Carbon capture and utilization for concrete production (CCU concrete) is estimated to sequester 0.1 to 1.4 gigatons of carbon dioxide (CO2) by 2050. However, existing estimates do not account for the CO2 impact from the capture, transport and utilization of CO2, change in compressive strength in CCU concrete and uncertainty and variability in CCU concrete production processes. By accounting for these factors, we determine the net CO2 benefit when CCU concrete produced from CO2 curing and mixing substitutes for conventional concrete. The results demonstrate a higher likelihood of the net CO2 benefit of CCU concrete being negative i.e. there is a net increase in CO2 in 56 to 68 of 99 published experimental datasets depending on the CO2 source. Ensuring an increase in compressive strength from CO2 curing and mixing and decreasing the electricity used in CO2 curing are promising strategies to increase the net CO2 benefit from CCU concrete.
The capture and utilization of carbon dioxide (CO2) to produce economically viable products offers the twin benefit of mitigating climate change and generating economically viable products1. Among the portfolio of products that can potentially utilize CO2, concrete offers several advantages including (i) a thermodynamically favorable reaction mechanism2,3 to sequester the CO2 as calcium or magnesium carbonates4,5 in ordinary Portland cement (OPC)6,7 (ii) the long term CO2 sequestration in the form of stable carbonate beyond the life-time of the infrastructure (>60 years) (iii) the significant sequestration potential due to the overall and expected growth in global cement production to meet increasing demand8. Concrete along with aggregates and chemicals/fuels are end-products with the potential to sequester the maximum quantity of CO2 (in gigatons)1,3.
Multiple emerging approaches such as carbonation of recycled concrete aggregates9, CO2 sequestration in alternative MgO based binders10, CO2 mineralization in industrial waste-derived aggregates and fillers11,12, and CO2 dissolution in mixing water13,14 have been investigated for CO2 utilization in concrete. However, this study focuses on the two approaches of CO2 mixing and CO2 curing as they are more extensively analyzed and applied for CO2 utilization in concrete (Supplementary information (SI) Section 2). In CO2 mixing, high-purity CO2 is injected into fresh concrete during batching and mixing. The CO2 binds to the calcium silicate clinker in OPC to form nano-scale CaCO3 particles15,16. In CO2 curing, CO2 is utilized as a curing agent5 to accelerate precast concrete fabrication. A review of CO2 curing and mixing studies reveals that the CO2 uptake potential in CO2 curing of precast concrete applications is significantly higher than in CO2 mixing (Supplementary Fig. 6).
A common assumption motivating research and commercial interests in CCU concrete is that the CO2 uptake during curing and mixing17,18,19,20 of CCU concrete lowers the CO2 burden of concrete production. Estimates show that 0.1–1.4 gigatons of CO2 can be utilized in concrete by 20501,3. However, a literature review (SI Section 13) demonstrates these estimates are not based on a comprehensive assessment that accounts for the change in compressive strength of concrete from CO2 utilization; the CO2 impact of capturing, transporting and utilizing CO2; the CO2 emissions from compensating for the energy penalty of CO2 capture and producing supplementary cementitious materials (SCM), which are by-products of coal electricity and pig iron production; the uncertainty and variability in inventory data and process parameters; and may not always be based on primary experimental data, which is required for a robust life cycle CO2 assessment.
CO2 curing can decrease the compressive strength of CCU concrete when compared to conventional concrete. For example, a review of 99 experimental datasets from existing literature shows that CCU concrete has a lower compressive strength than conventional concrete in 31 datasets (SI Section 2 Supplementary Fig. 3). In such cases, CCU concrete would require a greater amount of OPC than conventional concrete to produce the same compressive strength. OPC production is a major source of CO2 emissions. Therefore, increased OPC content in a concrete formulation leads to an increase in CO2 emissions from upstream cement production processes, which may outweigh the benefit of the CO2 captured and used in concrete production.
In addition, the CO2 impact of CCU concrete can be difficult to generalize due to the lack of consistency in the boundaries and scope of analysis. For example, the energy associated with the capture and transport of CO2 is included in certain studies21 while being excluded from others20,22,23. Moreover, the uncertainty and variability in data and process parameters, which is typical in the early stages of R&D, impacts the environmental assessment of emerging technologies such as CCU concrete24,25,26,27,28. Life cycle assessments (LCA) of CCU concrete rely on point values for process parameters rather than parameter distributions that provide a more realistic representation of uncertainty and variability16,21. The failure to account for uncertainty in the early stages of technology development can hinder research efforts to address hotspots and increase the CO2 benefit from CCU concrete25,26. An uncertainty assessment in the early stages of technology development can determine process parameters and inventory items that are the most significant contributors to the CO2 burden of CCU concrete and, thereby, help identify research strategies that are most effective in addressing the hotspots.
