An industrial demonstration study on CO2 mineralization curing for concrete.

A 10,000 ton-CO2/y mineralization curing (CMC) process was demonstrated in Jiaozuo city, China by retrofitting a traditional autoclaved curing plant. An industrial concrete formula with synergistic effects of aggregate gradation, early hydration, and alkali excitation was developed using local solid wastes resources. Approximately 90% of the raw materials, including fly ash, furnace blaster slag, steel slag, and carbide slag, came from coal-based industries. An extraordinary phenomenon of high-temperature accumulation from room temperature to 140°C was first observed in an industrial scale because of the rapid and strong exothermic carbonation reaction. A step pressure-equalizing procedure was developed to achieve a rapid carbonation rate, a high CO2 conversion ratio of >98%, and efficient carbonation exotherm recycling. The global warming potential life cycle analysis revealed that compared with autoclaved curing, CMC showed significantly decreased the emission of 182 kg CO2-Eq/m3-product, with direct CO2 sequestration accounting for ∼65% of the reduction.

Introduction

CO2 mineralization uses CO2 reactions with alkaline metals, such as calcium (Ca) and magnesium (Mg), to form stable carbonates, which is a CO2 fixation technology with a Gibbs energy change of <0 (Xie et al., 2015) and a global sequestration potential of 10 Gt (Xie et al., 2016). Ex situ mineral carbonation of natural-minerals and industrial-solid-wastes has recently gained popularity due to the accelerated carbonation rate and safety (Xie et al., 2015), as well as the ability to produce various high-value products without risk of leakage.

A large amount of alkaline and neutral materials could be employed as feedstocks for ex situ CO2 mineralization. Alkaline solid waste is a promising resource for CO2 mineralization in an eco-friendly circular economy model (Monkman and Shao, 2006; Greeshma et al., 2015; Wei et al., 2018; Zhan et al., 2013, 2014). A massive amount of solid wastes (including steel slag, fly ash, and other alkaline solid wastes) is generated, especially for developing countries, presenting enormous potential for CO2 neutrality. In China, for example, 42 million tons of CO2 could be sequestrated via steel slag, 80.66 million tons via fly ash from coal combustion, and 8.92 million tons via carbide slag annually (Li et al., 2007; Ning et al., 2020; Xie et al., 2015, 2016).

CO2 mineralization curing for concrete is one of the rapidly emerging technologies for large-scale CO2 and industrial solid wastes utilization owing to the advantage of the mild condition and valuable products (Pan et al., 2012; Zhang et al., 2020; Sanna et al., 2014; Liu et al., 2021). When compared with the 4-week air curing of building materials, the cementation process via mineralization curing can be achieved under a mild condition (<80°C, 0.1–40 bar) (Huang et al., 2019a; Wang et al., 2017, 2021) and completed in a matter of hours (Wang et al., 2017). Researchers have been working on developing flexible formulas, such as Wollastonite-Portland cement (He et al., 2020; Huang et al., 2019a; Wang et al., 2017), MgO-Portland cement (Vandeperre and Al-Tabbaa, 2007; Wang et al., 2020), Slag-Portland cement (Li et al., 2020a; Mo et al., 2016; Yi et al., 2020), and waste concrete aggregate (Liang et al., 2020; Tang et al., 2020; Zhan et al., 2013, 2016). However, for different products, the cumulative energy demand (CED-NRe) of the process varies between 2,000 and 4,000 MJ/m3 product (Huang et al., 2019b), which is highly dependent on the raw materials selection and source (Kim et al., 2013; Wang, 2020), preparing processes (Huijgen et al., 2007; Huijgen et al., 2006; IEA, 2013), and curing system (Liu et al., 2021). Furthermore, the performances (the level of density and compressive strength) of the products determine the economic benefits of the whole process. Thus, the challenge of process designs is to balance the competing demands of high product performance, low energy consumption, and high efficiency of carbon sequestration efficiency.

CMC technology was determined to be TRL-8 stage (NASA, 2012), and corresponding industrial tests have been conducted gradually for demonstration by different groups such as CarbonCure, CarbonBuilt, and Carbicrete. More than 1 Gt/y industrial solid waste was reused in China to produce building materials (Commission, 2014), such as autoclaved fly ash block, precast concretes, and aggregates. This study proposed a set of operational CMC processes and reported a 10,000-ton/y CO2 concrete curing demonstration in central China. Relevant results are provided in the following sections, including curing condition optimization, process design, continuous operation, product physicochemical characterization, and material and heat balance analysis. The net CO2 sequestration rate was evaluated through life cycle analysis (LCA), and the levelized cost of CO2 concrete was also reported.

