Fundamental Studies on CO2 Sequestration of Concrete Slurry Water Using Supercritical CO2

To prevent drastic climate change due to global warming, it is necessary to transition to a carbon-neutral society by reducing greenhouse gas emissions in all industrial sectors. This study aims to prepare measures to reduce the greenhouse gas in the cement industry, which is a large source of greenhouse gas emissions. The research uses supercritical CO2 carbonation to develop a carbon utilization fixation technology that uses concrete slurry water generated via concrete production as a new CO2 fixation source. Experiments were conducted using this concrete slurry water and supernatant water under different conditions of temperature (40 and 80 °C), pressure (100 and 150 bar), and reaction time (10 and 30 min). The results showed that reaction for 10 min was sufficient for complete carbonation at a sludge solids content of 5%. However, reaction products of supernatant water could not be identified due to the presence of Ca(HCO3)2 as an aqueous solution, warranting further research.


Introduction
Concrete is the most widely used construction material, and approximately 4100 million metric tons of concrete were produced globally in 2020 [1][2][3]. According to a United Nations (UN) report, the current world population is predicted to increase from 7.7 billion to 9.7 billion by 2050, i.e., an increase of nearly 2 billion [4]. Such population growth is expected to increase the demand for social infrastructure facilities and housing, especially in developing countries, which will lead to a sharp increase in concrete consumption [5,6].
Cement, the main binder material of concrete, is produced by mixing and pulverizing limestone (primary raw material; CaCO 3 ) with other clay minerals (SiO 2 , Al 2 O 3 , Fe 2 O 3 , etc.) and sintering at a high temperature above 1450 • C. The amount of CO 2 generated during sintering accounts for approximately 93% of the total emissions during cement manufacturing, where 33% originates from the combustion of fossil fuels and 60% from the decomposition of limestone. The process of limestone decomposition proceeds according to Equation (1) [7][8][9].
CaCO 3 + heat → CaO + CO 2 (1) Various other industries are also responsible for emitting a significant amount of greenhouse gases, and such increase in greenhouse gas emissions leads to intensified global warming and rapid climate change [10]. Carbon capture and sequestration (CCS) technology has been developed for capturing CO 2 emitted from industrial sectors and sequestering it in the Earth's strata to maintain environmental sustainability by preventing the average global temperature from reaching 1.5 • C above pre-industrial levels. However, problems such as leakage of CO 2 isolated in the stratum and groundwater pollution are recognized as concerns associated with CCS [11]. Therefore, extensive research has been directed to developing carbon capture and utilization (CCU) technology based on mineral carbonation, which chemically bonds the captured CO 2 as carbonate minerals and utilizes it as an effective resource [12]. Since typical conversion of CO 2 to carbonate is very slow and inefficient under the conditions of ambient temperature and pressure, there has recently been an increasing trend in research on maximizing the mineral carbonation reaction using supercritical CO 2 . Existing studies on mineral carbonation using supercritical CO 2 have revealed an absence of any significant effect of temperature in the supercritical state and have also shown that the carbonation efficiency increases with increasing pressure [12][13][14][15][16][17][18][19][20][21]. The temperature-pressure conditions explored in recent studies on mineral carbonation using supercritical CO 2 are summarized in Figure 1.
