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14th International Congress on the Chemistry of Cement (ICCC 2015)
13~16 October 2015, Beijing, China
1
A Comparative Study of the Reactivity of Calcium Silicates during Hydration and Carbonation
Reactions
Warda Ashraf
1
, Jan Olek1* and Vahit Atakan2
1. Lyles School of Civil Engineering, Purdue University, West Lafayette, IN 47907, USA
2 Director, Research and Development, Solidia Technologies, NJ, USA
Abstract
This work presents a comparative study of the hydration and carbonation reactions of three calcium
silicate phases. Triclinic tricalcium silicate and γ-dicalcium silicate were synthesized in laboratory
from stoichiometric mixtures of CaO and fumed silica (SiO2). The reactivity of the synthesized calcium
silicate systems were then monitored during both carbonation and hydration reactions. Along with the
synthesized calcium silicates, the reactivity of natural wollastonite was also investigated in this work.
For carbonation reaction, calcium silicate samples were first mixed with deionized water and then
exposed to CO2. After 24 hours of reaction the samples were analyzed using Fourier Transform
Infrared (FTIR) Spectroscopy, thermo gravimetric analysis (TGA) and scanning electron microscopy
(SEM) techniques. The presence of Ca(OH)2, CaCO3 and calcium silicate hydrates (C-S-H) was
identified in case of both hydrated and carbonated tricalcium silicate and dicalcium silicate
specimens. For wollastonite specimen, polymerized silica and CaCO3 were identified as the main
reaction products.
In order to compare the reactivity of the calcium silicate systems, the relative amounts of reaction
products were also determined using TGA and FTIR test methods. The relative amounts of reaction
products determined using these two methods were found to be in very good agreement with each
other. Using both of these test methods, higher amount of reaction products were found in case of
carbonation reaction than in the case of hydration reaction.
Originality
Many publications can be found in the literature that compare mechanical properties of the
conventional cement system while subjected to carbonation and hydration reactions. The originality of
this work lies in the fact is that it presents a comparative study on the reactivity of calcium silicate
samples during the hydration and carbonation reactions in terms of reaction products. The originality
of the work also lies in the fact that it studied the hydration and carbonations reactions of individual
calcium silicate systems such as tricalcium silicate (or dicalcium silicate) instead of the conventional
cement binders which is composed of both tri- and di – calcium silicates. Such experimental work
involving single calcium silicate system also allowed calculating the relative fractions of reaction
products using simple stoichiometric equations for both hydration and carbonation reactions.
Keywords: Hydration, Carbonation, Calcium Silicates, Reactivity, FTIR, TGA, SEM.
*Corresponding author:olek@purdue.edu, Tel: 1-765-494-5015
14th International Congress on the Chemistry of Cement (ICCC 2015)
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1. Introduction
Conventional portland cements contains around 75% of calcium silicates, namely – tricalcium silicate
((or C3S in cement chemist notation) and dicalcium silicate ((or C2S)) and thus both,
the strength development and the microstructure of hardened cement paste,are strongly linked to the
reactivity of these phases. Depending on the type and the duration of the curing process and the
exposure conditions, these calcium silicates will undergo both hydration and/or carbonation reactions.
Hydration reactions of calcium silicates has been thoroughly investigated over the last century and
most details of the reaction mechanisms are considered to be well understood. Contradictory to this,
carbonation reaction (or CO2 curing of calcium silicate) is a relatively new concept, which has only
been investigated during the last few decades (Shtepenko et al., 2006; Young et al. 1974). Recently,
the carbonation process of calcium silicates have gained renewed interest in connection with research
on alternative mechanisms of CO2 storage (Galan et al. 2010; Kashef-haghighi and Ghoshal 2010;
Pade and Guimaraes 2007) and also due to its potential for producing rapid strength gain materials
(Klemm and Berger 1972; Shao et al. 2014).
During the hydration reaction, both C3S and C2S react with water to form calcium silicate hydrate
(C-S-H) gel, which is the main source of strength in hardened cement paste. The hydration reaction of
C3S is faster than that of C2S and thus C3S is more desirable component of cement with respect to the
early strength development. On the other hand, other calcium silicates (e.g. α-monocalcium silicate
(wollastonite, CS)) may not exhibit any hydraulic properties at room temperature. The exact reasons
behind the differences in hydraulic reactivities of various calcium silicate phases are still not well
understood (Durgun et al. 2014).