Technology and process description

CO2 mineralization curing

The industrial scale CMC demonstration has sufficient technical theory support. The accelerated carbonation in chamber reactor is happened by the reaction of mid-pressure CO2 and alkali metal (Ca/Mg) in Ordinary Portland Cement (OPC)-based (Equation 1) and solid-wastes-based (Equation 2) materials. Different from the natural carbonation of building materials, concrete treated with CMC technology has higher carbonation depth associated with the enhanced performances.

In the CMC technology system, CO2 gas molecules diffuse through the micropore structure of the formed concrete and finally, fast react with the carbonated active component to form the stable CaCO3 microcrystal. The concrete microstructure is significantly affected by carbonation process, which is manifested by the two side effects: (1) the filling effect by carbonation production of CaCO3 microcrystal; (2) the carbonation cementitious effect improves the interface transition zone (Wang et al., 2017). In addition, the carbonation kinetics can be improved through significant interface continuity by adjusting the doping ratio of inert substances in the concrete system (Huang et al., 2019a). Therefore, kinetically, the CO2-diffusion-controlled carbonation process can be enhanced by active controlling of the CO2 partial pressure (Huang et al., 2019a; Wang et al., 2017; Yi et al., 2020).

The curing process is generally operated at room temperature; there is no need for the extra heat resource compared with the traditional autoclaved curing. Due to carbonation reaction heat release, the temperature rise could improve the gas diffusion and carbonation reaction kinetics (Zhan et al., 2013; Wang et al., 2021). Because the decomposition temperature of carbonates in concrete is >400°C (Khan et al., 2021), thermodynamically, the negative effect of heating process on CO2 sequestration is negligible during CMC process.

Industrial demonstration process

CMC process was supported by a fully equipped system retrofitted based on a traditional autoclaved curing plant in Henan Qiangnai New Materials Co., Ltd. Annually, the factory produces 200,000 tons of the block using a traditional autoclaved curing process, consuming 9,000 tons of steam and 630,000 m3 of natural gas.

The whole CMC system comprised filler blender unit, mandatory broken unit, brick machine unit, pre-curing room unit, CO2 curing chamber unit, gasification pry unit, and packaging and transportation unit (Figure 1). The raw materials, including coarse aggregate (slag), fine aggregate (ash), and cementitious material (cement), were first fed into the bin feeders in the appropriate proportion, aggregate gradation, and water to solid ratio. After stirring in the blender, the filler mixture was transferred to the mandatory broken unit, which was molded in the brick machine unit and distributed into the cubic mold after oscillating in the brick machine. Following a pre-curing process (natural condition), the ready brick was sent to the CO2 curing workshop for CO2 mineralization curing. The performance parameters of each unit are summarized in Table S1.

Figure 1

Schematic diagram of CMC process

The CO2 gas used in this study was supplied from the liquid-CO2 tanker. A gasification system, with a capacity of 500 m3/h (in 1.2 MPa and 20°C) and an electric auxiliary heat device was employed to provide a consistent supply of gaseous CO2 to the curing chamber. The step pressure-equalizing (PE) CO2 curing procedure was applied in this CMC demonstration to recycle the majority of the CO2 gas. For a one-step PE operation, after the curing process in chambers #1, #2 was completed with gas pressure P1; chambers #1, #2, #3, and #4 were connected via a gas separate cylinder, and the pressure was equalized to P2. At least half of the gas in chambers #1 and #2 were recycled to chambers #3 and #4 during the PE process. Following the PE operation, the residual gas in chambers #1 and #2 were evacuated to the atmosphere, followed by the unloading of concrete products from the chambers.

Results

Optimization of pre-curing process

The pre-curing process allows the block to undergo early hydration process, which influences the performance of CO2 mineralization, particularly CO2 uptake and compressive strength. The actual size sample (240 mm × 115 mm × 53 mm) and a smaller reactor (∼600L) were used in laboratory to optimize the pre-curing process. As demonstrated in Figure 2, as the pre-curing time is increased, the residual w/s decreases due to the water evaporation. A rapid water loss was observed in the first 24 h, accounting for more than 70% of the total water loss. CO2 uptake had a noticeable negative relationship with the residual w/s, which was attributed to changes in gas permeability in concrete blocks (Wang et al., 2017; Huang et al., 2019a; Yi et al., 2020). CO2 mineralization reactions accompanied by early hydration could result in denser microstructure and strength enhancement (Guo et al., 2019; Greeshma et al., 2015; Jang et al., 2016), demonstrating a positive correlation of compressive strength with CO2 uptake. Considering the improvement of CO2 mineralization performance and pre-curing efficiency, the optimal water loss could be controlled at 4–5 wt.% with a pre-curing time of about 24 h. Further increase in the water loss may cause serious concrete shrinkage (Li et al., 2020b) and decrease production efficiency.

