Concrete Blocks Manufactured Using Sequestered Carbon Dioxide. Abstract

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1 Concrete Blocks Manufactured Using Sequestered Carbon Dioxide Sean Monkman 1 and Mark MacDonald 1 Abstract A novel approach to beneficially using carbon dioxide to produce concrete blocks was investigated during a pilot demonstration. A retrofit system to inject gaseous carbon dioxide was attached to a conventional concrete mixer to produce blocks using a conventional medium weight block mix design and curing. The carbon dioxide was injected into the concrete during the mixing cycle. The gas was thereby absorbed into the concrete to form thermodynamically stable carbonate reaction products distributed throughout the concrete matrix. The effects of the absorbed carbon dioxide on the concrete temperature, compressive strength, density, and absorption were examined. The carbonated blocks required a simple mix design modification through a mix water increase. The carbonated blocks were found to display improved performance relative to the control blocks at 7, 28 and 56 days. The carbon dioxide was sequestered in the product as nano-scale carbonate reaction products. The treatment served to improve the environmental footprint of the treated blocks. Keywords: concrete masonry unit, block, carbon dioxide, CO2, sustainability, upcycling Introduction Carbon dioxide emissions are recognized as a significant issue for the cement and concrete industry. It is estimated that 5% of the world s annual CO 2 emissions are attributable to cement production [Damtoft 2008]. Portland cement clinker typically has embodied CO 2 on the order of 866 kg CO 2 e/t of clinker [Cement Sustainability Initiative 2009]. Cement production releases CO 2 due to the calcination of limestone that is heated to drive off CO 2 and yield reactive CaO phases. Carbon dioxide emissions are further associated with the energy required to operate the cement kiln. About 40% of the process emissions are associated with the energy consumption and 60% are associated with the calcination. One potential method to improve the environmental profile of concrete is to upcycle carbon dioxide into concrete products by treating them with CO 2 prior to the end of their processing, such as during the curing stage [Kashef-Haghighi 2015; Shao 2013; Shi 2012]. If an industrial process could successfully use carbon dioxide as a feedstock in the production of concrete building products there would be widely distributed carbon utilization that would effectively close the loop for some of carbon dioxide emitted during the cement production. The mechanism of the carbonation of freshly hydrating cement was systematically studied in the 1970s at the University of Illinois. The main calcium silicate phases in cement were shown to react in the presence of water to form calcium carbonate and calcium silicate hydrate gel [Berger 1972]: 3CaO SiO 2 + (3-x)CO 2 + yh 2 O xcao SiO 3 yh 2 O + (3-x)CaCO 3 [1]