The hydration reactions of C3S and C2S are illustrated, respectively, by equations 1 and 2
Both C3S and C2S were also found to produce C-S-H during the carbonation reaction as shown in
equations 3 and 4 (Young et al. 1974). The carbonation reaction of wollastonite forms polymerized
silica gel and calcium carbonate as shown in equation 5:
Although the carbonation of calcium silicate materials has gained recent interests, the relative
reactivity of these phases during the carbonation process is yet to be investigated. Thus, the objective
of this paper is to present a comparative study of the hydration and carbonation reactions of C3S, C2S
and CS phases. In addition, a brief comparison of the reaction products forming during the carbonation
and hydration reactions of these calcium silicates materials is also presented in this paper.
2. Experimental Setup
2.1 Synthesis of Calcium Silicates
Several methods for synthesizing pure calcium silicates phases can be found in literature (Goto et al.
1995,Berliner et al. 1997, andShtepenko et al. 2006) most of which involve sintering the
stoichiometric mixture of reactive lime and silica. As mentioned earlier, three types of calcium
silicates were utilized in this study. These included phase with high hydraulic reactivity (synthetic
C3S) phase with low hydraulic reactivity (synthetic γ-C2S phase) and non-hydraulic CS phase (natural
mineral (wollastonite)). The synthetic calcium silicate phases were prepared by sintering the
stoichiometric mixture of CaO and amorphous (fumed) silica (SiO2). The CaO used in the sintering
process was obtained by decomposing the reagent grade (99.9% pure) calcium carbonate (CaCO3) by
14th International Congress on the Chemistry of Cement (ICCC 2015)
13~16 October 2015, Beijing, China
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exposing it to a temperature of 1000 ºC for 3 to 4 hours. The C3S was prepared by exposing the
mixture of CaO and fumed silica (at 3:1 molar ratio) to a temperature of 1500 ºC for 4 hours and then
leaving the melt in the furnace until it cooled down to a room temperature. The resulting materials was
ground, sieved using # 200 (74 m) sieve and refried (three times in total) to maximize the chemical
reaction of available lime and silica. After each grinding, small amount of the powdered sample was
examined using X-ray diffraction (XRD) method to check for the presence of any free lime. The
process of preparation of γ-C2S was the same as that used to prepare C3S except in this case the molar
ratio of CaO and fumed silica was 2:1 and the sintering temperature was 1400 ºC. The natural
wollastonite (CS) used in this study was supplied by Solidia Technologies LLC.
The XRD patterns of both of the synthesized calcium silicate phases as well that of the wollastonite
are given in Figure 1. These patterns match reasonably well the published XRD patterns for,
respectively, The C3S, γ-C2S and wollastonite (Goto et. al. 1995, HRB 1972).
Figure 1 XRD patterns of the unreacted calcium silicate phases
2.2. Preparation of Test Specimens
The powdered calcium silicate materials were mixed with small amount of water (water to solid ratio =
0.40) and compacted into small discs (0.75 inch in diameter and 0.25 inch in height). For carbonation
reaction, these small discs were then placed in an enclosed chamber which was kept on the laboratory
bench and which was continuously purged with 99.9% pure CO2(Figure 2). To prevent the drying-up
of the specimens, additional water source was placed inside the chamber. For hydration reaction, the
discs were placed in a small enclosed box, which was placed on the laboratory bench. After 24 hours
of reaction, both types of specimens (i.e. the carbonated and the hydrated ones) were ground (using a
mortar and pestle)and used to perform the characterization tests described in the next section (section
2.3).
Intensity
20 30 40 50 60 70
C3S
γ-C2S
CS
14th International Congress on the Chemistry of Cement (ICCC 2015)
13~16 October 2015, Beijing, China
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Figure 2: Schematic showing the setup used for carbonation of the specimens.
2.3 Test Methods
The thermogravimetric analysis (TGA)was performed using the commercially available instrument
(TA model: Q50). The powdered sample was first kept under isothermal conditions for 10 minutes and
then the temperature was raised at the rate of 10 °C per minute up to 1000 °C. Approximately 35 to 50
mg of powder was tested for each specimen. Nitrogen (N2) was used as purge gas during the TGA
tests. The experimental data from TGA were used to calculate the weight fraction of calcium carbonate
in the sample.