Figure 2

Effect of pre-curing time on residual w/s, CO2 uptake, and compressive strength

(initial w/s = 0.18, 25°C, 4 h curing with 99% purity of CO2).

Performance of CO2 curing

Temperature and pressure evolution in the CO2 curing chamber

Figures 3A and 3B depict the temperature and pressure evolution in the CO2 curing chamber (∼150 m3) within 72 h, which included nine cycles of CO2 curing and 8 times of PE operation. It took about 6–7 h for gas intake and pressure to rise for each cycle. The chamber pressure dropped dramatically during the PE stage, from 0.6–0.7 MPa to 0.05–0.1 MPa, in 20–30 min. This is because the fresh concrete in the next group of chambers provided a high-adsorption driving force owing to the high mineralization kinetics in the initial stage (Wang et al., 2017; Huang et al., 2019a; Guo et al., 2019; Yi et al., 2020). In addition, the low residue CO2 pressure would result in a high CO2 conversion ratio. Meanwhile, an intriguing phenomenon of dramatic temperature rise was observed inside the curing chamber. The heat released by the carbonation reaction caused the chamber temperature to rise above 100°C. The highest curing temperature reached was over 140°C.

Figure 3

Pressure and temperature change within 72 h

(A) Pressure; (B) temperature; (C) pressure and temperature change of chamber #4 in the seventh curing cycle.

As an example, in the PE stage of a single curing cycle (Figure 3C), approximately 3 tons of CO2, or 50% of the total gas intake, was consumed in 30 min with the temperature increased to 45°C. The pressure plateau indicates a fast mineralization process during the early period of external gas supply (30–120 min). At the same time, because the majority of the carbonation reaction heat can be stored in high heat capacity bricks, the temperature also performed a plateau of about 90 min. The pressure dropped markedly during the gas-stop stage (350–430 min) without external gas intake, but the temperature kept rising, indicating that the concrete still had high reaction activity after 6 h of carbonation process.

Evaluation of CO2 curing kinetics

According to the CO2 curing kinetics (Figure 4A), the fastest carbonation happens during the PE stage. The CO2 uptake rate reached to 36 t/h in the very initial stage. In chambers #5 and #6, 60% of CO2 was adsorbed in 30min. Further analysis revealed that the CO2 uptake rate was controlled by the gas equalizing rate, as the slops of cures are close to 1 (Figure 4B). The steadily increasing CO2 uptake in the external gas supplying stage demonstrated a nonstop carbonation reaction lasting up to 450 min. The paralleled fitting curves in Figure 4C show that the CO2 uptake rate is kept around 0.02 t-CO2/min, slightly less than the gas intake rate (0.022–0.035 t-CO2/min). This phenomenon supports the hypothesis of product layer diffusion-controlled kinetics (Wang et al., 2017; Huang et al., 2019a; Yi et al., 2020), which is attributed to the fast densification effect of the carbonation reaction on the concrete surface.

Figure 4

CO2 uptake kinetic analysis of CO2 curing process

(A) The whole stage, (B) PE stage, and (C) external gas supplying stage.

Performance characterization of CO2 concrete

CO2 uptake

During the 72-h continuous curing operation, 101 tons of CO2 were supplied to the system, and 1.04 tons of residual CO2 were released to the atmosphere after pressure equalization, yielding a CO2 conversion rate of 98.97%. The annual CO2 sequestration at this capacity is approximately 11,000 tons (running 8000 h/year) with 5.3 wt.% CO2 uptake of concrete. Blocks at different spatial positions in the cross-section of the curing chamber (Figure 5A) were sampled to investigate the difference in CO2 uptake. The CO2 uptake value and phenolphthalein cross-section test showed that the carbonation depth of bricks was increased from 0.5 cm to 2.5 cm from the top position to the bottom position. The TGA results in Figure 5B illustrate the significant difference of CO2 uptake for bricks at a different position inside the chamber. This intriguing phenomenon is thought to be related to the uneven CO2 concentration distribution in the chamber. The partial pressure of CO2 in the lower part of the chamber may be higher because of the relatively lower position of CO2 inlet and higher density of CO2 gas as compared with the density of air.

Figure 5

CO2 uptake of different spatial positions

(A) Phenolphthalein test and (B) TG test results on different spatial positions.

