CO2-Reducing Cement Based on Calcium Silicates
Presented at the14th International Congress on the Chemistry of Cement
Beijing, China
October 2015
S. Sahu1*, S. Quinn1, V. Atakan1, N. DeCristofaro1, G. Walenta2
1. Solidia Technologies, 11 Colonial Drive, Piscataway, NJ-08854, USA
2. LCR, 95 rue du Montmurier BP 15, 38291 Saint-Quentin Fallavier Cedex, France
Abstract
A new type of cement is made from raw materials that are typically used to make ordinary Portland cement
(OPC) clinker. The kiln feed raw mix proportions are adjusted so as to produce a clinker rich in low-lime
calcium silicate phases such as CS and C3S2 instead of the high-lime phases C3S and C2S typical of OPC.
Production of such clinker requires 30% less limestone compared to OPC clinker and allows sintering at
temperature as low as 1200oC compared to 1450°C for OPC. The clinker is ground to make a cement of similar
fineness to OPC, and its total manufacturing CO2 emissions are estimated to be 30% less than for OPC. This
cement does not harden by hydration; rather, it hardens by carbonation. Its main carbonation products are
CaCO3 and SiO2. The CaCO3 is primarily formed as calcite with some vaterite. The SiO2 is present in a
non-crystalline form. Microstructural evaluation shows that silicon is less mobile than calcium during curing
and remains around the cement particles. However, the CaCO3 precipitates in the pore spaces of the cement
matrix.
This cement is a patented cement of Solidia Technologies and is commercially available as Solidia Cement
Originality
This is original work carried out at Solidia Technologies.
Keywords: CO2-reduction, Carbonation, CaCO3, CO2-emission, CS
* Corresponding author: Sada Sahu, [email protected]
™.
1. Introduction
The cement industry is a significant contributor to global greenhouse gas emissions. A 2005 study by
the World Resources institute (WRI) has reported that the cement industry was responsible for 3.8% of
the total global greenhouse gas emissions (WRI, 2005). The International Energy Agency (IEA) has
set a target for the cement industry to reduce its CO2 emissions from the 2.0 Gt emitted in 2007 to 1.55
Gt by 2050 (IEA, 2008). This must be accomplished despite a predicted 43 to 73% growth in
worldwide cement production (Barcelo L., et al, 2013). The specific CO2 emission per tonne of cement
produced must be reduced from 0.8 t to between 0.35 t and 0.42 t to meet this target. To achieve this
goal, a radical approach to cement manufacturing and CO2-funtional cement chemistry must be
undertaken.
1.1. Portland Cement and CO2 Emissions
Portland cement clinker is produced by burning a mixture of calcareous and siliceous raw materials in
a rotary kiln at high temperature. Calcareous material, such as limestone, and siliceous material, such
as sand, clay, or similarly composed materials are ground and processed at a sintering temperature of
~1450°C in a kiln. The clinker nodules produced by the sintering process are then ground and
combined with ~5% gypsum to produce finished cement. Portland cement production emits CO2 by
two direct mechanisms:
 The primary source of CO2 emission is the decomposition of CaCO3 to produce CaO(s) and
CO2(g).
 Heat necessary to achieve reaction and sintering temperature in the kiln is supplied
through the burning of fossil fuels.
CO2-emissions are also indirectly created through the use of electricity. Mining, crushing and grinding
of raw materials to create the raw meal feed to the kiln and final grinding of cement clinker contribute
to this category. Emissions from use of purchased electricity can vary widely depending on the source
of the electricity and are relatively small, about 10% of the total CO2 emissions. Due to this,
contributions to CO2 emissions from the use of electricity are omitted in the calculation here.
Portland cement clinker typically contains up to 70% CaO by weight. The calcination of limestone
used to achieve this proportion of CaO releases ~540 kg of CO2 gas per tonne of clinker produced
(Barcelo L., et al, 2013). The CO2 emitted from combustion within the kiln can vary depending on the
type and efficiency of the kiln as well as the fuel source. A high efficiency kiln with a five stage
pre-heater and pre-calciner has an efficiency of 58% which results in a CO2 emission of ~ 270 kg CO2
per tonne of clinker produced. An older wet-process kiln with an efficiency of 26% can have a CO2
emission from combustion as high as 600 kg CO2 per tonne of clinker produced (Barcelo L., et al,
2013). The contribution of calcination and combustion process of Portland clinker production yields
an associated specific CO2 emission of 810 kg to 1,146 kg per tonne of clinker produced.
