain(%) CaC03 Depth (x100%)
Figure 5.15 Effect of carbonation pressure
Batch BIO
Batch Bll
Figure 5.16 Cross section of BIO and Bll
7 0 -
7
6
5 CD T3
c 4 O) CD
3
2
1
0
£3 Thickness=12.7mm,cement (B1) E3Thickness=6.4mm,cement (B14) EThickness=12.2mm,slag (B4) E\ Thickness=6.4mm,slag (B15)
C02 time=15min Pressure=5bar
Concentration=100% One supply
54
4 6
4 2
wwm.
1 t
2.1
:-:-: 1.2
4.7
24
38
3 1
1 4 11
4 7 46
1.0 1.0 1.0 0.62T
™
MOR(MPa) Flexural Modulus(GPa)
Mass Gain(%) CaC03 Depth (x100%)
Figure 5.17 Effect of specimen thickness
Batch Bl
Batch B14
Figure 5.18 Cross section ofBl and B14
71
8
7
6
0) 5 -•o 3
I 4 03
3
2 H
1
0
C02 time=15min Pressure=5bar Concentration= 100% One supply
4.6
2.6
2.1 2.4 2.4
1-4 1.3
^Cement (B1) ^ Fiber cement (B6) BSlag (B4) EH Fiber slag (B7)
38
4.7 4.7 4.9
II 0.62 0 - 7
1.0 1.0
iH s :-:-:-:;
MOR(MPa) Flexural Mass Gain(%) CaC03 Depth Modulus(GPa) (x100%)
Figure 5.19 Effect of cellulose fibers
72-
6
5
4 CD
•o +-.
c 3 CD
2 ^
4.6
S Cement (B1) • Slag (B4)
P2 Ground waste cement (B18)
3.8
2.4 2.1
1.3 1.4 T l 1 . 1
C02 time=15min Pressure=5bar
4.7 Concentration^ 00% One supply
2.5
H
1.0
0.62 0.35
MOR(MPa) Flexural Mass Gain(%) CaC03 Depth Modulus(GPa) (x100%)
Figure 5.20 Effect of binder
Batch Bl
5tf/c/z 54
Batch Bl8
Figure 5.21 Cross section ofB21, B4 and B18
73
l^"**" ".?
• Batch B4
Batch Bl
Figure 5.22 SEM of batch B4 and Bl
74-
CD "a _3
' c en
10
9
8
7 H
6
5 H
4
3
2
1
0
• 0 El
15 min, one supply (B1) 120 min, one supply (B17) 120 min, continuous supply (B21) 180 min, continuous supply (B22)
6.5 6.5K:-:
7.2
Concentrat ion^ 00% Pressure=5bar
7 6
4.6 4 9
I 4 7
4.0 vl
2.4
K l 2 0
J i
3 8
0.62f
111
1.0 1.0 1.0
MOR(MPa) Flexural Mass Gain(%) CaC03 Depth Modulus(GPa) (x100%)
Figure 5.23 Carbonation at 100% CO2 concentration
Batch Bl 7
Batch B21
,
mm.
Batch B22
Figure 5.24 Cross section ofB17, B21 and B22
75-
CD o
E 03
Concentration=25% Pressure=5bar
35
^ 15 min, one supply (B9) S120 min, one supply (B16) B 120 min, continuous supply (B20)
1.7
2 9
1.0 0.7
,.
2.7
0.8
_d
1.2
0.12, 0.25 0.34
MOR(MPa) Flexural Mass Gain(%) CaC03 Depth Modulus(GPa) (x100%)
Figure 5.25 Carbonation at 25% CO2 concentration
Batch B9
T%ifr :;—,•*$>
ferff ..rf*? " UML."J> •*£**** tor*** •**> Vwtwemf
Batch B16
Batch B20
Figure 5.26 Cross section ofB9, B16 and B20
76.
