microstructure of 2 and 28-day cured portland limestone...

Indian Journal of Engineering & Materials Sciences

Vol. 17, August 2010, pp. 289-294

Microstructure of 2 and 28-day cured Portland limestone cement pastes

Gözde İnan Sezera*, Oğuzhan Çopuroğlub

& Kambiz Ramyara

aCivil Engineering Department, Faculty of Engineering, Ege University, 35100 İzmir, Turkey bMaterials Science Section, Faculty of Civil Engineering and Geosciences, Delft University of Technology, 2628CN, Delft, The

Netherlands

Received 18 November 2008; accepted 10 June 2010

In this study, microstructures of limestone cement pastes cured for 2 and 28 days are investigated. For this purpose,

limestone and clinker are separately ground in a ball-mill until obtained a given constant 32 µm sieve residue. The gypsum

is kept constant as 5% by weight of cement. Cement pastes with 0.5 water/binder ratios and 0, 6, 35% limestone/clinker

ratios are prepared. After 2 and 28 days standard curing, ESEM investigations are carried out on these specimens. It is found

that hydration products are accumulated around limestone particles. In 2 days, compared to the control specimen, pore

percentages of cements containing 6% limestone are decreased whereas pore percentages of cement pastes containing

35% limestone are increased. The difference between porosities of the control paste and limestone cement paste is decreased

at 28 days.

Keywords: Limestone, clinker, hardened cement paste microstructure

Pore structure is a very important microstructural

feature in porous solid because it affects physical,

mechanical and durability properties of the material.

Limestone addition influences the pore structure of

hardened cement pastes1,2

. Limestone changes

capillary porosity of cementitious materials due to

several physical effects. These effects are dilution

effect, filler effect and heterogeneous nucleation.

Limestone addition decreases cement content and

increases effective water/cement ratio. The filler

effect of limestone modifies initial porosity of the mix

and generally resulted to decrease in water

requirement with constant workability. Besides,

limestone particles act as nucleation sites and

increasing the early hydration of cement3. Limestone

creates a nuclei effect for precipitation of hydration

products and provides free dispersion of cement

particles. As a result, accelerates the rate of cement

hydration4-6

.

Limestone Portland cement (LPC) is a blended

cement bearing up to 35% limestone (calcite)

according to the European Standard EN 197-17. It has

a great potential in engineering applications and

contemporarily used for special conditions. For

instance, LPC is used vastly in the self-compacting

high performance concrete because of its filler effect

and prevent excessive heat development due to

hydration8. Furthermore, the usage of LPC known to

have noteworthy technical, economical and

environmental advantages such as increasing early

strength and workability as well as reducing water

requirement, decreasing production cost of concrete

and reducing the CO2 emission in the cement

production4,9,10

. It is expected that the world wide

production and the usage of limestone cement will be

increased. Thus, there is need to further research on

the effect of limestone on cement and concrete

properties.

Four types of portland limestone cements can be

produced according to the European Standard EN

197-1. These types are classified as type II/A-L, II/A-

LL, II/B-L and II/BLL. Types II/A-L, II/A-LL and

types II/B-L, II/BLL contain 6-20% and 21-35%

limestone, respectively. L and LL type cements can

contain maximum 0.2% and 0.5% total organic

carbon, respectively. This limitation is due to the fact

that excessive amount of organic carbon decreases

frost resistance of concrete7,11

.

In this study, ESEM investigations were carried out

for the assessment of microstructure of Portland

limestone cements after 2 and 28 days curing. There

are several investigations related with the effect of

clinker or limestone type on physical, chemical and

mechanical properties of Portland limestone ____________

*Corresponding author (E-mail: [email protected])

INDIAN J. ENG. MATER. SCI., AUGUST 2010

290

cement9,12,13

. However, there are relatively limited

number of studies on the microstructure of these

cements13,14

.

Materials and Methods

Two types of clinker and limestone were used in

the experimental study. The chemical compositions of

the clinkers and limestones are shown in Table 1.