Compressive strength

The compressive strength of bricks as shown in Figure 6A can be used to characterize the practical mechanical performance. The significant improvement in compressive performance due to CO2 curing could be attributed to the densification effect (Wang et al., 2017; Tang et al., 2020) of carbonation, which helped strengthen the microstructure. Because of the later hydration, the compressive strength could reach to 15 MPa after 1 week and increased to more than 20 MPa over the next 4–5 weeks, with a 70.5% strength enhancement compared with the samples that did not receive CO2 curing. The longer-lasting later hydration effect may be attributed to the denser product layer formed by carbonation, which has a higher water-retention property. As demonstrated in Figure 6B, the compressive strength was noticeably affected by spatial position. The densification effect during carbonation should increase the compressive strength with increased CO2 uptake, which is consistent with the pattern found in laboratory experiments in Section “Optimization of pre-curing process.”

Figure 6

Compressive strength of bricks

(A) Different age and (B) different spatial position.

Microscale characterization of carbonation product

Chemical composition

Because SiO2 is a relatively stable component, the carbonation degree of bricks could be semiquatitatively analyzed by using the ratio of relative characteristic peak intense of CaCO3 to SiO2. As illustrated in Figure 7A, the relative peak height of carbonation product CaCO3 (∼29.5°) increased significantly after CO2 mineralization curing. Meanwhile, the hydration products Ca(OH)2 (∼17.5°) and carbonation product CaCO3 (∼29.5°) were gradually increased with an increase in pre-curing time (24–48 h) after CO2 curing. According to the relative peak height of CaCO3 in Figure 7B, the higher the chamber temperature, the more the CaCO3 was generated, indicating that the carbonation reaction of concrete is primarily controlled by kinetics below 150°C. At the same time, the carbonation conversion rate is far from reaching the thermodynamic maximum.

Figure 7

XRD analysis of bricks

(A) Different pre-curing times and (B) different maximum curing temperatures.

Microstructure

Compared with the regular rod-like Ca(OH)2 crystal observed during the normal hydration process (Figure 8A), more calcium-modified silica gel was formed during 104°C CO2 curing, which may be due to the alkali excitation effect at high temperatures (Gómez-Casero et al., 2021; Aydın and Baradan, 2012) (Figure 8B). Carbonation products tended to form aragonite crystal and metastable vaterite rather than calcite as carbonation temperature increased (Figures 8C and 8D). Vaterite, a rare carbonation product in concrete, was only observed when the carbonation temperature exceeded 140°C. It is well known that vaterite is a metastable polymorph with the highest energy level of the three typical CaCO3 morphologies and that it is formed in a moist environment at high temperatures (>100°C) (López-Arce et al., 2011; Bjørge et al., 2019). Therefore, one can infer that CO2 curing at relatively high humidity and temperature could elevate the energy level of CaCO3 carbonation products and form relatively unstable polymorphs (aragonite and vaterite) (Bjørge et al., 2019; Wang et al., 2019).

Figure 8

SEM pictures of bricks

(A) Natural curing; (B) CO2 curing (max temperature = 104°C); (C) CO2 curing (max temperature = 125°C); (D) CO2 curing (max temperature = 144°C).

Mass balance and benefit analysis

Mass and heat balance analysis

In this demonstration, the boundary of the materials balance calculation was deposited as a single process line from raw materials (slag, ash, cement, and gaseous CO2) into the factory to the finished brick (light low-carbon concrete brick) out of the factory (Figure 9). During the 72-h demonstration, 1909 tons of materials were consumed, containing 50% of slags, 40% of ashes, and 10% of cement. Analysis of the change of water content in bricks revealed that the water evaporation during pre-curing was 26.22 tons. During the curing process, the CO2 intake during the 72-h test was 101 tons as determined by weighing the CO2 tank truck. Moreover, the residual CO2 was calculated as 1.04 tons using Equation 4, whereas the CO2 conversion rate was 98.9% (see Table S2). Water evaporation and precipitation (average moisture content change of bricks before and after curing) were calculated to be 39.06 tons during the CO2 curing process.

Figure 9

Materials flow balance schematic of CMC process

Figure 10 illustrates the Sankey plot of mass and heat flow during a curing period, based on the data from a typical curing chamber. The heat release was calculated using the HSC CHEMISTRY 6.0 reaction enthalpy database (see Table S3) and Equation 7. The total heat of the carbonation reaction was approximately 18.6 GJ, as the only heat source of the entire process. The sensible heat of solid and water in bricks dominates the heat flow, accounting for 57.5% and 16.7% of the total, respectively. The sensible heat of steel (chamber body and trolley), the heat dissipation, and latent heat of exhausted steam each accounted for 18.6%, 5.9%, and 1.2% of total heat, respectively.