Cement, dry C02(B21) I Cement, moist C02 (B24) I Mortar.dry C02 (B26) I Ground waste cement, dry CQ2 (B23) 79
Concentration^ 00% Pressure=5bar Time:r2 hours
Continuous supply 7 2
7 4
: • -
1 ! 3.2
1.0 1.0 1.0
0 4 5
MOR(MPa) Flexural Modulus(GPa)
Mass Gain(%) CaC03 Depth (x100%)
Figure 5.27 Carbonation at continuous CO2 supply of two hours
-71-
CD
3
c 4 o>
0 120min continuous supply paste(B21)
S3 7days air curing paste (B25)
0 120min continuous supply mortar,dry
C02 (B26) (3 7days mortar Air curing (B27)
MOR(MPa) Flexural Modulus(GPa)
Figure 5.28 Comparison of 2 -hours carbonation curing with 7-day air curing
Batch B26
Batch B27
Figure 5.29 Cross section of Mortar B26 and B27
78
60
50
CD •o 3
40
30
20
10
57 1
H I 53.e IB1 SB17 DB20 0B21 E3B26
Compressive strength air curing:
Paste(B25): 46.4 MPa Mortar(B27): 44.6 MPa
29 5
5 51 6 5, 1
Compressive Strength (MPa) right after
carbonation
Compressive strength (MPa) after 2 months
Figure 5.30 Compressive strength of carbonated concrete plates
79-
Chapter 6
Conclusions
Feasibility of carbon dioxide uptake by concrete products through early age curing
was studied. The process parameters examined included the CO2 concentration, the
carbonation time, the carbonation pressure, the type of binder, the chemical additive, the
thickness of the specimen, and the CO2 supply method. The performance of the
carbonated products was evaluated through the measurement of pressure drop,
temperature rise, strength development, mass gain and carbonation depth. The following
conclusions can be drawn:
(1) It is possible to carbonate the CaO-based cementitious materials in two hours
or less to fast produce concrete building products with sufficient strength and certain
amount of CO2 uptake. In general, higher CO2 concentration, longer carbonation time,
higher CO2 pressure can produce stronger products and promote more CO2 absorption.
(2) The best results obtained from this preliminary study are the Portland cement-
based concrete products carbonated two hours under 5 bar pressure with 100% CO2
concentration using continuous CO2 supply method. The two-hour compressive strengths
exceeded 40 MPa and the two-hour moduli of rupture (MOR) were over 6 MPa, both of
which satisfied the minimum requirement for concrete block and cement board
applications. The corresponding CO2 uptake by the concrete reached 7-8%. For 12.7 mm
thick plate samples, the carbonation went through the entire thickness in two hours.
(3) The manufactured C02 gas was used in this study. The 100% C02 was
employed to simulate the recovered CO2 from the exhaust gas, and the diluted CO2 at
-80-
25% and 50% concentration was to mimic the as-captured or partially recovered CO2.
Although the low CO2 concentration generated low strength gain and low mass gain, the
strength developed using 25% C02 in two-hour carbonation still satisfied the minimum
requirement for block application. Moreover, the partially carbonated concrete obtained
the full strength after left in the air two months for further curing. However, the C02
uptake was low with 25% concentration. A longer carbonation time (more than two hours)
or higher pressure (higher than 5 bar) would be a solution to improve the CO2 absorption,
if the concentration of the gas could not be enhanced in an economical way.
(4) The continuous CO2 supply method proved to be technically effective and
practically feasible in full scale concrete production using carbonation curing. The
continuous CO2 supply in carbonation process compensated the CO2 diminishing
automatically and promoted the CO2 absorption by concrete to the maximum. For the
full-scale real production, it is suggested that the carbon dioxide gas be injected into the
chamber first in a continuous supply manner at a given pressure and to a required time,
and then the valve be turned off to switch to one-time supply for concrete to consume all
the remaining CO2 gas before opening the chamber. The pressure drop in one-time supply
can be used to monitor the CO2 starvation and the strength gain. The carbonation time and
pressure can be calculated and programmed to make the process itself clean.
(5) Non-uniform carbonation along the thickness was a concern in the early
studies and had hindered the commercial applications of the technique. It was shown in
this research that if the carbonation time was allowed to be extended to more than two
hours, it was highly possible to achieve all solid along a 12.7 mm (0.5 inch) thickness. It
is a perfect geometry for cement board. However for concrete products with thicker
members, it is a challenge to obtain full carbonation. More studies are necessary to find
-81 -
the optimal combination of a variety of process parameters for total carbonation. If total
carbonation is not possible for thick members, it is interesting to know the performance of
a concrete with the skin made of carbonates and the core made of hydrates.
(6) The so called "full carbonation" in this study because of the all-solid hardness
developed along the entire 12.7 mm thickness did not mean the total carbonation. This
was confirmed by phenolphthalein solution treatment. A large number of red spots were
observed on the cross section although the hardness was proved by scratch using the steel
pen. The low mass gain was another evidence. If total carbonation is achieved, the mass
gain due to the CO2 uptake should reach approximately to 50% by weight of binder. The
highest mass gain obtained in this study was only about 8%.
(7) The mass gain calculated in this study was not accurate. The true C02 uptake
could be higher because the water evaporation during carbonation was significant and
was not considered in the calculation.
(8) Portland cement seemed to be the best binder for CO2 absorption for both mass
gain and strength development. Ladle slag and ground waste cement were not good for
strength, but absorbed certain amount of the carbon dioxide gas. A hybrid system to blend
cement with slag or waste cement would be efficient for carbonated concrete products to
consume more CO2 and maintain at least the same strength.
(9) The chemical additives did not make the same contribution to the bending
strength as it did to the compressive strength. For high content of CaO, fast reaction did
not produce more solid carbonates. Instead, the considerable heat generated resulted in a
delamination failure in plate specimens.