Limestones and clinkers were separately ground in a

ball-mill until obtaining a 24% constant 32 µm sieve

residue. 6 and 35% limestone/clinker ratios were used

in preparation of Portland limestone cements. The

gypsum inter-ground by clinker was kept constant as

5% by weight of cement. 50 mm cube cement pastes

with 0.5 water/binder ratios were prepared for the

microstructure analysis. After 2 and 28 days water

curing, specimens were kept in the stove for 45 min at

35°C then immediately vacuum impregnated by low

viscosity epoxy while the paste samples were still

practically non-carbonated. The next day, the samples

were ground with #1200 and #4000 sand papers and

polished with 6 µm, 3 µm, 1 µm and 0.25 µm

diamond paste on a lap wheel. Finally, the specimens

were soaked in an ultrasonic bath for 10 min to

remove residual polishing paste. Microscopic analysis

was carried out by Philips XL30 ESEM equipped

with EDS for energy dispersive X-Ray analysis.

Results and Discussion

Microstructures of specimens

Since the chemical compositions of clinkers and

limestones were very close to each other no

considerable difference was observed between the

microstructure of resultant cement pastes. Thus,

general evaluations were made independently from

clinker and limestone type.

Table 1—Chemical composition of the materials

% Clinker 1 Clinker 2 Limestone Limestone

(C1) (C2) 1 2

SiO2 19.01 20.27 0.99 0.00

Al2O3 5.07 4.56 0.78 0.19

Fe2O3 5.91 4.05 0.33 0.26

CaO 61.64 65.56 54.35 55.56

MgO 1.98 1.21 0.45 0.44

K2O 0.55 0.59 0.21 0.14

SO3 1.86 1.98 0.33 0.07

C3S 54.13 67.36 - -

C2S 13.67 7.30 - -

C3A 3.44 5.23 - -

C4AF 18.00 12.34 - -

Fig. 1—Image of AC1-%6L1 paste at 2 days


Fig. 3—Close up of image at Fig. 2

SEZER et al.: MICROSTRUCTURE OF LIMESTONE CEMENT PASTES

291

Microstructure of 2-day cured pastes

In limestone cements paste containing 6%

limestone, it was observed that clinker grains were

surrounded by calcium silicate hydrate gel (C-S-H)

and hydration products were accumulated around

limestone particles (Figs 1 and 2). Accumulation

around limestone particles can be attributed to

nucleation effect of limestone6,15

. This effect is clearly

seen in Fig. 3. Limestone particles attracted hydration

products from clinker particles to their surfaces. This

is why limestone cements have higher rate of

hydration at early ages than ordinary Portland cement.

As a result of accumulation of some of the hydration

products on the surface of limestone particles, the

thickness of hydration products coating unhydrated

clinker particles reduces. In this way, access of water

to unhydrated clinker gets easier and rate of hydration

increases.

There was no considerable difference between the

microstructure of limestone cement pastes containing

35% limestone and that of limestone cements

containing 6% limestone. Similar to 6% limestone

bearing pastes, in 35% limestone bearing pastes

clinker grains were surrounded by C-S-H (Fig. 4) and

hydration products were accumulated around

limestone particles (Fig. 5).

Microstructure of 28-day cured pastes

In limestone cement pastes containing 6% limestone,

it was observed that clinker grains were surrounded

by a denser C-S-H than 2-day cured specimens

(Figs 6 and 7). This is due to the increase in amount of

hydration products by ongoing hydration.

In limestone cement pastes containing 35%

limestone, it was observed that clinker grains were

also surrounded by a denser C-S-H (Fig. 8) and

hydration products were accumulated around

limestone particles (Fig. 9).

In 2-day cured specimens, densities of hydration

products around limestone particles were higher than

those around clinker particles (Fig. 2). However,

visual density of hydration products around limestone

particles was balanced with clinker particles in 28-day

cured specimens (Fig. 10). As it was expected,

limestone was found to be more effective at early ages

of hydration.