Materials 2022, 15, x FOR PEER REVIEW 2 of 10 mineral carbonation, which chemically bonds the captured CO2 as carbonate minerals and utilizes it as an effective resource [12]. Since typical conversion of CO2 to carbonate is very slow and inefficient under the conditions of ambient temperature and pressure, there has recently been an increasing trend in research on maximizing the mineral carbonation reaction using supercritical CO2. Existing studies on mineral carbonation using supercritical CO2 have revealed an absence of any significant effect of temperature in the supercritical state and have also shown that the carbonation efficiency increases with increasing pressure [12][13][14][15][16][17][18][19][20][21]. The temperature-pressure conditions explored in recent studies on mineral carbonation using supercritical CO2 are summarized in Figure 1. During concrete production, the generation of concrete slurry water is inevitable. Concrete slurry water refers to the surplus concrete returned from over orders or from failure to meet the requirements of construction sites and concrete cleaning requirements, which ensure that concrete does not adhere to truck agitators or mixers in batching plants at ready-mixed concrete factories. Since concrete slurry waste is strongly alkaline, it may contaminate the soil and water and, thus, needs to be neutralized or treated with recycling equipment, such as a filter press [22][23][24].
In general, concrete slurry waste is a dehydrated cake primarily obtained from air drying or filter pressing concrete slurry water upon separation into residual aggregate and coarse aggregate in a ready-mixed concrete factory. The waste contains a large amount of Ca 2+ derived from the non-hydrated cement component; therefore, it shows promise for application as a material for immobilizing a substantial amount of CO2 [25,26].
However, most reported studies on concrete slurry waste have primarily explored the use of a small amount of dry/wet concrete slurry waste as a cement substitute, and there is currently a lack of reported research on the use of concrete slurry waste for largescale CO2 capture.
In addition, for supernatant water that lies above the precipitate of concrete slurry water, hydration is delayed by 0.5-2 h or more due to the coating of cement gel formed around the non-hydrated cement. It is understood that the Ca 2+ concentration increases in the supernatant water during this induction period and that CO2 may react with the ions to be sequestered in the form of CaCO3 [27].
Therefore, in this study, we investigate the possibility of large-scale CO2 sequestration by concrete slurry water and supernatant water by utilizing a supercritical CO2 reaction and exploring the effects of temperature (40 and 80 °C), pressure (100 and 150 bar), and reaction time (10 and 30 min). During concrete production, the generation of concrete slurry water is inevitable. Concrete slurry water refers to the surplus concrete returned from over orders or from failure to meet the requirements of construction sites and concrete cleaning requirements, which ensure that concrete does not adhere to truck agitators or mixers in batching plants at ready-mixed concrete factories. Since concrete slurry waste is strongly alkaline, it may contaminate the soil and water and, thus, needs to be neutralized or treated with recycling equipment, such as a filter press [22][23][24].
In general, concrete slurry waste is a dehydrated cake primarily obtained from air drying or filter pressing concrete slurry water upon separation into residual aggregate and coarse aggregate in a ready-mixed concrete factory. The waste contains a large amount of Ca 2+ derived from the non-hydrated cement component; therefore, it shows promise for application as a material for immobilizing a substantial amount of CO 2 [25,26].
However, most reported studies on concrete slurry waste have primarily explored the use of a small amount of dry/wet concrete slurry waste as a cement substitute, and there is currently a lack of reported research on the use of concrete slurry waste for large-scale CO 2 capture.
In addition, for supernatant water that lies above the precipitate of concrete slurry water, hydration is delayed by 0.5-2 h or more due to the coating of cement gel formed around the non-hydrated cement. It is understood that the Ca 2+ concentration increases in the supernatant water during this induction period and that CO 2 may react with the ions to be sequestered in the form of CaCO 3 [27].
Therefore, in this study, we investigate the possibility of large-scale CO 2 sequestration by concrete slurry water and supernatant water by utilizing a supercritical CO 2 reaction and exploring the effects of temperature (40 and 80 • C), pressure (100 and 150 bar), and reaction time (10 and 30 min).