The Fourier transform infrared (FTIR) spectroscopy data presented in this paper were obtained using
the Thermo Nicolet Nexus 470 spectrophotometer, which was fitted with an attenuated total
reflectance (ATR) accessory. The frequency range was 4000 to 800 cm−1 at a resolution of 4 cm−1.
Each spectrum presented in this paper is an average of 36 scans.
The secondary electron (SE) SEM images were collected using a FEI NOVA nanoSEM FESEM. Prior
to SEM investigation, the samples were coated with platinum.
3. Results and Discussion
3.1 Thermogravimetric Analysis
The TGA curves for hydrated and carbonated calcium silicates are given in Figure -5. Both, in the case
of C3S and γC2S test specimens (shown, respectively, in Figure and Figure 4), the major mass
losses were observed to occur during three distinctive temperature ranges. The first mass loss occurred
in the temperature range from about 450 ºC to 500 ºC and it can be attributed to the dehydration of
Ca(OH)2 (as illustrated by equation 6). The second mass loss was observed in the temperature range
from about600ºC to 700 ºC and the third mass loss was observed in the range from 700 ºC to 850 ºC.
Both, the second and third peaks in the TGA graphs can be attributed to the decomposition of calcium
carbonate(see equation7).
The presence of two stages of mass loss in the temperature range from 600 ºC to 850 ºC indicates the
formation of two different forms of calcium carbonate during the carbonation process of C3S and
γ-C2S (Goto, Suenaga, and Kado 1995).
Outlet
Inlet
Samples
Pressure
Regulator
CO2
14th International Congress on the Chemistry of Cement (ICCC 2015)
13~16 October 2015, Beijing, China
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From Figure 5, it can be observed that the CS sample experienced only one major mass loss(in the
temperature range from around 600 to 800 ºC), which is due to the decomposition of CaCO3. The
Presence of CaCO3is expected in case of the carbonated samples as this compound is one of the major
reaction products (see equations 3 - 5). The hydrated samples were also found to contain small amount
of CaCO3, which most likely formed by atmospheric carbonation of the calcium hydroxide present in
hydrated test specimens.
Figure 3: TGA test results for hydrated and carbonated C3S.
Figure 4: TGA test results for hydrated and carbonated C2S.
14th International Congress on the Chemistry of Cement (ICCC 2015)
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Figure 5: TGA test results for hydrated and carbonated CS
Additional powdered samples were heated from 105 ºC to 1100 ºC to obtain the total mass loss (%).
The amount of water that is chemically bound in the calcium silica gel was then obtained by
subtracting the mass losses due to Ca(OH)2 and CaCO3 from the total mass loss. The amounts of
Ca(OH)2 and CaCO3 were calculated using, respectively, equations 6 and 7 and the TGA determined
mass loss values (i.e. the amount of the emitted CO2). However, since the exact nature of the
calcium-silica gels formed during the carbonation process is still unknown, the amount bound water
was reported instead.
The relative mass fractions of Ca(OH)2, CaCO3 and chemically bound water produced during the
hydration and carbonation reactions of the calcium silicates are given in Figure 6. The Ca(OH)2 was
found to be absent in both, the hydrated and carbonated CS specimens, indicating the non-hydraulic
property of the wollastonite. For both C3S and C2S systems, the amount of Ca(OH)2 in carbonated
samples was found to be higher than that found in the hydrated samples. The presence of substantial
amount of Ca(OH)2, even in the carbonated samples, has also been reported by other researchers
(Short et al. 2001). One additional interesting observation from Figure 6 is that the mass fractions of
the reaction products for all calcium silicates are higher in case of carbonation reaction than in the case
of hydration reaction. This finding suggests that, when compared over the same length of reaction
period (24 hours), higher amounts of calcium silicates (even the highly hydraulic ones) undergo
carbonation than hydration.
3.2 ATR-FTIR Test Results
3.2.1 Tricalcium silicate
Figure 77shows the FTIR spectra of, respectively, the unreacted, carbonated and hydrated C3S
specimens. To emphasize the changes in the spectra caused by the reactions, the spectrum of unreacted
C3S was subtracted from the spectra for hydrated and carbonated C3S specimens and the spectra
resulting from these subtractions are presented in Figure 8.
14th International Congress on the Chemistry of Cement (ICCC 2015)
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Figure 6: Relative mass fractions of chemical phases formed during the carbonation and hydration reactions of
pure calcium silicates.