To address these issues, we review 99 datasets from 19 publications to determine the range of potential net CO2 benefit associated with CCU concrete. The net CO2 benefit is defined as the difference between the lifecycle CO2 impact of producing conventional concrete and producing CCU concrete though CO2 curing or CO2 mixing. The net CO2 benefit accounts for the life cycle CO2 impact of the 13 upstream processes to capture, transport and utilize CO2 and produce and transport the materials used in concrete. The net CO2 benefit accounts for any changes in compressive strength when CCU concrete is produced though CO2 curing or CO2 mixing. We conduct a sensitivity analysis consisting of a scatter plot analysis and moment independent sensitivity analysis25,29,30 to determine the key processes with the most significant influence on the net CO2 benefit.
Results and discussion
Illustrative example: net CO2 benefit for dataset 1
We illustrate the interpretation of the results using a single dataset, which helps better understand the findings across the 99 datasets. Consider the dataset of conventional and CCU concrete production in the A1 scenario reported in ref. 31 wherein CO2 is used for curing and OPC is used as binder. Based on the inventory requirements presented in the dataset, the CO2 emissions for the 13 processes (Fig. 1 and Supplementary Table 1), the total life cycle CO2 emissions from producing CCU concrete (TOTCCU, Eq. 1) and conventional concrete (TOTConv, Eq. 5), and the net CO2 benefit (Eq. 6) are stochastically determined in 10,000 Monte Carlo runs.
The distribution in Fig. 2a quantifies the likelihood of the net CO2 benefit in dataset 1 being positive (right of the y-axis) in 10,000 Monte Carlo runs. If the net CO2 benefit is positive, then the total life cycle CO2 emissions from producing CCU concrete (TOTCCU) is lower than conventional concrete (TOTConv). For dataset 1 the likelihood is 0%, which signifies that TOTCCU is greater than TOTConv in all of the 10,000 Monte Carlo runs. Alternately, the distribution for the net CO2 benefit is always negative which indicates that, on a life cycle basis, producing CCU concrete is more CO2 intensive than conventional concrete in the 10,000 Monte Carlo runs.
The scatter plot analysis of the 10,000 Monte Carlo runs (Fig. 2b) determines the key drivers in the relationship between the difference in the CO2 emissions from the 13 conventional concrete (PConv) and CCU concrete (PCCU) processes and the net CO2 benefit. The scatter plot shows the 10,000 net CO2 benefit values on the ordinate and the 10,000 values of the difference between the CO2 emissions from the 13 processes on the abscissa. A visual inspection of the scatter plot reveals a difference in the slopes of the scattered points. A higher slope indicates higher sensitivity of the net CO2 benefit to the contributing process. The scatter plot for dataset 1 (Fig. 2b) demonstrates that the difference in the CO2 emissions from cement used (P1:OPC) has a significant slope. As the difference in CO2 emissions from OPC production are scattered in the lower left quadrant, the CO2 emissions from OPC production for CCU concrete is greater than conventional concrete. Therefore, for dataset 1, the difference between the CO2 emissions from OPC production is the most important reason for TOTCCU being greater than TOTConv (i.e., the net CO2 benefit being negative).
This finding is confirmed by a moment independent sensitivity analysis25,29,30, which determines δ indices for each of the 13 processes contributing to the net CO2 benefit (Fig. 2c). The δ index is a measure of the contribution from the difference in the CO2 emissions from a conventional and CCU concrete production process to the probability distribution function of the net CO2 benefit. The process with a greater δ index value has a greater contribution to the net CO2 benefit than a parameter with a lower δ index value. By convention, if the δ value is negative then the CO2 emissions from the process are greater in CCU concrete production than in conventional concrete production. The difference in the CO2 emissions from OPC production has the highest δ value and is, therefore, the most significant contributor to the net CO2 benefit. The negative value indicates that the CO2 emissions from OPC production are greater in CCU concrete than in conventional concrete. The finding from the sensitivity analysis for dataset 1 can be attributed to the compressive strength of CCU concrete (16–17.4 MPa) being lower than conventional concrete (18–18.6 MPa). The mean compressive strength of CCU concrete (16.7 MPa) is 9% lower than that of conventional concrete (18.3 MPa). This implies that in dataset 1 a greater mass of OPC is produced for CCU concrete to achieve the same compressive strength as conventional concrete. In dataset 1, the OPC produced per MPa for CCU concrete is 24.8 kg/MPa and for conventional concrete is 22.6 kg/MPa.