Figure 10

Sankey plot of mass and heat balance analysis in the CO2 curing chamber

Carbon LCA analysis—global warming potential

Considering the low reactivity of the block with CO2 after CMC and autoclave curing (The low free-CaO content in Table S4), passive CO2 absorption during the lifetime of the block was not calculated in this paper. Therefore, the “cradle to gate” is set as the LCA calculating boundary (ISO 14040 &14044) for the whole system, and the detailed boundary schematic diagram is shown in Figure 11. The raw material and external input GWP data were obtained from the world’s life cycle inventory (LCI) database—ecoinvent 3.7 (Sep. 2020) (Ecoivent, 2020). The database is now known as the largest LCI database, containing data from statistical and published technical literature. Figure 11 illustrates the allocation of resource input units for various industrial activities. The electric power emission data were obtained from the ecoinvent database based on the power source in Henan province, China; coal and diesel were based on global average emissions.

Figure 11

LCA calculation boundary

Figure 12 shows a significant difference in GWP between CO2 curing and autoclaved curing (Detailed calculation is provided in Table S5). Blocks with the same strength level (>15 MPa) are used for GWP analysis of different processes. The GWP of the CMC process is 58.51 kg CO2-Eq/m3, which is only 1/4 of the autoclaved curing process (241.30 kg CO2-Eq/m3). The vast differences were primarily due to the differences in material formulation and CO2 sequestration capacity between the two processes. The majority of CO2 emissions in raw materials came from cement manufacturing, making the GWP highly dependent on concrete mixture design. Therefore, using of industrial solid waste instead of cement significantly reduced GWP and also allows the solid-waste-based concrete to achieve a lower GWP index in the raw material unit. The contribution of transportation to GWP is negligible (less than 2%), and it is unaffected by changes in raw material freight volume distribution. GWP did not differ significantly at the end of manufacturing process because the processes of CO2 curing and autoclaved curing were similar. Moreover, when compared with autoclaved curing, CO2 curing had a significant carbon-negative effect with a GWP of −120 kg CO2-Eq/m3.

Figure 12

The GWP indicator results for CO2 curing and autoclaved curing processes

Technical economic analysis

The boundary, functional units, and allocations of technical economic analysis (TEA) were based on the LCI calculation for GWP provided in STAR Methods and Table S6. Of note, transportation costs are factored into raw materials prices. As demonstrated in Figure 13, the operational cost of the CMC process was 30.93 USD/m3, with raw material (74.25%), manufacture (8.23%), gas supply (9.42%), and labor (8.10%). Compared with the autoclaved curing process, CMC effectively reduced the proportion of cement in raw materials with a cost advantage of 2.09 USD/m3. A similar process structure resulted in comparable costs of manufacturing and labor. The cost of CO2 gas supply based on PSA (Union_Engineering, 2016) was, however, higher than that of steam.

Figure 13

The operational cost of CO2 curing and autoclaved curing processes

The operational cost of CO2 curing and autoclaved curing processes.

Considering the local production (lightweight concrete brick) price (∼36.3 USD/m3), the economic benefits of the two processes are 5.40 USD/m3 for CO2 curing and 5.31 USD/m3 for autoclaved curing. Notably, the product capacity of CO2 curing (6–8 h/cycle, as shown in Figure 5) is more than 1.5 times that of autoclaved curing (8–12 h/cycle), which can increase the plant revenues by more than 50% per year. As a CO2 utilization technology, the negative cost benefit of CMC technology can reach 45 USD/t-CO2 without a carbon tax. In addition, when integrated with existing coal-fired flue gas CO2 capture at ∼40 USD/t-CO2, CMC technology still has a high economic benefit of ∼35 USD/t-CO2.

Discussions

Uncertainty analysis

LCA uncertainty

As shown in the table in Figure 14, two scenarios are set up to predict the influence of potential changes in CMC process on GWP index. In the two scenarios, cement content, transportation distance, manufacture energy consumption, and CO2 uptake are set as ±50% of the base scenario. The GWP of cement in raw material is still dominant (201 kg CO2-eq in negative and 67 kg CO2-eq in positive), and the resulting change (Wang et al., 2017) in CO2 uptake (−180 kg CO2-eq in negative and −60 kg CO2-eq in positive) does not compensate for the emission. Although the sightly emission change in transportation distance is negligible to the GWP of CMC process, increasing transportation distance still elevates the raw material cost (from TEA). Therefore, a >50% GWP reduction can be achieved in the future by positively reducing cement content, optimizing CO2 uptake, and manufacture energy consumption by the deployment of renewable energy.