Figure 7: FTIR spectra for hydrated, carbonated unreactedC3Sspecimens.
As seen in Figure 7, the primary band for the unreacted C3S is located at around 829 cm-1. After 24
hours of carbonation of C3S, the major bands are observed at around 3640 cm-1, 1300 ~ 1500 cm-1,
1050 cm-1, 967 cm-1and 875 cm-1 (see Figures 7 and 8). The band at 3640 cm-1 is caused by the
1.95
1.73
1.07
0.59
1.22
1.64
1.22
1.40
5.75
9.56
2.81
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
Hydrated
Carbonated
Hydrated
Carbonated
Hydrated
Carbonated
C3S
C2S
CS
Weight Fraction (%)
Calcium silicate materials
Calcium carbonate
Portlandite
Chemcally bound water
1.20 0.85
1413 cm-1
3641 cm-1 965 cm-1
872 cm-1
3640 cm-1 1409 cm-1 965 cm-1
872 cm-1
829 cm-1
4000 3500 3000 2500 2000 1500 1000
Wavenumbers(cm-1)
C3S
24 hours carbonated C3S
24 hours hydrated C3S
Absorbance
14th International Congress on the Chemistry of Cement (ICCC 2015)
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stretching vibration of the hydroxyl ion present in the Ca(OH)2(Hughes et al. 1995, Arnold et al. 2006,
Chollet and Horgnies 2011, Fernández-Carrasco et al. 2012, and Yu et al. 1999). The band located in
the range of 1300 cm-1 to 1500 cm-1 is due to the asymmetric stretching (ν3) of C-O bond present in
CaCO3 and the band located at around 875 cm-1 corresponds to the out of plane bending vibration (ν2)
of the same C-O bond (Yu et al. 1999).
Figure 8: FTIR spectra for hydrated and carbonatedC3Sspecimens after subtracting the spectra of unreacted C3S.
The bands located in the range of 950 to 1100 cm-1 are associated with the calcium silicate phases that
formed during the hydration and carbonation reactions of C3S. For carbonated C3S sample, the major
bands are located in the range from 950 cm-1to 970 cm-1 and from 1050 cm-1to 1060 cm-1, both of
which are due to the asymmetric stretching of Si-O bonds present in the C-S-H. The exact positions of
these bands are dependent on the calcium to silica (Ca/Si) atomic ratio in the C-S-H phase, which also
indicates differences in the degree of polymerization of the C-S-H (Yu et al. 1999). It can be also
observed from Figure 8 that, in comparison to the hydrated C3S sample, the carbonated C3S sample
has have higher proportion of the band located around 1050 cm-1 compared to the band located at
around 967 cm-1. The ratio of integral area under the band at 1050 cm-1 to that of the band at 967
cm-1 was found to be 9.60 and 2.86 for carbonated C3S and hydrated C3S samples, respectively. This
finding indicates that the C-S-H formed during the carbonation process of C3S may have lower Ca/Si
atomic ratio and higher degree of polymerization compared to the C-S-H formed during the hydration
of C3S.
3.2.2 Dicalcium silicate (γ-C2S)
The FTIR spectra for unreacted γ-C2S, hydrated γ-C2S and carbonated γ-C2S are shown in Figure 9.
The spectra obtained for hydrated γ-C2S and carbonated γ-C2S after subtracting the spectrum of
unreacted γ-C2S are given in Figure 10. For the unreacted γ-C2S, two major bands are observed, one
at 846 cm-1 and at 929 cm-1. The FTIR spectra of hydrated γ-C2S sample (Figure 9 and Figure 10)
showed only minor differences compared to the spectrum from unreacted γ-C2S specimen, indicating
that the overall degree of reaction was low. In contrast, the FTIR spectrum for carbonated γ-C2S
specimen was significantly different from that observed for unreacted specimen. Specifically, the
FTIR spectra of carbonated γ-C2S specimen reveal the presence of CaCO3, Ca(OH)2 and C-S-H as
evident from the presence of the bands located at 1418 cm-1, 872 cm-1, 3641 cm-1 and 1068 cm-1. The
C-S-H formed in the carbonated C2S sample showed only one broad band at 1068 cm-1and the band
one would normally expect for C-S-H (i.e. one located at around 950 cm-1to 970 cm-1) was completely
1409 cm-1
1413 cm-1 1050 cm-1 958
cm-1
1052 cm-1 967 cm-1
872 cm-1
Wavenumbers (cm-1)900
12001500
24 hours
carbonated C3S
24 hours
hydrated C3S
Absorbance
14th International Congress on the Chemistry of Cement (ICCC 2015)
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absent. This change in the band position indicates that the C-S-H formed during the carbonation of
γ-C2S has a lower Ca/Si atomic ratio and higher degree of polymerization compared with the C-S-H
formed in the carbonated C3S sample.