Using a mean value of 0.948 kg CO2/kg OPC for the life cycle CO2 footprint of OPC (Supplementary Table 2), the CO2 emissions from OPC production for CCU concrete is 23.5 kg CO2/MPa and for conventional concrete is 21.4 kg CO2/MPa. Therefore, the difference between the CO2 emissions from OPC production for CCU and conventional concrete is 2.1 kg CO2/MPa, which is greater than the CO2 utilized in CCU concrete (1.1 kg CO2/MPa, SI Section 1). As a result, the life cycle CO2 emissions from the increased OPC production in CCU concrete is greater than the CO2 utilized for curing the CCU concrete.
Increased binder use undermines CO2 benefit of CCU concrete
We extend the analysis conducted for dataset 1 to the remaining 98 datasets (Fig. 3). The 99 datasets are divided into 4 categories. Category 1 has 50 datasets wherein CO2 is used for curing and the binder consists of only OPC. Category 2 has 20 datasets wherein CO2 is used for curing and the binder consists of a mix of OPC and supplementary cementitious materials (SCM). Category 3 has 8 datasets wherein CO2 is used for mixing and the binder consists of only OPC. Category 4 has 21 datasets wherein CO2 is used for mixing and the binder consists of a mix of OPC and SCM.
A visual inspection of the slopes in the scatter plot in Fig. 4 and the δ indices in Fig. 5 reveal that the net CO2 benefit is most sensitive to the amount of OPC produced and used in the design mix (e.g., P1 in datasets 1, 2, 3, and 4 in Fig. 5), the energy used for CO2 curing (e.g., P12 in datasets 21, 22, and 23 in Fig. 5) and SCM produced and used (e.g., P5 in datasets 51, 52, 53, and 54 in Fig. 5).
The plots with the red background in Figs. 3, 4, and 5 demonstrate that the net CO2 benefit is negative (i.e., CCU concrete has higher life cycle CO2 emissions than conventional concrete) with at least a 50% likelihood in 56 out of the 99 datasets. The compressive strength in CCU concrete decreases due to CO2 curing when compared to conventionally cured concrete. The OPC and SCM consumed to produce the same compressive strength is greater in CCU concrete than in conventional concrete. Therefore, the results demonstrate the CO2 burden of increased OPC and SCM consumption for CCU concrete outweighs the benefit of the CO2 that is captured and used in CCU curing. Additionally, in category 1 datasets, the electricity use in the CO2 curing process is the second key contributor to the increase in the total life cycle CO2 emissions from CCU concrete (e.g., datasets 46, 47, and 48 in Fig. 5).
The results in Fig. 5 also demonstrate that the life cycle CO2 emissions from capturing, compressing, transporting and vaporizing CO2, and the CO2 emissions from producing fine aggregate, coarse aggregate, water and steam curing are not significant contributors to the net CO2 benefit. This can be attributed to the mass of CO2 utilized in concrete being lower than the mass of the cement and coarse and fine aggregate (Supplementary Fig. 1) and the life cycle CO2 intensity of coarse and fine aggregate being significantly lower than cement (Supplementary Table 2).
To investigate the change in results when CO2 intensity of OPC production decreases, we determine the net CO2 benefit of CCU concrete when CO2 is captured from a cement plant (SI Section 11). The results show that CCU concrete has higher life cycle CO2 emissions than conventional concrete (i.e., negative CO2 benefit) in 44 out of the 99 datasets (Supplementary Fig. S4 in SI Section 11) when compared to 56 out of the 99 datasets in the baseline scenario (Fig. 5). Therefore, when CO2 is captured from a cement plant, there is a lower likelihood of CCU concrete producing a negative net CO2 benefit than when CO2 is captured from a power plant. The difference in the results can be attributed to the reduced CO2 intensity of cement production due to CO2 capture at the cement plant.
Results across CO2 use, binder type, and allocation method
Fig. 6 summarizes the results depending on whether CO2 is used for curing or mixing, SCM is used as a binder material or not, and the type of allocation method used to determine the CO2 emissions from producing the SCM material. Two types of SCMs are used in concrete production—ground granulated blast furnace slag and fly ash. Slag is a by-product of pig-iron production and fly ash is a by-product of electricity generation in coal power plants. As a result, we need a method to allocate total CO2 emissions between slag and pig-iron and between fly-ash and coal electricity. We use three methods—system expansion (SE), mass-based (MA), and economic value-based allocation (EA)—to allocate and account for the life cycle CO2 emissions from producing the SCMs (process P5, SI Sections 4, 5, and 6).