Figure 14

Scenario modeling of GWP in CMC process

TEA uncertainty

The potential green premium for CMC technology can be obtained through the incentives from carbon trading by recycling CO2 and operational cost reduction from renewable energy deployment. Hence, we use the most competitive price index ($/m3-Block) to measure the future economics of CMC technology (Figure 15). Based on the TEA data, a Cost-Price model was established to analyze the future changes of block price. For concrete manufacturers using CMC technology, the decision on minimum pricing can be made in combination with carbon tax under the premise of ensuring income (5.4$/m3-Block).

Figure 15

Green premium analysis for CMC technology

Before the large-scale deployment of renewable energy, the contribution to operational cost reduction is limited, but the incentives from carbon trading will help concrete manufacturers quickly reap the benefits (blue line in Figure 15). Suppose in the future carbon trading scenario (Reference from: European Union Allowance, EUA ∼100 $/t-CO2 (ESMA, 2021)), compared with the traditional autoclaved curing process, CMC can achieve ∼$12/t-Block of benefits through negative carbon benefits, which is equivalent to a 1/3 reduction in operational cost. In the scenario of large-scale deployment of renewable energy, if the operational cost reduced by 50% by getting cheaper raw materials, manufacturers can still benefit from CMC technology even if the blocks do not enter the market (Price = 0).

Contribution of this study

This industrial CMC process demonstrates the vast demand for industrial solid waste from the block defined as a “low-carbon” building material. This study discusses the adaptability of raw materials and product property index, which helps wider audience to have a concrete concept of low-carbon building materials from the preparation method and product performance.

In the study of CO2 curing kinetics at the industrial scale as mentioned in Section “Performance of CO2 curing,” favorable product performances can be obtained at different CO2 partial pressures (from 0.1 MPa to 0.6 MPa). This demonstrates that a flexible carbon source choosing can be realized by combining CMC with impure CO2 gas from cement mill, steel mill, and coal chemistry factory upstream. This helps to make CO2 more valuable for developing countries to form a huge carbon sink close to the source of CO2 emissions.

A full-scale CMC industrial demonstration provides a set of complete process parameters, mass and heat balance data, LCA data, and TEA data for assessment by the third party, which help in the future industrial design.

Limitations of the study

In actual industrial application, CO2 may come from a variety of scenarios (flue gas CO2 of ∼12% from coal-fired power plant or high concentration CO2 from coal chemistry). Therefore, it is worth to study the integration process of CMC and different CO2 gas sources and explore the potential cost reductions. In the future, CMC technology should be compatible with a wider range of building material product orientation (precast concrete unit, ready-mixed concrete, and aggregates). It is necessary to systematically study the technical process, product performance, LCA, and TEA of different building material products through more demonstrations.

STAR★Methods

Key resources table

Resource availability

Lead contact

Further information and requests for resources should be directed to and will be fulfilled by the lead contact, Tao Wang (oatgnaw@zju.edu.cn).

Materials availability

This study did not generate new unique reagents.

Method details

Raw materials

The filler powder was made up of a 5:4:1 mixture of waste slag, waste ash, and cement. Steel slag was delivered by Henan Jiyuan Iron & Steel Co. LTD., located 75 km away from the demonstration base. The steel manufacturing method was converter steelmaking with water quenching. Blaster furnace slag and fly ash were supplied by Jiaozuo Wanfang Electric Power Co., LTD, located 25 km away from the base. Carbide slag was a by-product of calcium carbide obtained from Henan Lianchuang Chemical Co. LTD., located 70 km away from the base. P.O. 42.5 cement from Jiaozuo Qianye Cement Co. LTD. (30 km away) was used for filler preparation. CO2 was purchased from Henan Xinlianxin Fertilizer Co. LTD., in the form of a liquid with a purity of more than 90%, transported by tanker. The particle size distribution of waste slag and waste ash were shown in Table S7 and Figure S1 respectively, and the element composition of the above raw materials are listed in Table S8.

Physicochemical characterization

The microstructure and morphology of the samples were comprehensively characterized through Scanning electron microscopy (SEM), X-ray diffraction (XRD), and Thermogravimetry (TG) tests. The samples were sprayed with gold for 2 minutes before the SEM test (SU-3500, Hitachi). The CuKα radiation PANalytical X-ray diffractometer was used for XRD analysis with 2θ° from 10° to 70°. Jade 5.0 was used to process curves and determine peaks. The powder sample was taken from the surface of the specimen. The carbonation degree of bricks was determined using TG/DSC (NETZSCH STA 449 F3). The operating temperature ranges between 10 and 1000°C, with a 10°C/min heating rate and a nitrogen flow rate of 50 mL/min. The weight loss between 400-800°C was attributed to the decarbonation of metastable CaCO3 (particularly, amorphous calcium carbonate and vaterite) and decrystallization of calcite and aragonite (Khan et al., 2021). The element composition of raw materials was determined by an X-ray Fluorescence spectroscope (XRF, Shimadzu Corporation, XRF-1800).