Figure 9: FTIR spectra for unreacted, hydrated and carbonated γ-C2S.
Figure 10: FTIR spectra for hydrated and carbonated γ-C2S samples after subtracting the spectra of unreacted
γ-C2S.
3641 cm-1
3641 cm-1 1411 cm-1
1414 cm-1
950 cm-1
846 cm-1
846 cm-1
929 cm-1
929 cm-1
1000150020002500300035004000 Wavenumbers (cm-1)
γ-C2S
24 hours hydrated γ-C2S
24 hours carbonated γ-C2S
Absorbance
9001200
1500
1414 cm-1
1411 cm-1 1068 cm-1 872 cm-1
961 cm-1
24 hours carbonated γ-C2S
24 hours hydrated γ-C2S
Wavenumbers(cm-1)
Absorbance
14th International Congress on the Chemistry of Cement (ICCC 2015)
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3.2.3 Wollastonite (CS)
The FTIR spectra for unreacted, hydrated and carbonated wollastonite (CS) specimens are shown in
Figure 11. The major bands for the unreacted CS samples are located at 896 cm-1, 1010 cm-1 and at
1060 cm-1. Due to the inosilicate structure of wollastonite, the presence of bands at higher calcium
silica region (1100 cm-1) is expected for unreacted CS samples due to the presence of higher amount of
the bridging tetrahedra (Hansen et al. 2003). FTIR spectra for both carbonated and hydrated CS
samples showed only minor changes compared to the spectrum of the unreacted CS. The most
significant change involved the formation of the band at around 1432 cm-1 and the increase of the
height of the peak at 1060cm-1. Nonetheless, the changes in the carbonated CS system were more
prominent than that of hydrated CS system indicating higher reactivity of CS system in case of
carbonation reaction than in the case of hydration reaction.
Figure 11: FTIR spectra for unreacted, hydrated and carbonated CS.
3.2.4 Comparison of the reactivity based on the FTIR spectra
As mentioned previously, the spectra of the original (unreacted) specimens were subtracted from the
spectra of the hydrated/ carbonated samples to improve the ease of comparison of changes in the FTIR
spectra resulting from the reaction. In general, additional bands were found to form in the region of
1300 cm-1 to 1500 cm-1 and at 950 cm-1 to 1200 cm-1 in FTIR spectra obtained after subtraction,
irrespective of the type of calcium silicate minerals used. The integrated areas under the bands „1300
cm-1 to 1500 cm-1‟ and „950 cm-1 to 1200 cm-1‟ were considered to be representative of the amounts of
CaCO3 and calcium silica gel, respectively, formed during the carbonation and hydration reactions of
the calcium silicates. These integrated peak areas are presented in Figure 12 for all reacted calcium
silicate samples. The variation of different phases appears to be similar to that of obtained from TGA
test (Figure 6). For all calcium silicate minerals, the amount of reaction products was found to be
higher in case of carbonation reaction than in the case of hydration reaction. This observation then
also supports the results of TGA tests. Additionally, similar to what was seen in the TGA results, the
1432 cm-1
1432 cm-1
1440 cm-1
1059 cm-1
1059 cm-1
1058 cm-1
1010 cm-1
1009 cm-1
1009 cm-1
896
cm-1
896
cm-1
896
cm-1
90010001100120013001400150016001700
CS
24 hours hydrated CS
24 hours carbonated CS
Absorbance
Wavenumbers (cm-1)
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carbonated γ-C2S specimens are shown to contain higher amount of reaction product than the
carbonated C3S specimens.
Figure 12: Integral peak areas of FTIR spectra for hydrated and carbonated calcium silicates.
3.3 The Microstructure of the Carbonated and Hydrated Calcium Silicates
Secondary electron images showing the microstructure of the carbonated and hydrated calcium silicate
specimens are shown in Figure 13. After 24 hours of hydration, a large amount of reaction products
(Ca(OH)2 and C-S-H) were identified in C3S specimen, (Figure 13(a)). The carbonated C3S sample
was found to contain spherical reaction products (Figure 13 (b)) which appear to be amorphous
calcium carbonates.