The overall results in Fig. 6 demonstrate that in 36 (EA in “Overall”) to 43 (SE in “Overall”) of the 99 datasets reported in the literature, CCU concrete production has a lower life cycle CO2 emission than conventional concrete production. In these cases, CCU concrete substituting conventional concrete lowers CO2 emissions. Negative CO2 net benefit values are obtained in the remaining 56 to 63 datasets. A similar analysis for CO2 capture from a NGCC power plant shows that the net CO2 benefit is negative in 61, 65, and 68 of the 99 datasets when SE, MA, and EA are used, respectively (SI Section 10). The overall results demonstrate that the CO2 benefit of CCU concrete production is negative in 56–68 of the 99 datasets depending on whether CO2 is captured from a coal or a NGCC power plant and when SE, MA or EA is used. As a result, there is a higher likelihood of the net CO2 benefit of CCU concrete being negative. Consequently, not all CCU concretes can help realize sequestering 0.1 to 1.4 gigatons/year of CO2 in concrete by 20501,3.
The results of the sensitivity analysis are similar across categories 1, 2, 3, and 4 as the production and use of binder material (OPC in categories 1 and 3 and OPC + SCM in Categories 2 and 4) has the most significant influence on the net CO2 benefit.
The overall results on the number of datasets in which there is a positive net CO2 benefit (i.e., CCU is less CO2 intensive than conventional concrete) is not significantly impacted by the choice of system expansion SE, MA or EA. CCU is less CO2 intensive than conventional concrete in a minimum of 36 datasets when EA is used and a maximum of 43 datasets when SE is used. However, for category 2 and 4 datasets, the choice of SE, MA or EA impacts the results from the δ indices sensitivity analysis. When SE or MA is used in category 2 datasets, SCM use is a key contributor to the net CO2 benefit of CCU concrete. When EA is used in category 2 datasets, the CO2 emissions from the CO2 curing process is the key contributor. When SE is used in category 4 datasets, SCM use is a key contributor to the difference in the total CO2 emissions between conventional and CCU concrete. When EA or MA is used in category 4 datasets, OPC use is the key contributor. However, the choice of SE, MA or EA is an artifact of the method of analysis—not actually changing real CO2 emissions—and, therefore, is not within the purview of engineering strategies to improve processes and decrease the CO2 emissions from producing CCU concrete.
The findings from this analysis are based on the design mixes, material usage, compressive strength, and parameters such as the CO2 curing duration, water to cement and SCM to cement ratios obtained from the 99 datasets (SI Section 2 Supplementary Figs. 1 to 5). It is important to note that the findings do not preclude future research (as discussed below) from optimizing the design mixes, curing processes and material properties to increase the net CO2 benefit from CCU concrete.
Strategies to improve the net CO2 benefit of CCU concrete
An R&D agenda focused on the following items, which are within the control of the CCU concrete production process, can increase the net CO2 benefit.
Ensure increase in compressive strength from CO2 curing: a key priority is to determine a CO2 curing protocol that consistently increases the compressive strength of CCU concrete. An increase in compressive strength implies that a smaller quantity of carbon intensive binder material is used in CCU concrete to achieve the same compressive strength as conventional concrete (i.e., lower quantity of OPC or SCM is consumed on a kg per MPa basis). Fine tuning the curing process such as duration of the pre-hydration and post-carbonation water compensation are promising candidates to restore the reduction in 28-day compressive strength observed for CO2-cured concrete32,33,34. For example, a longer duration of pre-hydration may enhance 28-day compressive strength but decreases CO2 uptake at early age. Further investigations are needed for enhancing consistency of CO2-cured concrete production and for implementing the laboratory strategies in field applications.
Decrease the CO2 emissions from the CO2 curing process: electricity use, which is the key contributor to the CO2 emitted during the CO2 curing process, can be lowered by streamlining the curing process. Future research can investigate and standardize promising options, such as natural drying or waste heat drying for pre-curing of CO2-cured concrete35 with the end goal of accelerating adoption in industry.
Improve understanding on the impact of CO2 curing on durability: the findings of this research are based on the compressive strength property of CCU concrete, which is limited from a lifecycle perspective. Prior studies show that construction and repair frequencies are key drivers in determining concrete life cycle CO2 impacts36,37. Therefore, the effect of CCU on concrete durability must be considered when analyzing life cycle CO2 emissions. Preliminary lab scale studies demonstrate that CO2 curing improves durability related parameters such as permeability, sorptivity and sulfate and acid resistance22,38,39,40. However, the variability in the curing conditions and the design mixes used in the studies should be accounted for to ensure that durability gains can be consistently realized when CO2 curing of concrete is adopted at a commercial scale. Future work can prioritize standardizing the CO2 curing protocol (e.g., the steam curing time, pre-hydration time, post-hydration time), and study the resulting durability impact on different design mixes (e.g., use of different SCMs), with the overall goal of identifying optimal curing conditions and design mixes to maximize durability. This applies to ready-mix concrete and general precast applications with end-products such as masonry units32,35, pipes22, and pavers41. In addition, CO2 curing for reinforced concrete needs further investigation due to the possibility of increased risk of steel reinforcement corrosion led by concrete carbonation. Moreover, CCU can be potentially synergized with established strategies for concrete crack width control, e.g., engineered cementitious composites with microfiber reinforcement, to further promoting concrete durability42,43.
