Presented at the14th International Congress on the Chemistry of Cement. Beijing, China. October 2015

Transcription

1 CO 2 -Reducing Cement Based on Calcium Silicates Presented at the14th International Congress on the Chemistry of Cement Beijing, China October 2015 S. Sahu 1 *, S. Quinn 1, V. Atakan 1, N. DeCristofaro 1, G. Walenta 2 1. Solidia Technologies, 11 Colonial Drive, Piscataway, NJ-08854, USA 2. LCR, 95 rue du Montmurier BP 15, 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 C 3 S 2 instead of the high-lime phases C 3 S and C 2 S typical of OPC. Production of such clinker requires 30% less limestone compared to OPC clinker and allows sintering at temperature as low as 1200 o C compared to 1450 C for OPC. The clinker is ground to make a cement of similar fineness to OPC, and its total manufacturing CO 2 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 CaCO 3 and SiO 2. The CaCO 3 is primarily formed as calcite with some vaterite. The SiO 2 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 CaCO 3 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: CO 2 -reduction, Carbonation, CaCO 3, CO 2 -emission, CS * Corresponding author: Sada Sahu, ssahu@solidiatech.com

2 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 CO 2 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 CO 2 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 CO 2 -funtional cement chemistry must be undertaken Portland Cement and CO 2 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 CO 2 by two direct mechanisms: The primary source of CO 2 emission is the decomposition of CaCO 3 to produce CaO (s) and CO 2(g). Heat necessary to achieve reaction and sintering temperature in the kiln is supplied through the burning of fossil fuels. CO 2 -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 CO 2 emissions. Due to this, contributions to CO 2 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 CO 2 gas per tonne of clinker produced (Barcelo L., et al, 2013). The CO 2 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 CO 2 emission of ~ 270 kg CO 2 per tonne of clinker produced. An older wet-process kiln with an efficiency of 26% can have a CO 2 emission from combustion as high as 600 kg CO 2 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 CO 2 emission of 810 kg to 1,146 kg per tonne of clinker produced Carbonatable Calcium Silicate Cement

3 A new type of cement, carbonatable calcium silicate cement (CCSC), has a chemistry and functionality that allows it to have a significantly reduced CO 2 footprint compared to Portland cement. In CCSC, low-lime calcium silicate phases such as wollastonite/pseudowollastonite (CaO SiO 2, CS) and rankinite (3CaO 2SiO 2, C 3 S 2 ) are preferred to the high-lime alite (3CaO SiO 2, C 3 S), belite (2CaO SiO 2, C 2 S) tricalcium aluminate (3CaO Al 2 O 3, C 3 A), and tetracalcium aluminate ferrite (4CaO Al 2 O 3 Fe 2 O 3, C 4 AF) phases of Portland cement Carbonatable Calcium Silicate Cement CO 2 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 CO 2 emitted during the production of Portland cement clinker in a high efficiency kiln may be due to the calcination of CaCO 3 within the raw materials. For CCSC, the lime content can be reduced from 70% to 45%, which translates to a reduction of CO 2 emissions from 540 to 370 kg per tonne of clinker during the calcination process. The remaining CO 2 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 CO 2 emissions as calculated by reduction of CaCO 3 in the raw mix and an estimated 30% fuel savings are shown in Table 2. The details of the CO 2 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 Reaction slightly depending on phase composition. Portland cement clinker ΔH (GJ/t) Carbonatable calcium silicate clinker ΔH (GJ/t) Calcination Decomposition of clays Formation of clinker phases Total Table 2: Summary of CO 2 emissions as reported for Portland cement clinker compared to the predicted CO 2 emissions for a carbonatable calcium silicate clinker. CO 2 Emission source Per tonne Portland Per tonne carbonatable calcium cement clinker silicate clinker Change Calcination 540 kg 375 kg % Combustion 270 kg 190 kg %

4 Total CO 2 Emission 810 kg 565 kg % 2. Experimental 2.2. Synthesis of Carbonatable Calcium Silicate Cement 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 Finish Milling The resultant clinker was crushed and ground to a Blaine fineness of approximately 490 m 2 /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) 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 Cubix 3 diffractomer (Almeno, NL) with 1600W Cu K α radiation from Θ, /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 Formulation of Concrete Concrete was prepared using the mixture proportions provided in Table 3. The water/cement mass ratio (w/c) was maintained at 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/m 3 ) Cement 18% Sand 31% /4 Aggregate 26% /8 Aggregate 25% 593.3

5 Figure 1: CCSC concrete cylinder after carbonation 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 CO 2(g) requires mild temperatures (< 100 C) and a high concentration of CO 2 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. CO 2 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 CO 2. 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 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 SiO 2 Al 2 O 3 Fe 2 O 3 MgO Na 2 O K 2 O SO 3 LOI 950 C Trace Concentration (mass %) 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.

6 Table 5: The average phase composition of CCSC as measured by X-ray diffraction. Component Formula Concentration (mass %) Pseudowollastonite CaSiO Wollastonite CaSiO Rankinite Ca 3 Si 2 O Belite Ca 2 SiO Amorphous 22.2 Melilite (Ca,Na,K) 2 (Al,Mg,Fe 2+ )[(Al,Si)SiO 7 ] 30.5 Bredigite Ca 7 Mg(SiO 4 ) Silica SiO Lime CaO 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. d 10 d 50 d µm µm µm 3.4. Carbonation Reaction of Concrete The results of compressive strength measurements are provided in Table 7. Bulk density (kg/m 3 ) Figure 2: Particle size distribution of the cement. Table 7: Test results of compressive strength (3 samples were tested). Compressive strength (MPa) ± 3.4 Wollastonite/pseudowollastonite (CaSiO 3 ) and rankinite (Ca 3 Si 2 O 7 ) react with CO 2 as described in equations 1-2. CaSiO 3(s) + CO 2(g) H 2O CaCO 3(s) + SiO 2(s) (1)

7 Ca 3 Si 2 O 7(s) + 3CO 2(g) H 2O 3CaCO 3(s) + 2SiO 2(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 CaCO 3(s) and SiO 2(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 (Ca 2+ ) ions, as well as the carbonate (CO 3 2- ) and bicarbonate (HCO 3 - ) ions formed from dissolved CO 2(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 CaCO 3 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 CaSiO Rankinite Ca 3 Si 2 O 7 7.9

8 Melilite (Ca,Na,K) 2 (Al,Mg,Fe 2+ )[(Al,Si)SiO 7 ] 41.8 Silica SiO Calcite CaCO 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 CaCO 3 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 CaCO 3 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.

9 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 CO 2 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 CO 2 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 CO 2 per tonne of this cement during the CO 2 -curing. The carbonation of the cement produces amorphous SiO 2 and crystalline CaCO 3. The CaCO 3 polymorphs formed were primarily calcite but vaterite and/or aragonite may be present in some circumstances. The proportions of the CaCO 3 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 CO 2. Acknowledgements Part of this work was supported US Department of Energy s (DOE) National Energy Technology Laboratory (NETL). References - World Resources Institute (WRI), Navigating the numbers. Greenhouse Gas Data and International Climate Policy. ISBN

10 - International Energy Agency (IEA), Energy technology perspectives 2008, scenarios and strategies to Paris. ISBN Barcelo L., et al, Cement and carbon emissions [on-line].materials and Structures. DOI: /s DeCristofaro N., Sahu S., CO 2 -Reducing Cement, World Cement, January. - Atakan V., et al, Why CO 2 matters advances in a new class of cement. ZKG, pp Sahu S., DeCristofaro N., White Paper on Solidia Cement TM [on-line]. On: [Accessed 2 nd February 2015].