The microstructure of hydrated γ-C2S sample (Figure 13 (c)) was similar to that of completely
unreacted γ-C2S sample (not shown) and did not contain any significant amounts of reaction products.
However, in case of carbonated γ-C2S samples, there appears to be some reaction products (in the form
of distinctive small, (“fuzzy”), round particles (Figure 13 (d)). Additionally, while investigating the
microstructure of carbonated γ-C2S samples, some areas containing large amount Ca(OH)2 and C-S-H
were also observed which were not present in case of hydrated γ-C2S samples.
After 24 hours of hydration, the discs samples prepared from natural wollastonite (CS) did not
hardened and hence the SE-SEM images were collected from the hydrated powdered wollastonite.
Some laths-like phases were found around the wollastonite grains, but considering the morphology and
appearance of these products, it is suspected that they represent a silica phase that leached out from the
original grains. A high magnification image of the carbonated wollastonite grain is given in Figure 13
(e). The surface of the carbonated wollastonite grain appears to be different from that of hydrated
wollastonite grain, indicating higher amount of ionic activity on the surface. The same image also
shows a formation of a distinctive layer on the wollastonite grains, which, based on the other research
(Daval et al. 2009 (a), Daval et al. 2009(b)) is likely a silica rich phase.
Hydrated
Carbonated
Hydrated
Carbonated
Hydrated
Carbonated
C3S
C2S
CS
1300 cm-1 to 1500 cm-1 : amount of CaCO3
950 cm-1 to 1200 cm-1 : amount of calcium silica gel
Integrated absorbance
(arbitrary unit)
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Figure 13: Secondary images of: (a) hydrated C3S, (b) carbonated C3S, (c) hydrated γ-C2S, (d) carbonated γ-C2S,
(e) hydrated wollastonite and (f) carbonated wollastonite after 24 hours of corresponding reactions.
(a)
(b)
(c)
(d)
(e)
(f)
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4. Summary and Conclusions
The synthetic C3S, γ-C2S and natural CS samples were subjected to carbonation and hydration
reactions at room temperature for 24 hours. The reaction products were then analyzed using the TGA,
ATR-FTIR and SEM test methods. The major findings from this work are summarized below:
(i). In case of C3S and γ-C2S specimens, the main reaction products were identified as Ca(OH)2,
CaCO3 and C-S-H for both hydration and carbonation reactions. For the CS specimens, the
main reaction products were CaCO3 and polymerized silica rich phase.
(ii). The relative amounts and types of reaction products determined from TGA tests results were
found to be a close match to those determined from FTIR spectra. This suggests that
ATR-FTIR test technique can be a very useful tool to investigate the chemical changes in
cement - based materials.
(iii). Using both TGA and FTIR methods, the amount of reaction products formed during the
carbonation reaction of all calcium silicate phases investigated in this study was found to be
higher than that generated during the hydration reaction. This finding suggests that, in the case
of reactions lasting 24 hours and taking place at room temperatures the carbonation process
was more effective than the hydration process, even for nominally highly hydraulic material.
(iv). From FTIR analysis, it was observed that the C-S-H formed during the carbonation of C3S had
a higher degree of polymerization than the C-S-H formed during the hydration reaction.
(v). As observed from the analysis of the FTIR spectra, the C-S-H formed due to the carbonation
of γ-C2S had higher degree of silica polymerization than the C-S-H formed during the
carbonation of C3S samples. Again, the silica polymerization of the carbonated wollastonite
sample (band position >1100 cm-1) was found to be the highest among all specimens tested
during this study. These observations suggest that the degree of silica polymerization is a
function of initial Ca/Si ratio. Similar conclusion was reported by other researchers
(Shtepenko et al. 2006).
Finally, it is important to note that the concluding points mentioned here are based only on the test
results from systems reacting for only 24 hours. However, the ongoing tests, which focus on materials
undergoing reactions for up to 72 hours, show very similar trends. In addition, the results obtained
from different test methods also show very consistent trends, thus increasing the level of confidence in
the findings reported in this paper.
Acknowledgements
The Solidia Technologies LLC is gratefully acknowledged for providing the partial funding and the
wollastonite samples required to complete this study.
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