Quantification and statistical analysis

Calculating methods

CO2 conversion rate

The CO2 conversion rate, C, is calculated from Equation 3, where, min and mout denote the inlet and outlet CO2 gas mass, respectively. The inlet gas mass was determined by weighing the liquid CO2 trunk over the weighbridge. The outlet CO2 mass was calculated by Equation 4, where, p and T denote the pressure (Pa) and temperature (K) after CO2 curing in the chambers, respectively; V denotes the volume of the curing chamber (m3); MCO2 denotes the molar mass of CO2 (g/mol); z denotes the compressibility factor of CO2 obtained from Aspen 8.0 property database (PRSK, pure substance, and UNIFAC model). R denotes the gas constant, which is 8.314 m3/(mol·K).

CO2 uptake during curing process

The real-time change of CO2 uptake Ct (wt %) in chambers during the CO2 curing process is calculated by Equation 5, where, Q denotes the average CO2 flow from a Vortex shedding flowmeter (VSF) in the front of gas separate cylinder (ton/min); pt and Tt denote the gauge pressure (Pa) and temperature (K) at time t in the chambers, respectively. ps denotes the saturation pressure of steam at Tt (Pa).

The CO2 uptake Ct (wt %) in the next chambers in the PE stage is calculated by Equation 6. p1 and T1 denote the gauge pressure (Pa) and temperature (K) in the finished chambers at the initial time t0, respectively. p1t and p2t denotes the pressures at time t in the finished chambers and the next chambers (Pa), respectively; T1t and T2t denotes the temperatures at time t in the finished chambers and the next chambers (K), respectively; ps1, ps1t, and ps2t denote the saturation pressure (Pa) of steam at T1, T1t and T2t, respectively (Because of the cooling effect of the PE, liquid water is continuously separated so that the humidity in the chambers is kept at saturation levels).

CO2 mineralization reaction heat

CO2 mineralization reaction heat E (kJ/100g-material) is obtained by Equation 7. ΔHi denotes theoretical enthalpy change (from HSC CHEMISTRY 6.0 database in Table S3) of hydroxide i (corresponding to oxide i) in reaction with CO2 (kJ/mol); C denotes the CO2 uptake of dry raw materials (g-CO2/100 g-material); MCO2 denotes the molar mass of CO2 (g/mol); N denotes the total amount of substance of activated oxide in 100 g raw material; Ni denotes the amount of substance of activated oxide i.

Ni is calculated by Equation 8, where, λj denotes the proportion of raw material j (%); ωj,i denotes the the proportion of active oxide i in dry raw material j (%); Mi denotes the molar mass active oxide i (g/mol). Here, i represents CaO, MgO, K2O, and Na2O; j represents steel slag, furnace blaster slag, fly ash, carbide slag, and cement, respectively. The oxide proportion is referred from Table S8.

Compressive strength

The uniaxial compressive strength of brick samples under natural curing and CO2 curing were tested using the universal tester, with a precision of 0.1 MPa. In the present study, the compressive strength after CO2 curing and natural curing were tested after 3d, 7d, 28d and 45d hydration in air. The displacement control was set to the universal tester with a loading speed of 2mm/min. The compressive strength is calculated by Equation 9, where, Rc and Fc denote the compressive strength (MPa) and the peak load (kN) when the sample is destroyed, respectively; A denotes the area (mm2) of the sample compression surface. The ultimate strength is the average value of one group of three parallel specimens for each condition.

Calculation of LCI

Mixture design

The mixture design was used to compare the GWP difference between CO2 curing and autoclaved curing (Table S9). Considering the original working condition of autoclaved curing, the steam consumption was approximately 45 kg/m3-concrete (8.7 m3-steam/m3-concrete with 180°C and 1 MPa, the density = 5.16 kg/m3).

Functional units

Ecoinvent v3.7 was used to obtain basic unit data and to calculate LCI. Based on variations in manufacturing processes, energy requirements for mixing, crushing, and molding processes as well as other plant energy requirements were obtained from the electricity functional unit (power plant). The corresponding mechanical energy consumed during the transportation of raw materials was obtained from the diesel functional unit (diesel engine); the energy consumed in steam heating during the autoclaved curing process came from the coal-fired functional unit (boiler). The electric power emission data was obtained from the ecoinvent database, according to the power source in Henan province, China; coal and diesel were based on global average emissions. In the actual operation, the curing chamber exhibited good thermal insulation as well as pressure holding performance, and no additional energy was input for pressure and temperature control (CCP and CCT are 0).