Materials
Concrete slurry water was obtained from a ready-mixed concrete factory owned by company "Y", located in Gyeonggi-do, South Korea. The concrete slurry water extraction is illustrated in Figure 2. The concrete slurry water used in experiments was separated into supernatant water and concrete slurry waste to obtain equal sludge solids content. The concrete slurry waste was dried at 105 • C until it reached a constant weight. Then, the dried concrete slurry waste was pulverized to control the particle size to 75 µm or below using a 200 mesh. Mineral carbonation using a supercritical CO 2 reaction was initiated by diluting 2 kg of supernatant water and 100 g of dried concrete slurry waste. The chemical compositions of the supernatant water and concrete slurry waste are presented in Tables 1 and 2,

Materials
Concrete slurry water was obtained from a ready-mixed concrete factory owned by company "Y", located in Gyeonggi-do, South Korea. The concrete slurry water extraction is illustrated in Figure 2. The concrete slurry water used in experiments was separated into supernatant water and concrete slurry waste to obtain equal sludge solids content. The concrete slurry waste was dried at 105 °C until it reached a constant weight. Then, the dried concrete slurry waste was pulverized to control the particle size to 75 µm or below using a 200 mesh. Mineral carbonation using a supercritical CO2 reaction was initiated by diluting 2 kg of supernatant water and 100 g of dried concrete slurry waste. The chemical compositions of the supernatant water and concrete slurry waste are presented in Tables 1 and 2, respectively.   Figure 3 shows a schematic representation of the supercritical CO2 reactor, comprising a gas booster (Maximator, Nordhausen, Germany) and a reactor (PHOS-ENTECH, Seoul, Korea). The reactor is equipped with an electrical heater for temperature control, an agitator for mixing, and a thermocouple and a pressure gauge for temperature and pressure measurements, respectively. The gas booster is connected to an air compressor that maintains the CO2 in its supercritical state by pressurizing the reactor vessel with CO2 gas at high pressure.
The maximum operating temperature and pressure of the supercritical CO2 reactor were 80 °C and 200 bar, respectively, and the internal volume of the reactor was 4 L. In addition, the agitator was designed to enable control of the rotation speed up to 400 rpm.   Figure 3 shows a schematic representation of the supercritical CO 2 reactor, comprising a gas booster (Maximator, Nordhausen, Germany) and a reactor (PHOS-ENTECH, Seoul, Korea). The reactor is equipped with an electrical heater for temperature control, an agitator for mixing, and a thermocouple and a pressure gauge for temperature and pressure measurements, respectively. The gas booster is connected to an air compressor that maintains the CO 2 in its supercritical state by pressurizing the reactor vessel with CO 2 gas at high pressure.

Supercritical CO2 Carbonation
Carbonation experiments using supercritical CO2 were performed using the following algorithm:


The sample diluted with supernatant water and concrete slurry waste is added to the reactor, which is then assembled. The concrete slurry water used in this study is shown in Figure 4.  The electric heater is powered on. Once the reactor vessel reaches the target temperature, CO2 is injected until the desired pressure is achieved.  Once the CO2 inside the vessel reaches the target pressure, the agitator is operated at 200 rpm to perform accelerated carbonation over the specified reaction time, while maintaining the temperature and pressure.  After the specified reaction time has elapsed, CO2 is released, the reactor is disassembled, and the sample is retrieved.   The maximum operating temperature and pressure of the supercritical CO 2 reactor were 80 • C and 200 bar, respectively, and the internal volume of the reactor was 4 L. In addition, the agitator was designed to enable control of the rotation speed up to 400 rpm.

Supercritical CO 2 Carbonation
Carbonation experiments using supercritical CO 2 were performed using the following algorithm: • The sample diluted with supernatant water and concrete slurry waste is added to the reactor, which is then assembled. The concrete slurry water used in this study is shown in Figure 4.

•
The electric heater is powered on. Once the reactor vessel reaches the target temperature, CO 2 is injected until the desired pressure is achieved.

•
Once the CO 2 inside the vessel reaches the target pressure, the agitator is operated at 200 rpm to perform accelerated carbonation over the specified reaction time, while maintaining the temperature and pressure.

•
After the specified reaction time has elapsed, CO 2 is released, the reactor is disassembled, and the sample is retrieved.

Supercritical CO2 Carbonation
Carbonation experiments using supercritical CO2 were performed using the following algorithm:


The sample diluted with supernatant water and concrete slurry waste is added to the reactor, which is then assembled. The concrete slurry water used in this study is shown in Figure 4.  The electric heater is powered on. Once the reactor vessel reaches the target temperature, CO2 is injected until the desired pressure is achieved.  Once the CO2 inside the vessel reaches the target pressure, the agitator is operated at 200 rpm to perform accelerated carbonation over the specified reaction time, while maintaining the temperature and pressure.  After the specified reaction time has elapsed, CO2 is released, the reactor is disassembled, and the sample is retrieved.  The supernatant water and concrete slurry waste are separated from the sample. The concrete slurry waste is dried at 105 °C until it reaches a constant weight.

Chemical Analysis
To investigate the mineralogical phase transition of the concrete slurry waste upon carbonation by supercritical CO 2 , SEM, XRD, and TG-DTA analyses and pH measurements were performed on samples before and after the carbonation reaction. The pH measurements were conducted on the eluate after mixing the sample with distilled water at a ratio of 1:5. To quantify the amount of CaCO 3 produced from the reaction, TG-DTA measurements were performed at a ramp rate of 10 • C/min over a temperature range from 25 to 1000 • C.