2 2CaO SiO 2 + (2-x)CO 2 + yh 2 O xcao SiO 3 yh 2 O + (2-x)CaCO 3 [2] The carbonation reaction is exothermic. The carbonation heats of reaction for the main calcium silicate phases are 347 kj/mol for C 3 S and 184 kj/mol for β-c 2 S [Goodbrake 1979]. The calcium silicate hydrate (C-S-H) gel that forms is understood [Berger 1972] to be intermixed with CaCO 3. C-S-H gel formation occurs even in an ideal case of β-c 2 S and C 3 S exposed to a 100% CO 2 at 1 atm given the observation that the amount of carbonate that forms does not exactly correspond to the amount of calcium silicate involved in the reaction [Goodbrake 1979]. Since carbonation reactions proceed starting with the dissolution of carbon dioxide in water to form carbonic acid, the initial vigorous carbonation stages of carbonation can be described accordingly [Young 1974]: 3CaO SiO (H 2 CO 3 ) 1.4CaO SiO 2 0.6(H 2 O) + 1.2(CaCO 3 ) + 0.6(H 2 O) [3] The reaction of carbon dioxide with a mature concrete microstructure is acknowledged as a durability issue given effects such as shrinkage, reduced pore solution ph and carbonation induced corrosion. In contrast, a carbonation reaction integrated into concrete production reacts CO 2 with freshly hydrating cement, rather than the hydration phases present in mature concrete, and does not have the same effects. Concrete Production Details Materials and Experimental Methods A trial run was conducted whereby carbon dioxide was integrated into the production of concrete masonry units. The testing used the mix design outlined in Table 1. The medium weight mix design was used to make standard 8 (200 mm) concrete blocks weighing about 17.7 kg. The experiment used a Besser Ultrapac producing four blocks at a time. The mixer was a Besser Rapid Pan Mixer. A carbon dioxide injection system was retrofit to supply carbon dioxide gas into the mixer. Table 1. Block mix design Component Type Target Quantity Sand fine 1960 kg Silica Sand fine 567 kg Gravel coarse 311 kg Cement cement 454 kg The target mix design, expressed as proportions of the solid inputs and assuming SSD aggregates, was 9.4% coarse aggregate, 76.2% fine aggregate, 13.7% cement. The target water, expressed in the same manner, was 7.9%. The cement analysis is presented in Table 2. Rainbloc 80 admixture 1479 ml The mixing cycle started with delivering the aggregates over 30 seconds. At 44 seconds a prewet water addition was made. At 60 seconds the cement was added. The carbon dioxide injection was synchronized with the addition of the cement and lasted 3 minutes. The injection duration was chosen to align with, and avoid extending, the overall mixing process. At 112 seconds an initial water addition was made followed by a final water addition at 160 seconds. In a typical mix, the free water associated with the aggregates accounted for 59% of the total mix water, while the prewet, initial and final water additions were 14%, 5% and 23%

3 Table 2. Cement analysis Metric respectively. Any post-mixing water addition took place after the CO 2 injection was completed. Carbon Dioxide Injection CO 2 gas was delivered by injecting it directly into the mixer. The mixer was effectively enclosed an ensured the injected gas was contained. Sensors monitored locations in the vicinity of the mixer for indications that the mixer had been overfilled (either by an excessive dose or excessive fill rate). No overspill issues were observed in regards to the examined gas dosages. K 2 O (%) Mn 2 O 3 (%) SrO (%) The carbon dioxide used in the experiment was purchased as liquid CO 2 from Air Liquide. Industrially supplied carbon dioxide is typically captured as a byproduct from an industrial process (e.g. ammonia, fertilizer or ethanol production). Consequently, carbon dioxide purchased from an industrial supplier SO 3 (%) 3.88 represents emitted carbon dioxide that has been diverted and BaO (%) 0.05 captured to meet market demand. Any carbon dioxide captured ZnO (%) Cr 2 O 3 (%) in the concrete carbonation process using industrially supplied carbon dioxide can represent a net sequestration benefit. Any carbon dioxide from the carbonation process that is lost to the atmosphere (as opposed to being absorbed by the concrete) is taken to be returning to the destination it had prior to the industrial capture. The use of purchased carbon dioxide does come with an environmental cost given that the gas must be captured, liquefied, and transported. Therefore net sequestration would have to be assessed on a case-by-case basis according to the specific capture scheme and transportation requirements. Analysis and Testing Value Blaine fineness (m²/kg) 501 Free CaO (%) 1.45 CaO (%) Na 2 Oe (%) 0.41 LOI 975 (%) 1.98 SiO 2 (%) Al 2 O 3 (%) 5.47 TiO 2 (%) 0.29 P 2 O 5 (%) 0.13 Fe 2 O 3 (%) 2.23 MgO (%) 2.70 Na 2 O (%) 0.34 The fresh concrete was assessed visually and via feedback from the machine and production personnel. Samples were taken periodically to assess the water content of the concrete and monitor whether the carbon dioxide had any impact on the wetness of the fresh product. Concrete performance was assessed through compressive strength testing at 7, 28 and nominal 56 days at the company s testing lab with 5 blocks for each test condition at each test age. Water absorption and density tests were made at 28 days. Strength, absorption and density tests were performed on full blocks. Blocks were capped for compression testing. The samples for analyzing the carbon dioxide content of the concrete were created by taking a fresh sample from the production line, drying the concrete on a hot plate to remove the water, and subsequently sieving the material through a 160 µm sieve. Samples of the raw materials were examined to determine how much of each component passes a 160 µm sieve and the carbon content of the passing material. This information, along with the concrete mix design, allows for a determination of the cement content in a sieved sample. The total carbon contents of the carbonated samples can then be compared to a control to determine the amount of sequestered CO 2. Two samples were processed for each batch and provided two