Determination of pore percentage of pastes using

image analysis techniques In this study, pore proportions of paste from ESEM

photos were determined for the purpose of

demonstrating the effect of limestone on pore

structure of hydrated cement paste. For this purpose,

simple image analysis on these photos was employed

via Qwin16

, commercial image analysis software.

The general process is depicted in Figs 11-13. The

proposed method is given as:

(i) A number of morphological operations including

inversion and image closing applied for a better

visualization of the pores

(ii) The pores, which were black in color (due to low

atomic number of epoxy) were detected and

transformed to a binary image (Fig. 12)

Fig. 5—Another image of AC1-%35L1 paste at 2 days



292

Fig. 6—Image of AC2-6L2 paste at 28 days





Fig. 11—Micro photo of unprocessed C1 control at 2 days

SEZER et al.: MICROSTRUCTURE OF LIMESTONE CEMENT PASTES

293

(iii) Opening and closing operations on binary image,

which were the combination of consecutive

erosion and dilation operations; were employed in

order to remove artifacts from the image and

(iv) The holes in the thresholded areas were filled via

morphological operations. Consequently, binary

images became ready for determination of pore

percentages (Fig. 13).

Pore proportions of cements are presented in

Table 2. The values shown in Table 2 are the average

of pore percentages values obtained from minimum

three ESEM photos of related cements.

Regardless of the clinker and limestone types, pore

percentages of cements containing 6% limestone at 2

days were less than the related control paste. This can

be attributed to the increase in visual density of

structure by filling effect of limestone. Besides,

nucleation effect of limestone particles may accelerate

the hydration of limestone cement and resultantly

decrease pore volume.

Regardless of the clinker and limestone types, pore

percentages of cements containing 35% limestone at 2

days were higher than the related control cement. This

may be due to the decrease in clinker content with

increasing limestone inclusion and consequently, an

increase in the effective W/C ratio. Obviously, all the

unbind water results in porosity3.

Pore percentages of cements containing C2 clinker

were generally less than the cements containing C1

clinker. Since C3S/C2S ratio of C2 clinker is higher

than that of C1 clinker, the rate of hydration of this

cement will be obviously higher. Thus, more

hydration products of cements containing C2 clinker

will be formed at the ages considered in this study.

Change in limestone type did not significantly affect

pore percentages of cements.

Conclusions The following conclusions may be drawn from this

study:

(i) Upon limestone addition, hydration products of

cement were accumulated around limestone

particles. In 2-day cured specimens, visual

densities of hydration products around limestone

particles were higher than clinker particles. The

visual density of hydration products around

Fig. 12—Black thresholding was applied to the image

Fig. 13—Obtained pores in cement paste

Table 2—Pore percentages of cement pastes

Cement Pore percentage* Cement Pore percentage*

2 days 28 days 2 days 28 days

AC1-

Control 25.36

17.92 AC2-

Control

23.44 18.33

AC1-

%6L1

25.05 18.33 AC2-

%6L1

22.62 22.49

AC1-

%35L1

28.66 23,81 AC2-

%35L1

25.14 21.54

AC1-

%6L2

22.67 22.26 AC2-

%6L2

20.35 19.78

AC1-

%35L2

29.89 23.50 AC2-

%35L2

27.78 21.19

*Reported values are the average of min three photos


294

limestone particles was balanced with those

around clinker particles at 28-days.

(ii) In 2 days, porosity of cement pastes containing

6% limestone were lower whereas pore

percentages of cements containing 35% limestone

were higher than control paste.

(iii)With a few exceptions, porosity of cements

containing C2 clinker were generally higher than

those containing C1 clinker. This was attributed

to the higher C3S/C2S ratio of C1 cement

compared to that of C2 cement.

(iv) Change in limestone type did not significantly

affect porosity of cements.

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microstructure of 2 and 28-day cured portland limestone...

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