1.2. Carbonatable Calcium Silicate Cement
A new type of cement, carbonatable calcium silicate cement (CCSC), has a chemistry and
functionality that allows it to have a significantly reduced CO2 footprint compared to Portland cement.
In CCSC, low-lime calcium silicate phases such as wollastonite/pseudowollastonite (CaO∙SiO2, CS)
and rankinite (3CaO∙2SiO2, C3S2) are preferred to the high-lime alite (3CaO∙SiO2, C3S), belite
(2CaO∙SiO2, C2S) tricalcium aluminate (3CaO∙Al2O3, C3A), and tetracalcium aluminate ferrite
(4CaO∙Al2O3∙Fe2O3, C4AF) phases of Portland cement.
1.3. Carbonatable Calcium Silicate Cement CO2 Emissions
Since the high-lime phases present in Portland cement clinker can be omitted, the requirement for CaO
in the clinker is reduced. As discussed in section 1.1, up to ~67% of CO2 emitted during the production
of Portland cement clinker in a high efficiency kiln may be due to the calcination of CaCO3 within the
raw materials. For CCSC, the lime content can be reduced from 70% to 45%, which translates to a
reduction of CO2 emissions from 540 to 370 kg per tonne of clinker during the calcination process.
The remaining CO2 emissions are due to the combustion of carbon-rich fuels within the kiln. The
calcination of limestone is a strongly endothermic reaction and consumes most of the heat input, so the
reduction in kiln feed limestone content also leads to a significant reduction in kiln fuel requirements.
The formation of clinker phases is also more exothermic for CCSC than for OPC, which further
reduces kiln fuel requirements. In addition, there is a small extra savings due to the reduction in the
peak sintering temperature from ~1450°C needed to make OPC clinker, down to as low as ~1200°C
for CCSC; however, since the cement kiln is an efficient counter-current heat exchanger this peak
temperature change only affects fuel consumption by a few percent. The combined effect is a potential
kiln fuel savings of ~30%. The calculated heats of reaction are summarized in Table 1 (Sahu S.,
DeCristofaro N., 2013). In this calculation several assumptions were made, so the real number may be
slightly different than what is reported here. The CO2 emissions as calculated by reduction of CaCO3
in the raw mix and an estimated 30% fuel savings are shown in Table 2. The details of the CO2 saving
opportunities are also discussed in previous publications (DeCristofaro N., Sahu S., 2014, Atakan V., et al,
2014).
Table 1: Reaction enthalpies calculated for Portland cement clinker and a theoretical carbonatable calcium
silicate clinker. Carbonatable calcium silicate clinker numbers are based on a modeled clinker and may vary
slightly depending on phase composition.
Reaction
Portland cement clinker
Carbonatable calcium silicate clinker
ΔH (GJ/t)
ΔH (GJ/t)
Calcination
+2.138
+1.514
Decomposition of clays
+0.063
+0.075
Formation of clinker phases
-0.377
-0.538
Total
+1.757
+1.051
Table 2: Summary of CO2 emissions as reported for Portland cement clinker compared to the predicted CO2
emissions for a carbonatable calcium silicate clinker.
CO2 Emission source
Per tonne Portland
Per tonne carbonatable calcium
cement clinker
silicate clinker
Calcination
540 kg
375 kg
-30.6 %
Combustion
270 kg
190 kg
-29.6 %
Change
Total CO2 Emission
810 kg
565 kg
-30.2 %
Experimental
2.2. Synthesis of Carbonatable Calcium Silicate Cement
2.
CCSC was produced using a rotary cement kiln. Limestone and sand typical of ordinary Portland cement
production were ground to 82% passing 200 mesh. The ground raw material was processed in a rotary kiln with
4-stage preheater. The burning zone temperature was maintained at ~1260°C.
2.3. Finish Milling
The resultant clinker was crushed and ground to a Blaine fineness of approximately 490 m2/kg by means of a
closed-circuit ball mill equipped with a separator. The particle size of the ground cement was measured by laser
diffraction in water suspension (Malvern Mastersizer 2000, Malvern, UK).