4 bakeoff estimates of the mix water content. These samples were then subjected to carbon analysis whereupon they were tested in duplicate. Thus the reported bakeoff values represent an average of two samples. The CO 2 uptake numbers are an average of four readings. Trial Details The trials started by investigating the impact of a carbon dioxide injection on the block production and fresh concrete properties without making any attendant mix or process adjustments. The same mix design was used for bot the control and the carbonated batch and no adjustments to the machine (e.g. compaction time, feed drawer open time) were made. A CO 2 dosage of 1.5% by mass of cement was used (abbreviated bwc or by weight of cement). The operator concluded that the blocks made with the carbonated mix appeared to be dry. A fresh block was weighed and it appeared that the carbonated blocks were underweight. The carbonated mix was concluded to require additional mix water so the procedure was modified to include an assessment of the fresh batch in the mixer. A final fine adjustment of the mix water was made after the normal mixing and CO 2 injection and prior to discharge. A second carbonated second trial batch was produced wherein the mix water was increase to achieve the desired look of the finished block. Fresh concrete analysis considered six control batches averaged to represent a baseline. Strength, density and absorption comparisons were made against a blended control made of two batches contributing 8 strength specimens five from one set and three from the other. Fresh Properties Results and Discussion A summary of the batch conditions and water contents can be found in Table 3. The water is reported in two ways. The mixer final is a sensor reading that monitors the water content of the concrete during mixing. The bakeoff value is associated with the water content determined by drying during the carbonate quantification sample preparation. The temperature of the as-received bakeoff sample is also noted. Code Condition Mix Adjustment Table 3. Block production variables and water contents Mixer final Water Bakeoff Temperature ( C) CO 2 Uptake (%bwc) Control Control % 6.54% % CO % 5.67% % CO 2 More water 8.39% 6.56% It was observed that when the CO 2 was introduced without any mix water adjustment (batch 504) the detectable water in the mix, according to the sensor, decreased by about 12%. The bakeoff value suggested a 13% decline in volatisable water. It is not concluded that the mix

5 water ceased disappeared from the mix, but rather that the CO 2 treatment promoted an increase in the bound water possibly through incorporation the silica-gel reaction products that co-formed alongside the carbonates or through the increase in wetting surface area due to the solid product formation. In the increased water batch (batch 605) the apparent water was increased 8% according to the sensor and was equivalent to the control according to the bakeoff. Taken together these two carbonated mix results suggest that if no modification is applied alongside the carbon dioxide injection then the apparent water content of the carbonated batch will decrease. Further, given that the mix water increase was based upon a visual assessment to achieve a carbonated concrete that looked the same as the control baseline concrete, the required mix water increase, for the mix carbonated with a dose of 1.5% bwc, is on the order of 8%. The temperature reading indicated the exothermic nature of the carbonation reaction. Relative to the baseline control concrete temperature, the non-adjusted carbonated batch increased 3.3 C while water adjusted batch increased 4.6 C. These temperature rises can be taken as indication of the reaction given that the CO 2 uptake for the latter batch was higher than for the former. The efficiency of the uptake in the first trial was 70% while the efficiency of the second carbonated mix was 93%. Block density and absorption The product density and absorption was measured at 28 days. The presented data comprises six control blocks from two different runs, as compared to three blocks for each of the carbonated trial mixes. The data is presented to show the average and spread of the collected data. The density data is presented in Figure 1 while the absorption data is in Figure Density (kg/m 3 ) Absorption (kg/m 3 ) Control CO2 504 CO2 605 Condition 100 Control CO2 504 CO2 605 Condition Figure 1. Density (kg/m 3 ) of three test conditions