The system boundary (Fig. 1) assumes that the CO2 captured from the power plant is used for CCU concrete production without any intermediate storage. In practice, the total CO2 captured from a power plant may be significantly greater than the maximum utilization capacity at a CCU concrete production plant. In such cases, the excess captured CO2 may be temporarily stored for future utilization in CCU concrete production or routed towards other utilization pathways. Given that CO2 utilization is an emerging field and in the early stages of commercialization, there is a lack of time-sensitive data on how the captured CO2 feedstock is either temporarily stored or immediately allocated to other utilization pathways. As a result, a system boundary that incorporates the time-sensitive utilization of CO2 captured from a power plant is beyond the scope of this work and is a topic for future research.
The transport of CO2 through pipelines has a lower CO2 impact than road-based transport using semi-trailer trucks44, which is modeled in this analysis. To quantify the maximum possible gains from shifting to a less carbon intensive mode of CO2 transport, we conduct a scenario analysis with the optimistic assumption that the CO2 impact of CO2 transportation is zero (SI Section 12). Despite this optimistic assumption of zero-carbon CO2 transport, CCU concrete has a lower CO2 impact than conventional concrete in 44 of the 99 datasets, which is similar to the 43 of the 99 datasets obtained in the baseline scenario (Supplementary Fig. 10 versus Fig. 5). As a result, a shift from road to pipeline based CO2 transport will not impact the findings from this analysis.
This analysis focusses on the use of pure CO2 and two approaches of CO2 utilization—curing and mixing—as they are more extensively investigated (e.g., 99 datasets used in this study) than alternate approaches such as concrete curing with flue gas21,45,46,47, carbonation of recycled concrete aggregates9, CO2 sequestration in alternative MgO based binders10, and CO2 dissolution in mixing water13,14. The increased availability of experimental data is necessary to robustly quantify the net CO2 benefit of CCU concrete and account for the impact of data uncertainty and process variability on the results. For example, further experimental research can generate data on the variability in the compressive strength of flue-gas cured concrete properties when the 2-week curing time is reduced47 and the CO2 concentration in the flue gas is varied45. With increased availability of inventory and process data from future experimental research, the life cycle approach presented in this study can be extended to quantify the net CO2 benefit of CCU concrete produced from flue gas and other alternate approaches of CO2 utilization.
The impact of CO2 curing or CO2 mixing on the lifetime behaviors and geography specific factors are important practical considerations when using CCU concrete in commercial applications. The lifetime behaviors are impacted by the variations in the geographical sources of raw materials, mix type, product type and the service environment in which the concrete is deployed. For example, the impact of CO2 curing on concrete lifespan would differ substantially between concrete with and without steel reinforcement, due to the heightened steel corrosion48 caused by the CO2-induced pH reduction49. Additionally, geography specific factors such as sulfate-rich soils50, cold regions39, or acidic environments22 can impact the durability of CCU concrete. The findings of this study can be further complemented by future research which quantifies the impact of variations in the lifetime and the geography specific factors on the net CO2 benefit of CCU concrete.
Literature review to categorize CO2 use in concrete
We conducted a literature review to obtain the 99 datasets from 19 studies presenting life cycle material and energy inventory data and process parameters for the production of CCU and conventional concrete. The literature review identified the 19 studies16,19,22,23,31,32,33,35,38,40,51,52,53,54,55,56,57,58,59 as they were the only ones to report the following three items (i) the design mix consisting of the energy and material inventory required for the production of conventional and CCU concrete (SI Section 2). The energy and material inventory are required to determine the life cycle CO2 impact of producing conventional and CCU concrete; (ii) the quantity of CO2 used in mixing or curing of concrete. This is required to determine the life cycle CO2 impact of capturing, transporting, and utilizing the CO2 used in producing CCU concrete; and (iii) the compressive strength of CCU and conventional concrete at the end of 28 days, which helps account for the change in the material property between conventional and CCU concrete. The 28-day compressive strength is among the most widely used technical parameters for assessing concrete quality, categorizing concrete mix designs60 and forms the basis for concrete structural design61,62 and is, therefore, chosen as the functional property based on which conventional and CCU concrete are compared. Based on whether CO2 is used in CCU concrete for curing or mixing and if SCM was used in the design mix, the 99 datasets were organized into four categories.