PH Measurement
The pH measurement results before and after supercritical CO 2 carbonation are shown in Figure 5. Prior to reaction, the supernatant water and concrete slurry waste had a pH above 12, while after reaction the pH was significantly lower (9.0-9.3). Moreover, pH measurements on supercritical CO 2 carbonation revealed no temperature and pressure dependence, but the pH was shown to decrease by 2% as the reaction time increased from 10 to 30 min. In general, the pH of high-purity CaCO 3 is 9.4; therefore, the pH of the products of the supercritical CO 2 carbonation reaction (9.0-9.3) indicate the conversion of CO 2 to CaCO 3 via the reaction shown in Equation (2).
CaO + CO 2 → CaCO 3 (2) In contrast, no products were formed from supernatant water and pH measurements could not be performed. As shown in Equation (3), CaCO 3 converts to Ca(HCO 3 ) 2 when CO 2 is supplied continuously. In generally Ca(HCO 3 ) 2 exists as an aqueous solution. It is understood that no products were formed due to the presence of Ca(HCO 3 ) 2 in its aqueous solution state because an excessive amount of CO 2 is available under supercritical CO 2 conditions of supernatant water that contains a small amount of Ca 2+ (800 mg/L). However, since the reaction shown in Equation (2) is reversible, Ca(HCO 3 ) 2 reverts to CaCO 3 at room temperature, along with release of CO 2 gas, and further research is required to clarify this point [28,29].

Chemical Analysis
To investigate the mineralogical phase transition of the concrete slurry waste upon carbonation by supercritical CO2, SEM, XRD, and TG-DTA analyses and pH measurements were performed on samples before and after the carbonation reaction. The pH measurements were conducted on the eluate after mixing the sample with distilled water at a ratio of 1:5. To quantify the amount of CaCO3 produced from the reaction, TG-DTA measurements were performed at a ramp rate of 10 °C/min over a temperature range from 25 to 1000 °C.

pH Measurement
The pH measurement results before and after supercritical CO2 carbonation are shown in Figure 5. Prior to reaction, the supernatant water and concrete slurry waste had a pH above 12, while after reaction the pH was significantly lower (9.0-9.3). Moreover, pH measurements on supercritical CO2 carbonation revealed no temperature and pressure dependence, but the pH was shown to decrease by 2% as the reaction time increased from 10 to 30 min. In general, the pH of high-purity CaCO3 is 9.4; therefore, the pH of the products of the supercritical CO2 carbonation reaction (9.0-9.3) indicate the conversion of CO2 to CaCO3 via the reaction shown in Equation (2).
In contrast, no products were formed from supernatant water and pH measurements could not be performed. As shown in Equation (3), CaCO3 converts to Ca(HCO3)2 when CO2 is supplied continuously. In generally Ca(HCO3)2 exists as an aqueous solution. It is understood that no products were formed due to the presence of Ca(HCO3)2 in its aqueous solution state because an excessive amount of CO2 is available under supercritical CO2 conditions of supernatant water that contains a small amount of Ca 2+ (800 mg/L). However, since the reaction shown in Equation (2) is reversible, Ca(HCO3)2 reverts to CaCO3 at room temperature, along with release of CO2 gas, and further research is required to clarify this point [28,29].  Figure 6 shows the SEM characterization results of concrete slurry waste produced during supercritical CO 2 carbonation. In general, in the case of Ca(OH) 2 , a microcrystalline layer of CaCO 3 is densely formed on the surface of the particle during the carbonation reaction with supercritical CO 2 and diffuses inwards to approach complete carbonation as the reaction proceeds [13]. In this study, particles of the concrete slurry waste before reaction showed spherical morphology without any crystal layers on the surface; however, after supercritical CO 2 carbonation, this morphology changed to microcrystalline layers on the surface of the particle regardless of temperature, pressure, and reaction time, indicating the progression of carbonation by supercritical CO 2 .

SEM
Materials 2022, 15, x FOR PEER REVIEW 6 of 10 Figure 6 shows the SEM characterization results of concrete slurry waste produced during supercritical CO2 carbonation. In general, in the case of Ca(OH)2, a microcrystalline layer of CaCO3 is densely formed on the surface of the particle during the carbonation reaction with supercritical CO2 and diffuses inwards to approach complete carbonation as the reaction proceeds [13]. In this study, particles of the concrete slurry waste before reaction showed spherical morphology without any crystal layers on the surface; however, after supercritical CO2 carbonation, this morphology changed to microcrystalline layers on the surface of the particle regardless of temperature, pressure, and reaction time, indicating the progression of carbonation by supercritical CO2.

XRD Measurement Results
XRD patterns obtained before and after supercritical CO2 carbonation are displayed in Figures 7 and 8, respectively. Peaks indicating the presence of Ca(OH)2 and a small amount of calcite were found in the XRD pattern of concrete slurry waste before reaction; after reaction, the Ca(OH)2 peak disappeared, while peaks reflecting the presence of calcite and a small amount of aragonite were detected. Further, comparing the XRD patterns of the reaction products obtained at different reaction temperatures, pressures, and times showed that similar peaks appeared irrespective of the reaction conditions. The three crystal structures of CaCO3 are aragonite, vaterite, and calcite, among which calcite is the most stable form [21,30]. According to reported studies, calcite is mainly formed when Ca 2+ /CO3 2− ≤ 1. The supercritical CO2 conditions in this study also produce a large amount of CO3 2− and, therefore, a decrease in Ca 2+ /CO3 2− ratio indicates that calcite is the main reaction product [16].