6 Figure 2. Absorption (kg/m 3 ) of three test conditions The density mirrors the water content observations. The application of the carbon dioxide without making any mix design or process tweak produced a block that had a significantly lower density. The carbonated block was 5.0% less dense than the control. The observation that the concrete appeared to be drier paralleled the density conclusion indicating that it was more difficult to effectively compact the carbonated material. The water adjustment successfully allowed a block to be produced with a density equivalent to the control. Beyond water adjustments it is expected the machine could be adjusted to improve the compaction of the blocks, but this was not the main focus of the trial. The water absorption analysis showed a benefit for the carbonation in both cases. Batch 504 (no extra water) had a water absorption 11.3% less than the control while the adjustment in batch 605 decreased the absorption by 17.8%. It is interesting that the lower density product had a lower absorption than the control given that it had not been compacted as well. Compressive Strength The compressive strength test results at 7, 28 and 56 days are presented in Figure 3. The strength in MPa is presented with bars labeled with their relative strength as compared to the control at the same age. 35 Compressive Strength (MPa) % 100% 119% 100% 118% 76% 68% 62% 7 day 28 day Control CO2 504 CO % 56 day Condition Figure 3. Average compressive strength (MPa) for each condition at three test ages The strengths of the carbonated product in Batch 504 were reduced, as would be expected based upon the density being 5% lower than the control. At 7 days the strength was 62% of the control, but it had improved to 76% and 19.1 MPa at 56 days. While the blocks were weaker than the control they did exceed the strength limit (13.1 MPa) established in ASTM C- 90 Standard Specification for Loadbearing Concrete Masonry Units for medium weight products.

7 Table 4. Emissions impact summary Metric Value Block mass (kg) 17.7 Cement fraction 12.9% Cement per block (g) 2282 Clinker per block (g) 2145 Clinker CO 2 emissions per block (g) 1858 CO 2 dose per block (g) 34.2 Emissions reduction The water increase used in Batch 605 with the resulted in block strengths there were better than the control at all ages. It was a consistent 18 to 19% higher than the control at each test interval. Recalling that the density of batch 605 was functionally equivalent to that of the control it is clear that the strength benefit can be attributed to the carbonation process. It is possible that the finely distributed carbonation products have improved the cementitious matrix. It is also possible that the side effect of the carbonation allowing for 8% higher mix water content allowed for greater hydration in the carbonated batch. The carbon dioxide absorbed into the concrete can effectively reduce the embodied carbon emissions. An emission impact summary, presented in Table 4, outlines, on a per block basis, a comparison of the carbon dioxide absorbed in contrast to the CO 2 emitted due to the production of the cement. If the block mass and mix design are known, then the total emissions related to the cement can be determined. The 17.7 kg block is found to be 12.9% (by wet mass) cement and thus there are 2282 g of cement in each block. The cement was suggested by the supplier to be 94% clinker. If the emissions intensity of the clinker is assumed to be a generic 866 kg CO 2 e/tonne of clinker produced [Barcelo 2014] then the clinker emissions for each block reach 1858 g. A generic carbonation uptake scenario can allow for an overall carbon dioxide absorption and net emissions offset to be calculated. Further testing with the 1.5% dose collected additional data that revealed additional uptake efficiencies of 95%, 95%, 83%, 79% and 79%. Thus, along with the 93% uptake presented earlier, an overall average efficiency can be taken to be 88%. A 1.5% dose by weight of cement means that 34.2 g of CO 2 are dosed per block while the uptake efficiency means that 30.1 g are bound as stable carbonate reaction products in the block. The difference is a loss representing the 12% inefficiency.