SCM took the form of either ground granulated blast furnace slag, which is a by-product of the pig-iron production63, or fly ash, which is a by-product of electricity generation in coal power plants.
The use of CO2 during mixing or curing changes the compressive strength of CCU concrete when compared to concrete produced through conventional mixing or curing. In addition, an energy penalty (Ep kWh) is incurred for CCU concrete in power plants due to the energy associated with capturing the CO2, which is used in the curing or mixing of CCU concrete (φCCU, kg CO2). Ep is not incurred when conventional concrete is produced since there is no CO2 capture. Therefore, the net CO2 benefit of substituting CCU concrete for conventional concrete should account for the CO2 impact from the change in the compressive strength and Ep, which is incurred in power plants only when CO2 is captured.
As a result, we use a functional unit of concrete with 1 MPa compressive strength and 1 m3 of volume and Ep kWh of electricity.
The functional unit accounts for the change in compressive strength and ensures consistency by normalizing the materials and energy consumed for producing 1 m3 of CCU and conventional concrete to 1 MPa of compressive strength. The inclusion of Ep kWh of electricity in the functional unit accounts for the difference in CO2 emissions from electricity generation without CO2 capture in the conventional concrete pathway and with CO2 capture in the CCU concrete pathway. Ep is determined based on the mass of CO2 captured from the power plant (Supplementary Table 1 Process 8).
CCU concrete production—system boundary and CO2 emissions
The literature review revealed that the total life cycle CO2 emissions from producing CCU concrete is the sum of the CO2 emissions from 13 key processes required to capture, transport, and utilize CO2 and produce the materials required in the design mix of concrete (Fig. 1).
The expression used to determine the total life cycle CO2 emissions from producing CCU concrete based on the CO2 emissions from the 13 processes is presented in Eq. 1. The 13 expressions within parenthesis in Eq. 1 correspond to the CO2 emissions from the 13 processes (Fig. 1).
Process 1 to 4—Ordinary Portland cement (C), coarse aggregate (CA), fine aggregate (FA), and water (W) production: The CO2 impact is the product of (i) the life cycle CO2 emissions from producing the material (φC, φFA, φCA and φW in kg CO2/kg material) and (ii) and the mass of material used in the design mix normalized to the compressive strength of CCU concrete (CCCU, CACCU, FACCU and WCCU in kg material/MPa/m3). The material used and the compressive strength are obtained from the literature review (SI Section 2) and φC, φFA, φCA, and φW are obtained from the ecoinvent database (Supplementary Table 2).
Process 5—SCM production: SCMCCU represents the mass of SCM used in the design mix normalized to the compressive strength of CCU concrete (in kg material/MPa/m3).
Slag and fly ash, which are co-products of iron-ore production and electricity generation from coal, are used as SCM in the design mix of concrete. Three methods—system expansion (SE), economic value-based allocation (EA) and mass-based allocation (MA)—are widely used in LCA to determine the CO2 emissions of co-products being generated by a single system.
In SE, the CO2 emissions from producing a required mass of slag is determined by expanding the system to include the production of a corresponding mass of iron-ore (based on a ratio of iron-ore to slag, SI Section 4). In the case of MA and EA, the total CO2 emission from a process producing both iron-ore and slag is allocated between iron-ore and slag based on the mass and economic value of the co-products, respectively (SI Sections 5 and 6). To explore variability in the CO2 emissions from CCU concrete production based on the allocation method, this analysis applies the three methods when determining the CO2 emissions for slag and fly ash.
The CO2 impact of slag (φSCM_slag in kg CO2/kg slag) is determined from Eq. 2
φIO is the life cycle CO2 emissions from producing 1 kg of iron ore and is 2.2 kg CO2/kg iron ore (SI Section 4).
When fly ash is used as the SCM, the CO2 impact per kg of fly ash (φSCM_ash in kg CO2/kg fly ash) is determined from Eq. 3
The value of Allocash is 1, 0.02 or 0.06 when SE, MA or EA is chosen, respectively (SI Sections 4, 5 and 6). φElec_Coal, which is the life cycle CO2 emission from producing 1 kWh of coal electricity, is 1.25 kg CO2/kWh (SI Section 4). αCap is 0.1 if CO2 is captured at a coal plant and used in CCU concrete production. αCap is 1 if there is no carbon capture at a coal plant i.e., when CO2 is captured from a combined cycle natural gas plant and used in CCU concrete production.