(10) Cellulose fibers did not significantly improve the C02 absorption. This
phenomenon is worthwhile a further study since other studies pointed out the cellulose
- 8 2 -
could be a good C02 absorbent. Cellulose fibers are commonly used in cement fiberboard
production with a large percentage for strength and toughness. The surface treatment of
the fibers is economically feasible and could be tailored to the CO2 uptake.
(11) Porosity is an important parameter for carbonation. Mortar mix with sand
was more porous than the paste mix. The mortar mix developed similar strength as paste
mix but gained a higher percentage of CO2 uptake. It demonstrated the carbonation
process could be an ideal curing method for concrete block production because of its
porous nature of the product. Compared to the currently used steam curing, carbonation
dose not require pre-setting period and thus can shorten the curing time substantially.
(12) The presence of water in carbonation is critical. The moist CO2 could be
another approach to improve the carbonation efficiency and requires a further
investigation.
- 8 3 .
References:
Bukowski, J.M. and Berger R.L. (1979), "Reactivity and Strength Development of
Activated Non-Hydraulic Calcium Silicates", Cement and Concrete Research. Vol.9,
pp57-68.
Halmann, M.M. and Steinberg, M. (1999) "Greenhouse Gas Carbon Dioxide Mitigation * ,
Lewis Publishers, CRC Pressed, NY, pp5-19.
Hermawan, D. and Hata, T, (1998), "Development Technology of Rapid Production of
High-Strength Cement-Bonded Particleboard by Using Gaseous or Supercritical Carbon
Dioxide Curing", Proceedings of inorganic-Bonded Wood and Fiber Composite
Materials, Vol.7 pp70-87.
Mourits, F. (2001), "Capture and sequestration of greenhouse gases in Canada", Natural
Resources Canada Annu. Rev. Energy Environ, ppl-13.
RILEM (1988), "edition CPC 18 Measurement of haiurdened concrete carbonation
depth" Technical Recommendations for the Testing and Use of Construction Materials
1994,pp453-455.
Simatupang, M.H. and Habighorst, C. (1995), Invextigations on the Influence of the
Addition of Carbon Dioxide on the Production and Properties of Rapidly Set Wood-
Cement Composites, Cement & Concrete Composites, 17, pp 187-197.
Teramura, S. and Isu, N. (Nov.2000), "New Building Material from Waste Concrete by
Carbonation", Journal of Materials in Civil Engineering, pp288-293.
Wagh, A.S., Singh, D., and Knox, L.Jr (April 1995), "Lab studying Greenhouse Effect on
Concrete Setting", Concrete International, pp41-42.
• 8 4 -
Worrel, E., Price, L., Martin, N (2001), "Carbon Dioxide Emissions from the Global
Cement Industry", Annu. Rev. Energy Environ. 26, pp 303-329.
Young, J.F., Berger, R.L. and Breese, J. (1974) "Accelerated Curing of Compacted
Calcium Silicate Mortars on Exposure to C02" Journal of The American Ceramic Society,
Vol.57, No.9. pp394-397.
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26
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259.7
259.6
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268
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265.8
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267.5
267.8
266.3
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3.4
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0.9
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169
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163
15
0
143
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226
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16
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64.3
59.3
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12.8
12
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12.7
12
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12.6
12
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12.7
12
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12.6
12
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12.7
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B15
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B15
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B14
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2.8|
26
2.5
262.
9 13
4.4
137.
1 13
4.1
135.
8 13
6 13
5 26
5 26
5.7
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2 M
ass
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-
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Mas
s ga
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oo
204
121
27.6
18
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26.5
CM OO
r-
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242
213
184
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d (N
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12.7
12
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6.4
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6.5
6.4
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12.7
12
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12.7
T
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ness
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m)
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2.5
1.5
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2.6
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All
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Thi
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1.9
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Bot
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0.94
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1.13
89
0.97
596
3.51
6937
2.
7521
65
1.77
1985
4.
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4 4.
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6 4.
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8 2.
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66
1.90
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56
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4.3
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Pre
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1.7
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B
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B
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B
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B
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B
18-3
B
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B
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B
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B
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B
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0 24
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36
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36.0
70
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70.8
70
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CO CO
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g)
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240
240
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259
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9.3
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8 25
9.8
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9 25
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2 19
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281
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9 |M
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5.5
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128
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147
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3.7
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12.7
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12.7
12
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12.6
12
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12.7
12
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11.9
16
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12.7
12
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12.7
|T
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m)
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165
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7.2
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7.5
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9 34
4 13
8 14
5 12
4 49
8 52
9 54
9 46
9 59
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12
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12.4
12
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12
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12
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mm
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3.9
3.8
1.7
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1.7
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ar)
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ssur
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ar)
15m
in
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|Pre
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120m
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6.6
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