8 Dose efficiency 88% Uptake per unit (g) 30.1 Direct emissions offset through uptake 1.62% Under these assumptions, the absorbed amount of carbon dioxide represents a direct offset of about 1.62% of the emissions from the clinker production. A net sequestration consideration requires a detailed analysis including the emissions required to implement the technology. A reasonable estimate can be made by considering the energy to capture and compress the CO 2 and the distance the CO 2 had to be transported. Additional factors are relevant (such as the creation and transport of the hardware for the technology) but are considered minor in the face of the gas-related aspects. A summary is presented in Table 5. Table 5. Net environmental impact summary Factor Amount Per 1000 blocks CO 2 dosed (g) 34,250 CO 2 absorbed (g) 30,140 CO 2 e gas processing (g) 3,522 CO 2 e gas transport (g) 479 Total CO 2 emissions (kg) 4001 Net CO 2 utilised (kg) 26,139 The closest industrial CO 2 source to the trial site is 63 miles away in Whiting, Indiana. The transportation emissions are taken to be 222 g CO 2 /tonne mile of freight [United States Environmental Protection Agency 2011]. The grid emissions associated with electrical power produced in Indiana are lb CO 2 e/mwh (685.6 g CO 2 /kwh) [United States Environmental Protection Agency 2014]. The energy required to capture carbon dioxide is on the order of 150 kwh/tonne. Net efficacy, % 86.7 For 1000 blocks a 1.5% bwc dose would inject 34,250 g of CO 2. The total absorption would be 30,140 g of CO 2. The gas processing and transport is calculated with respect to the total injected amount. The carbon dioxide emissions associated with the energy required to capture and compress the CO 2 is 3,522 g. The transportation of the liquid CO 2 63 miles results in carbon dioxide emissions of 479 g. No energy is required to vaporize the liquid CO 2 at the concrete plant if an atmospheric vaporizer is employed. The technology to carbonate 1000 blocks would result CO 2 emissions of 4,001 g. The net utilization, 26,139 g, is then the difference between the total carbon dioxide absorbed and total process emissions. This means that 13.3% of the absorbed CO 2 cannot be associated with an environmental benefit due to the associated emissions. However, it suggests that the net efficacy of the CO 2 utilization is 86.7%. A sensitivity analysis can suggest how location-specific inputs can affect the sequestration efficacy. Certain locations are further from sources of industrial CO 2 than the present case. If the liquid CO 2 transport distance was increased to 600 miles (reasonable in markets where industrial gases are shipped in from distant areas) then the increase in transport emissions reduces the estimated efficacy to 73.2%. However, if the distance was kept at 63 miles then the effect of grid emissions on carbon dioxide processing emissions can be examined. An example of low grid emissions can be found in New York where parts of the state see lb CO 2 e/mwh (248.7 g CO 2 e/kwh). The reduced gas processing emissions would increase the sequestration efficacy to 94.2%.

9 An example of very low grid emissions can be found in Quebec at 5.1 lb CO 2 e/mwh (2.3 g CO 2 e/kwh) [Environment Canada 2014] where the efficacy would reach 98.4%. On the other hand, Colorado has high grid emissions at lb CO 2 e/mwh (864.7 g CO 2 /kwh). The efficacy would decrease to 83.7%. These estimates are consistent with a previously examined case [Monkman 2010]. On the order of 80-90% of the carbon dioxide absorbed by the concrete would represent a net removal of CO 2 from the atmosphere while the balance would be offset by the emissions required to employ the technology. Next steps Laboratory scale testing has shown that observed material performance results are sensitive to mix design and cement type. A deeper understanding of the variation associated with the material aspects is required to suitably predict the potential technology performance. Although the trial examined water increases as a way to compensate for reduced product mass it is acknowledged that adjusting the machine settings is a potential alternate or complementary strategy that would be evident during an extended run of the technology. The overall sequestration benefit, on a per unit basis, would increase if a higher uptake was achieved. Additional trials would uncover if the dose and uptake could be increased and thereby improve the overall environmental impact as considered by an overall amount of CO 2 utilized. Finally, material science impacts that contribute to the performance improvement require further investigation. The role that the carbonates play in the strength and absorption benefits is intriguing. Conclusions Concrete blocks were produced using upcycled carbon dioxide. The gas was injected during the mixing stage whereupon it formed stable carbonate reaction products. A small temperature increase was associated with the carbonation. The treated concrete appeared drier and was harder to compact. A mix water increase of 8% produced a block with the desired appearance and density. The carbonated blocks with the increased water were shown to be 19% stronger and have a water absorption 18% lower than the control. The CO 2 absorbed in one block represents a direct offset of about 1.62% of the emissions from the cement therein. The emissions associated with undertaking the technology (namely the gas processing and gas transport) suggest that the 86.7% of the CO 2 absorbed in the process can represent a net environmental benefit. Acknowledgements