Process 6—Material Transportation: The CO2 emissions from material transport is the product of the 5 materials used in the design mix (MCCU in kg/MPa/m3), the CO2 intensity of the mode of transportation used (φM in kg CO2 per kg-km) and the distance over which the materials are transported (DM in km). MCCU represents CCCU, FACCU, CACCU, WCCU and SCMCCU from processes 1 to 5. DM values for road, rail, ocean and barge transport are obtained from the national average values for the US concrete industry (SI Section 7)60. φM for the four transportation modes are obtained from the Ecoinvent database (SI Section 7).
Process 7—Monoethanolamine (MEA) Production: The CO2 impact of carbon capture is the product of the mass of CO2 which is captured and used in the curing or mixing of CCU concrete (φCCU, kg CO2) and the life cycle CO2 emissions from producing a monoethanolamine (MEA) post-combustion CO2 capture system (φMEA). φMEA is obtained from the literature review of 21 studies44,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83 (SI Section 3).
MEA systems are considered as they capture CO2 with a high efficiency (90%)64,65,84, capture CO2 from dilute concentrations85, are retrofittable to power plants currently in operation and are a commercially mature technology86,87. The power sector accounts for 28% of the overall CO2 emissions in the U.S88 and is, therefore, a good candidate for carbon capture. As a result, we consider CO2 capture from power plants. Post-combustion capture is considered as it more commonly deployed than oxy-fuel and pre-combustion systems65,85. The reader can refer to65,85 for further details on the underlying physical principles of carbon capture using MEA, which is beyond the scope of this work.
Process 8—Power plant electricity generation: When CCU concrete is produced, the total CO2 emissions from the power plant is the sum of two components.
Allocelec quantifies the allocation of CO2 emissions from a coal power plant between the co-products of electricity and fly ash, which is used as SCM in concrete production in certain datasets. Allocelec is 0.98 or 0.94 as economic or mass allocation allocates 0.02 and 0.06 of the total CO2 emissions from the coal power plant to the co-product of fly ash (SI Sections 5 and 6). Allocelec is 1 when electricity is sourced from a combined cycle natural gas power plant or when system boundary expansion is used (instead of economic or mass allocation). φNot Cap accounts for the 10% of CO2 which is not captured as the capture efficiency of the MEA system is 90%64,65,84.
The second component accounts for the CO2 emissions from compensating for the energy penalty (Ep in kWh), which is incurred when CO2 is captured from a power plant. The second component is the product of Ep and the CO2 intensity of the electricity used to compensate for Ep (φAvg in kg CO2/kWh).
Ep is quantified as follows
φCCU is the mass of CO2, which is captured from a power plant and utilized in CCU concrete production. heatccu represents the heat required to regenerate the MEA (2.7 to 3.3 MJ/kg CO2, Supplementary Table 5), which could have alternately been used to generate electricity in the power plant70,89,90,91. hte is the heat to electricity factor (0.09 to 0.25, Supplementary Table 5), which is used to determine the electricity equivalent of heatccu. Epump is the electricity required to power the pumps and fans in the carbon capture unit (16.6 to 30.6 × 10−3 kWh/kg CO2, Supplementary Table 5) and Eliq is the electricity required to liquify the captured CO2 (0.089 kWh/kg CO2, SI Section 3 “CO2 Liquefaction”)).
This analysis follows the standards recommended by the National Energy Technology Laboratory (NETL)92 to determine the CO2 intensity of the electricity used to compensate for the energy penalty. NETL recommends that the energy penalty is compensated through an external electricity source, which is representative of the grid-mix of the region in which the analysis is carried out92. φAvg varies between 0.38 and to 0.56 kg CO2/kWh, which represents the lower and upper limit of the average CO2 intensity of electricity generated in the different grid regions in the US in 202092.
Process 9—CO2 Transportation: This analysis assumes that the captured CO2 is transported in a semi-trailer truck (SI Section 3 “CO2 Transportation”) as it is necessary to supply the CO2 from the site of capture to geographically disperse concrete curing or mixing facilities, which are primarily accessible by road21. The CO2 emissions from transporting CO2 is the product of the total weight (φCCU plus the tare weight), the distance over which the transport occurs (DCO2 in km) and the CO2 intensity of transportation emissions of a semi-trailer truck (φT = 112 g CO2 per ton km, Supplementary Table 11). The transport of 1 kg of CO2 necessitates the transport of an additional tare weight (Tw) of 0.4 kg in the onward trip to the CCU concrete production facility (Supplementary Table 7). In the return trip, we account for the CO2 emissions from the transport of only the tare weight. As a result, Tw equals 0.8. We assume DCO2 to be 810 km, which is equal to the longest distance by which CO2 can be transported in the U.S93.