Calculation of GWP

The GWP indices were calculated as:Whereby, in Equation 10, IGWP represents the GWP index value; IRGWP, IMGWP, and ITGWP respectively denote the evaluation data of the raw materials unit, manufacture unit, and transportation unit; i and j respectively represent different orders of raw material types and manufacture processes. In Equation 11, W and IR represent the mass (kg-material/m3) and GWP-LCI values of raw materials i (kg-CO2/kg-material); In Equation 12, IM represents the GWP-LCI value of manufacture step j (kg-CO2/m3), while C denotes CO2 uptake pre unit concrete product (kg-CO2/m3); In Equation 13, D represents the transportation distance of raw material i (km), while IT represents the GWP-LCI index value for raw material I, transported by freight vehicles (kg-CO2/(kg-material· km)).

Allocations

With regards to raw materials, production data of Portland-cement was obtained from the ecoinvent database, according to the the cut-off by classification principle. To determine the potential environmental benefits of solid wastes (waste slag and ash in this study) utilization, the “allocation by economic value” is used to calculation the GWP. The result showed the allocation coefficient of waste is no more than 5% (2.3% for slag and 1% for ash) (Chen et al., 2010), so that the GWP of solid waste (∼10−2 kg-CO2/Eq) is neglected in this study. The gas source was a liquid CO2 tank truck with 99% purity. Given that CO2 gas compression and purification were necessary processes in CO2 capture, in this study, the relevant energy consumption was not included in the life cycle boundary. With regards to transportation, freight vehicle was set as the standard loading capacity with 32 t, and diesel fuel consumption for 100 kilometers was 32 L. It was assumed that CO2 was transported through pipelines without quality and economic allocation. During the manufacturing processes, we took the electric power consumption (averaged to per cubic product) of the whole plant when operating at full load for the two curing processes. The GWP calculation are shown in Table S5.

Calculation of economic benefit

Life cycle cost (CLCC) was calculated as:whereby, in Equation 14, CLCC represents the LCC of CMC; CRaw material, CManufacture, CGas supply, and CLabor are the costs of raw materials unit, manufacture unit, gas supply unit, as well as labor unit, respectively. In Equation 15, i and j respectively represent different orders of process types and index types; W and P denote the functional value and the price of index j. The calculations and units are shown in Table S6.

REAGENT or RESOURCE	SOURCE	IDENTIFIER
Deposited data
CO2 emission factor (Coking coal, Default)	2006 IPCC Guidelines for National Greenhouse Gas Inventories	https://www.ipcc-nggip.iges.or.jp/public/2006gl/index.html
CO2 emission factor (Diesel oil, Default)	2006 IPCC Guidelines for National Greenhouse Gas Inventories	https://www.ipcc-nggip.iges.or.jp/public/2006gl/index.html
Cement CO2 emission (GWP)	The Life Cycle Inventory Data Version 3.7 - ecoinvent	https://www.ecoinvent.org/database/
Electric power CO2 emission in Henan province, China (GWP)	The Life Cycle Inventory Data Version 3.7 - ecoinvent	https://www.ecoinvent.org/database/
Electricity price in Henan province, China	State Grid Henan Electric Power Company	http://www.ha.sgcc.com.cn/
Labor price in Henan province, China	People’s Government of Henan Province	https://www.henan.gov.cn/
Cost of CO2 capture by pressure swing adsorption process	UNION_ENGINEERING. 2016. COST EFFECTIVE SOLUTION FOR OIL & GAS. The United Kingdom: Union engineering.	https://union.dk/news-pr/all-news/cost-effective-solution-for-oilgas/
European Union Allowance	Preliminary report - Emission Allowances and derivatives thereof	https://www.esma.europa.eu/sites/default/files/library/esma70-445-7_preliminary_report_on_emission_allowances.pdf
[Database]: [Dataset for “An Industrial Demonstration Study on CO2 Mineralization Curing for Concrete”]	Mendeley Data https://doi.org/10.17632/7x27pg63hd.1	https://data.mendeley.com/datasets/7x27pg63hd/1
Software and algorithms
Reaction enthalpy database	HSC Chemistry	Metso Outotec
Compressibility factor of CO2 (PRSK, pure substance, and UNIFACmodel)	Aspen 8.0 property database	Aspentech
Microsoft Excel	Microsoft	Excel v.16.0