10 The authors wish to recognize the funding support from Sustainable Development Technology Canada (SDTC) and the National Research Council of Canada s Industrial Research Assistance Program (IRAP). Further, the support of the industrial host was vital to the success of the research and is greatly appreciated.

11 References Barcelo 2014: Barcelo, L., J. Kline, G. Walenta, E. Gartner. "Cement and carbon emissions". Materials and Structures, 47(6), 2014 pp Berger 1972: Berger, R. L., J. F. Young, K. Leung. "Acceleration of Hydration of Calcium Silicates by Carbon-Dioxide Treatment". Nature: Physical Science, 240(Dec 8), 1972 pp Cement Sustainability Initiative 2009: Cement Sustainability Initiative. Cement Industry Energy and CO2 Performance - Getting The Numbers Right. World Business Council for Sustainable Development, Damtoft 2008: Damtoft, J. S., J. Lukasik, D. Herfort, D. Sorrentino, E. M. Gartner. "Sustainable development and climate change initiatives". Special Issue The 12th International Congress on the Chemistry of Cement. Montreal, Canada, July , 38(2), 2008 pp Environment Canada 2014: Environment Canada. Greenhouse Gas Division, Environment Canada, National Inventory Report, : Greenhouse Gas Sources and Sinks in Canada. Ottawa, Goodbrake 1979: Goodbrake, C. J., J. F. Young, R. L. Berger. "Reaction of Beta- Dicalcium Silicate and Tricalcium Silicate with Carbon Dioxide and Water Vapor". Journal of the American Ceramic Society, 62(3-4), 1979 pp Kashef-Haghighi 2015: Kashef-Haghighi, S., Y. Shao, S. Ghoshal. "Mathematical modeling of CO2 uptake by concrete during accelerated carbonation curing". Cement and Concrete Research, 2015 doi: /j.cemconres Monkman 2010: Monkman, S., Y. Shao. "Integration of carbon sequestration into curing process of precast concrete". Canadian Journal of Civil Engineering, 37(2), 2010 pp Shao 2013: Shao, Y., V. Rostami, Z. He, A. Boyd. "Accelerated Carbonation of Portland Limestone Cement". Journal of Materials in Civil Engineering, 26(1), 2013 pp Shi 2012: Shi, C., F. He, Y. Wu. "Effect of pre-conditioning on CO2 curing of lightweight concrete blocks mixtures". Construction and Building Materials, 26(1), 2012 pp United States Environmental Protection Agency 2011: United States Environmental Protection Agency. "EPA and NHTSA Adopt First-Ever Program to Reduce Greenhouse Gas Emissions and Improve Fuel Efficiency of Medium- and Heavy- Duty Vehicles". Office of Transportation and Air Quality, August United States Environmental Protection Agency 2014: United States Environmental Protection Agency. "egrid 9th edition Version 1.0 Year 2010 Summary Tables". In Emissions & Generation Resource Integrated Database (egrid) Young 1974: Young, J. F., R. L. Berger, J. Breese. "Accelerated Curing of Compacted Calcium Silicate Mortars on Exposure to CO2". Journal of the American Ceramic Society, 57(9), 1974 pp

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