Processes 10 and 11—Vaporization and injection of CO2: After transportation, the liquified CO2 needs to be vaporized to a gaseous state and injected into the concrete sample for curing or mixing94. The CO2 emissions from vaporizing (φVap) and injecting CO2 (φInj) is the product of φCCU (kg CO2), φAvg (kg CO2/kWh) and the electricity required to vaporize (5.3 × 10−3 kWh/kg CO2, SI Section 3) and inject CO2 (37 × 10−3 kWh/kg CO2)16, respectively. η is the CO2 absorption efficiency and represents the portion of the total CO2 which is absorbed during mixing or curing of concrete (datasets 71 to 99). η varies between 50% and 85% during mixing16,19,52. For curing, η is equal to 1 (i.e.,100% absorption) as the curing datasets (datasets 1 to 70) report CO2 utilized as the ratio of the mass of CO2 absorbed to the mass of cement.
Processes 12 and 13—CO2 and steam curing: The CO2 emissions from CO2 curing of the concrete sample (φCO2_Cur) is the product of φCCU (kg CO2), φAvg (kg CO2/kWh), the electrical power requirements of the curing chamber (PCO2_Cur = 38.8 kW/m3 of concrete)35,95 and the duration of curing (tCO2_Cur in hours, SI Section 2), which is determined from the literature review38,96. φCO2_Cur is normalized to the compressive strength of the concrete sample. In some datasets, a combination of steam and CO2 curing is used for the production of CCU concrete. In this case, the analysis includes the CO2 emissions from steam curing of CCU concrete. The CO2 emissions from steam curing (φStm_Cur) is the product of CO2 intensity of steam curing (39.55 kg CO2/m3/h, Supplementary Table 8) and the duration of steam curing (tstm_Cur in hours), which is determined from the literature (Supplementary Table 1 Process 13). φStm_Cur is normalized to the compressive strength of the concrete sample.
When CO2 is used for mixing of concrete (datasets in category 3 and 4), the CO2 emissions from CO2 and steam curing are assumed to be zero as CO2 curing of concrete is not conducted.
Conventional concrete production CO2 emissions
The total life cycle CO2 emissions from producing conventional concrete (TOTConv) are similarly quantified in Eq. 5.
(Ep * φPow_Plnt* Allocelec) quantifies the CO2 emissions from generating Ep kWh of electricity in a power plant without carbon capture. φPow_Plnt is the CO2 intensity of electricity generated in a coal or NGCC plant (kg CO2/kWh, SI Supplementary Table 1).
Net CO2 benefit and sensitivity analysis
TOTCCU and TOTConv are driven by the CO2 emissions from the 13 processes, which, in turn, are impacted by the uncertainty and variability in the underlying parameters (Supplementary Table 1).
In the scatter plot analysis, 10,000 values are stochastically generated for the material and inventory items and the parameters for the 13 processes, which are obtained from the dataset (ranges and relationships presented in Supplementary Table 1). The stochastically generated values are applied in Eqs. 1, 5, and 6 to determine the CO2 emissions from the 13 processes for conventional and CCU concrete and the net CO2 benefit. The net CO2 benefit is plotted on the ordinate. The difference between the CO2 emissions for each of the 13 contributing processes in conventional and concrete is plotted on the abscissa.
To further verify the results, this analysis conducts a moment independent sensitivity analysis25,29,30,97 to determine the process (from the 13 processes) having the most impact on the net CO2 benefit. The moment independent sensitivity analysis determines the δ index for each of the 13 processes. The δ index quantifies the relative contribution of each of the 13 processes to the probability distribution function of the net CO2 benefit. The moment independent sensitivity analysis offers methodological advantages as it accounts for the correlation between the input parameters for the 13 processes and is applicable when the input parameters and the output are not linearly related98. This study determines the δ indices over 10,000 Monte Carlo runs based on the approach presented in Wei, Lu, and Yuan97.
All the datasets utilized and analyzed in this study are included in section 2 of the Supplementary information file.
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The authors thank Helaine Hunscher and Dr. Christophe Mangin from the University of Michigan for feedback, which improved the quality of the manuscript. This work was supported by the Global CO2 Initiative, Center for Sustainable Systems (CSS), School for Environment and Sustainability (SEAS) and the Blue Sky Program of the College of Engineering at the University of Michigan. D.R. conducted and completed the research presented in this manuscript at the Center for Sustainable Systems, University of Michigan, USA. D.R. is currently affiliated with the National Renewable Energy Laboratory (NREL), USA.
The authors declare no competing interests.
Peer review information Nature Communications thanks Mahmoud Reda Taha and other anonymous reviewers for their contributions to the peer review of this work. Peer review reports are available.
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Ravikumar, D., Zhang, D., Keoleian, G. et al. Carbon dioxide utilization in concrete curing or mixing might not produce a net climate benefit. Nat Commun 12, 855 (2021). https://doi.org/10.1038/s41467-021-21148-w
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