XRD Measurement Results
XRD patterns obtained before and after supercritical CO 2 carbonation are displayed in Figures 7 and 8, respectively. Peaks indicating the presence of Ca(OH) 2 and a small amount of calcite were found in the XRD pattern of concrete slurry waste before reaction; after reaction, the Ca(OH) 2 peak disappeared, while peaks reflecting the presence of calcite and a small amount of aragonite were detected. Further, comparing the XRD patterns of the reaction products obtained at different reaction temperatures, pressures, and times showed that similar peaks appeared irrespective of the reaction conditions. The three crystal structures of CaCO 3 are aragonite, vaterite, and calcite, among which calcite is the most stable form [21,30]. According to reported studies, calcite is mainly formed when Ca 2+ /CO 3 2− ≤ 1. The supercritical CO 2 conditions in this study also produce a large amount of CO 3 2− and, therefore, a decrease in Ca 2+ /CO 3 2− ratio indicates that calcite is the main reaction product [16].     Figure 9 and Table 3 present the TG-DTA results obtained before and after supercritical CO2 carbonation. Generally, the major cement hydrate, Ca(OH)2, undergoes pyrolysis near 450-550 °C and reacts with CO2 to form CaCO3, which pyrolyzes at 600-800 °C. The TG-DTA measurement on the sample before supercritical CO2 carbonation revealed small weight losses for Ca(OH)2 and CaCO3. Measurements after the reaction showed no signs of Ca(OH)2 weight loss, but showed signs of CaCO3 weight loss regardless of the temperature, pressure, and reaction time. Therefore, it was confirmed that the sludge with a solids content of 5% underwent complete carbonation in only 10 min via supercritical CO2 carbonation.  Table 3 present the TG-DTA results obtained before and after supercritical CO 2 carbonation. Generally, the major cement hydrate, Ca(OH) 2 , undergoes pyrolysis near 450-550 • C and reacts with CO 2 to form CaCO 3 , which pyrolyzes at 600-800 • C. The TG-DTA measurement on the sample before supercritical CO 2 carbonation revealed small weight losses for Ca(OH) 2 and CaCO 3 . Measurements after the reaction showed no signs of Ca(OH) 2 weight loss, but showed signs of CaCO 3 weight loss regardless of the temperature, pressure, and reaction time. Therefore, it was confirmed that the sludge with a solids content of 5% underwent complete carbonation in only 10 min via supercritical CO 2 carbonation.

Conclusions
In this study, supercritical CO 2 reactions were shown to reduce CO 2 emissions when utilized in the developed carbon capture and utilization technology using concrete slurry water produced during concrete manufacturing as a new means for CO 2 sequestration. The major findings are as follows: • For supernatant water under supercritical CO 2 conditions, reaction products could not be confirmed due to Ca(HCO 3 ) 2 existing as an aqueous solution in the presence of an excess amount of injected CO 2 . However, since the reaction is reversible, CaCO 3 can precipitate from aqueous Ca(HCO 3 ) 2 over time at room temperature, along with the release of CO 2 gas; hence, further research is necessary.

•
Mineral carbonation of concrete slurry water by supercritical CO 2 demonstrated that complete carbonation can be achieved in only 10 min of reaction at a sludge solids content of 5%. However, the reaction seemed to be independent of the supercritical CO 2 temperature and pressure at 5% sludge solids content. Further investigation on the change in the reaction with respect to sludge solids content is to be performed.

•
In future studies, quantitative analyses on supercritical CO 2 carbonation (optimal temperature, pressure, and reaction time) and CO 2 storage capacity with respect to sludge solids content will be conducted.

•
Collating the results, it is possible to fix CO 2 via supercritical CO 2 carbonation, and the implementation of a new CO 2 fixation source with concrete slurry water is considered viable. The implementation of supercritical CO 2 mineral carbonation technology using concrete slurry water will enable carbon-neutrality to be achieved by reducing greenhouse gas emissions from not only cement industries, but all industrial sectors.

Data Availability Statement:
The data presented in this study are available upon request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.