2.4. Chemical and Phase Analysis
The clinker was sampled and analyzed for elemental composition by X-ray fluorescence (XRF) and phase
composition by X-ray diffraction (XRD). Clinker samples were made into fused glass beads for XRF analysis
and measured using a Panalytical Axios WDS spectrometer (Alemo, NL). Data was collected using a
Panalytical Cubix3 diffractomer (Almeno, NL) with 1600W Cu Kα radiation from 5° - 65° 2Θ, 0.017°/s, 60s per
step. Amorphous content was determined by comparison of the collected pattern to a crystalline rutile standard.
Silica was present as quartz, cristobalite and trydimite.
2.5. Formulation of Concrete
Concrete was prepared using the mixture proportions provided in Table 3. The water/cement mass ratio (w/c)
was maintained at 0.29. A high-range water reducer was used at dosage rate of 10 ml/kg of cement. Concrete
cylinders were cast and carbonated (as detailed below) and compressive strengths were measured using the
ASTM C39 method. A carbonated cylinder is shown in Figure 1.
Table 3: Mixture proportion of concrete samples.
Component
Dry mass %
Proportion (kg/m3)
Cement
18%
427.2
Sand
31%
735.7
1/4” Aggregate
26%
617.1
3/8” Aggregate
25%
593.3
Figure 1: CCSC concrete cylinder after carbonation.
2.6. Carbonation Reaction of Cement and Concrete
Carbonation of the concrete cylinders was carried out in a closed container. The reaction of
carbonatable calcium silicate phases with CO2(g) requires mild temperatures (< 100°C) and a high
concentration of CO2 to proceed. Moisture was always present during carbonation process.
A sample of cement was carbonated in an aqueous environment. Distilled, deionized water was heated
to 60°C in a reactor with a condenser to maintain the water level. CO2 was bubbled through the reactor
using a diffuser and a mineral oil bubbler was used at the reactor outlet to maintain a high
concentration of CO2. The cement sample was introduced to the reactor to achieve water to solids ratio
of 10:1 and stirred for one hour. After the reaction period the solids were extracted using a vacuum
filter with 0.22 µm filter paper and immediately dried. The reacted cement was analyzed by XRD to
observe the change in the phase composition upon carbonation.
2.7. Scanning Electron Microscopy (SEM)
A small cut portion of a concrete cylinder was dried and epoxy impregnated and cured. After curing
one of the surfaces was polished to ¼ micron finish. The polished surface was sputter coated with
carbon and examined under a SEM in backscattered mode. A Tescan, Mira 3 Field Emission SEM at
20 keV was used to acquire the images.
3. Results and Discussion
3.1. Chemical Composition of the Clinker
The average elemental composition of the clinker as measured by XRF and expressed as oxides, is shown in
Table 4, together with the measured ignition loss.
Table 4: The average composition of CCSC.
Component
CaO
SiO2
Al2O3
Fe2O3
MgO
Na2O
K2O
SO3
6.00
2.47
2.03
0.14
1.00
1.06
LOI950°C
Trace
0.27
0.23
Concentration
(mass %)
42.76
43.20
3.2. Phase Composition of the Clinker
The average phase composition of the clinker as determined by quantitative X-ray diffraction with
Reitveld refinement is shown in Table 5.
Table 5: The average phase composition of CCSC as measured by X-ray diffraction.
Component
Formula
Concentration
(mass %)
Pseudowollastonite
CaSiO3
22.3
Wollastonite
CaSiO3
0.2
Rankinite
Ca3Si2O7
18.1
Belite
Ca2SiO4
1.3
Amorphous
22.2
2+
Melilite
(Ca,Na,K)2(Al,Mg,Fe )[(Al,Si)SiO7]
30.5
Bredigite
Ca7Mg(SiO4)4
0.3
Silica
SiO2
5.0
Lime
CaO
0.1
3.3. Particle size of the cement
The particle size distribution of the cement is provided in Figure 2. The statistics of the particle size
distribution is provided in Table 6.
Table 6: Particle size distribution statistics.
d10
d50
d90
1.60 µm
13.42 µm
33.94 µm
Figure 2: Particle size distribution of the cement.
3.4. Carbonation Reaction of Concrete
The results of compressive strength measurements are provided in Table 7.
Table 7: Test results of compressive strength (3 samples were tested).
Bulk density (kg/m3)
Compressive strength (MPa)
2436
56.4 ± 3.4
Wollastonite/pseudowollastonite (CaSiO3) and rankinite (Ca3Si2O7) react with CO2 as described in
equations 1-2.
𝐻2 𝑂
CaSiO3(s) + CO2(g) →
CaCO3(s) + SiO2(s)
(1)
𝐻2 𝑂
Ca3Si2O7(s) + 3CO2(g) →
3CaCO3(s) + 2SiO2(s)
(2)
Additionally, the amorphous component of the cement can carbonate to some extent, depending on its
calcium content and bulk chemistry. The formation of CaCO3(s) and SiO2(s) is associated with a net
solid volume increase. This reaction is what gives CCSC the ability to generate strength, similarly to
the solid volume increase produced by hydration in Portland cements. A thin layer of water present on
the surface of the cement particles is necessary to catalyze the carbonation process by allowing the
transport of calcium (Ca2+) ions, as well as the carbonate (CO32-) and bicarbonate (HCO3-) ions formed
from dissolved CO2(g). Silica has low solubility in this aqueous phase and so is not transported over
long distances. Water is not consumed in the reaction but slowly evaporates during curing.
The diffraction pattern obtained from measurement of the carbonated cement is shown in Figure 3. Results
of Rietveld refinement are given in Table 8. The results show that calcite with small amounts of aragonite
and vaterite were formed. The proportions of CaCO3 polymorphs formed are controlled by impurities
present in the cement and the reaction conditions used. The silica formed during the carbonation process is
present as an amorphous calcium-depleted rim around the residual uncarbonated particles and thus was not
detected by XRD.
Figure 3: XRD patterns collected for cement before and after carbonation. The patterns were normalized and
truncated for clarity. The dissolution of pseudowollastonite, rankinite (CS, C3S2) and precipitation of calcite (C) is
apparent. The major peak associated with melilite (M) is unchanged by the carbonation reaction. Only a small
amount of belite was present in the unreacted cement and is not visible at this scale.
Table 8: Quantification of carbonated CCSC XRD results. Only the crystalline components were quantified.
Component
Formula
Concentration (mass %)
Pseudowollastonite
CaSiO3
13.6
Rankinite
Ca3Si2O7
7.9
Melilite
(Ca,Na,K)2(Al,Mg,Fe2+)[(Al,Si)SiO7]
41.8
Silica
SiO2
5.7
Calcite
CaCO3
31.1
Scanning electron microscopic (SEM) images of the hardened CCSC concrete collected in backscattered
electron (BSE) imaging mode are provided in Figures 4 and 5. High brightness reactive phase particles are
generally surrounded by a dark rim of calcium depleted amorphous silica. The space between cement
particles is filled with CaCO3 particles of intermediate brightness. The dark area is the porosity left after the
carbonation process. The carbonation of CCSC produces a densified material through the direct
precipitation of CaCO3 in the pores, where it acts to bind together the components of the concrete.
Figure 4: FESEM-BSE image showing the microstructure of the carbonated paste area.
Figure 5: FESEM-BSE image showing the microstructure of the carbonated area around an unreacted cement
particle.
4. Conclusions
The production of CCSC typically emits about 30% less CO2 than the production of Portland cement, and
the total energy consumption is also about 30% less. Concretes made by carbonating CCSC can achieve a
reduction in carbon footprint by as much as 70% compared to conventional OPC-based concretes. This is
achieved a) by reducing the CO2 emitted during cement production from 810 kg per tonne of OPC clinker
to 565 kg per tonne of CCSC clinker; and b) by consuming up to 300 kg of CO2 per tonne of this cement
during the CO2-curing.
The carbonation of the cement produces amorphous SiO2 and crystalline CaCO3. The CaCO3 polymorphs
formed were primarily calcite but vaterite and/or aragonite may be present in some circumstances. The
proportions of the CaCO3 polymorphs formed depend on impurities in the cement and the reaction
conditions used. The carbonation reaction results in a net increase in the solid volume of the cement, which
densifies the system by filling the pores with reaction products and allows CCSC to produce concretes with
good mechanical properties. Depending on the application, the cement content and the cement quality, a
final carbonated concrete product may contain up to 7 wt.% of sequestered CO2.
Acknowledgements
Part of this work was supported US Department of Energy’s (DOE) National Energy Technology
Laboratory (NETL).
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