Effect of Mixing and Curing System on Carbon Fixation Amount and Performance of Circulating Fluidized Bed Fly Ash Cement Cementitious Material System

Abstract

The use of industrial solid waste to capture and fix CO₂ is a promising technology for CO₂ sequestration. A thermogravimetric analyzer and CO₂ cement hydration mixing device were used to study the effects of mixing method, curing system, temperature, CO₂ concentration and other factors on the carbon fixation amount and performance of the circulating fluidized bed fly ash cement-based material system. The results showed that the carbon fixation and early strength of the cementitious materials could be improved by adding CO₂ in the stirring process and making CO₂ directly participate in the process reaction. The cementing materials samples prepared with CO₂ were cured in a standard curing box for 2 days and a carbon atmosphere for 1 day, the carbon fixation amount of the cementing material was increased by 33% and the compressive strength of the cementing material was also improved. This is because under the combined action of carbon mixing and carbon curing, the prepared binding materials produced more Ca(OH)₂ in the early stage, and it reacts with the introduced CO₂ to form CaCO₃. The strength of the calcium carbonate crystals is higher than the strength of the earlier stage of cement, and at the same time, the samples would solidify more CO₂. Considering the carbon fixation amount, sample performance and solid waste utilization rate, the best conditions for the cementing materials are as follows: the content of the circulating fluidized bed fly ash (CFA) was 35%, the concentration of carbon curing was 30%, the curing temperature was 40 ℃, the water-binder ratio was 0.4, and the carbon fixation amount of the cementing material could reach about 20%. The use of CFBFA to solidify and storge CO₂ is not only a new way to utilize high value-added fly ash resources, but also beneficial for reducing industrial carbon dioxide emissions.

1. Introduction

Owing to the rapid advancement of industry, atmospheric carbon dioxide levels have escalated from a pre-industrial concentration of 280 ppm to a current peak of 412 ppm. It is imperative to maintain atmospheric carbon dioxide below 550 ppm [1]. Achieving this goal by 2050 necessitates a reduction in global carbon dioxide emissions by 50%–60% [2,3,4,5]. Carbon neutrality is defined as the process whereby enterprises, groups, or individuals calculate their total greenhouse gas emissions over a specified period and neutralize their carbon footprint through afforestation, energy conservation, and emission reduction measures, thereby attaining a state of “zero carbon dioxide emissions”. Currently, strategies for CO₂ emission reduction adopted by various countries include enhancing existing energy utilization efficiency, augmenting renewable energy use, and implementing carbon capture, storage, and utilization (CCUS). Utilizing alkaline material minerals for CO₂ sequestration represents a novel storage concept, offering beneficial applications for rapidly accumulating waste [6].

The production process in many industrial sectors generates substantial solid waste [7]. In the metallurgical industry, examples include magnesium slag, lead–zinc slag, and steel slag. Additionally, coal combustion yields solid wastes such as fly ash and bottom ash, while municipal solid waste incineration produces fly ash, bottom ash, and air pollution control residues. These by-products are considered potential materials for CO₂ solidification. In mineral sequestration, atmospheric CO₂ is absorbed by alkaline oxides and hydroxides in the material, forming a stable carbonate. Mineral carbonation is an acid–base reaction in which the acid (H₂CO₃) formed by dissolving CO₂ in water is neutralized by a solid base (basic mineral) [8,9].

Fly ash, a byproduct of thermal power generation, is extensively utilized in cement and concrete production [10]. Liu Wei [11] investigated the direct solidification of CO₂ by fly ash in a circulating fluidized bed, revealing that at 600 °C with a water–cement ratio of 0.2, 60 g of CO₂ can be solidified per kilogram of fly ash. This process results in the formation of a dense carbonate layer on the surface due to the Ca²⁺ in coal ash. Alessandro Mazzella [12] examined the CO₂ absorption capacity of fly ash under varying pressures and temperatures, finding that at 115 bar and 45 °C, the absorption per kilogram of dry ash could exceed 180 g. At lower pressures (<7.5 bar), the CO₂ capture rate appears to increase solely with temperature. The maximum carbonization efficiency, contingent on the initial CaO content, reached 74%. Analysis of the reaction equations of various minerals in cement indicates that C₃S, C₂S, C₄AF, and C₃A can all react with CO₂. Gibbs free energy calculations show the reaction rates of these minerals with CO₂ in the order: C₃S > C₄AF > C₂S > C₃A. During the cement hydration reaction, the sequence is C₃A > C₃S > C₄AF > C₂S, with C₃A exhibiting minimal reaction with CO₂. Hence, to enhance carbon fixation during cement hydration, it is crucial to increase the CO₂ amount in the initial hydration phase. Literature suggests that elevating CO₂ concentration and pressure during the reaction can boost the carbonation of gelling materials [13,14,15,16]. The impact of temperature on CO₂-cured cement-based materials is complex, as temperature influences the carbonization reaction bidirectionally. On one hand, it reduces the solubility of CO₂ and calcium ions in the cement solution, diminishing reactant concentration and inhibiting carbonation [16,17]. Conversely, a temperature rise speeds up CO₂ gas diffusion in the cement matrix, thus accelerating ion movement in the carbonization reaction, and consequently increasing the chemical reaction speed [18,19].

Recent studies on concrete carbonation curing have focused on optimizing curing conditions to enhance the carbonation reaction [20,21]. However, early-stage hydration carbonation, particularly during the mixing process, has been less explored. This study introduces CO₂ in the cement mixing process, termed carbon mixing, and adds circulating fluidized bed fly ash with a high specific surface area to improve the carbonization process [22]. This aims to enhance the carbon fixation capacity and performance of the cementitious material system through an appropriate curing system. The goal is to provide technical support for industrial production of high carbon-fixed concrete prefabricated products, reducing carbon emissions and economic expenditures in high-emitting plants.

2. Experiment

2.1. Raw Materials

The circulating fluidized bed fly ash raw material was produced by Huangling Power Plant, the specific surface area of the specimen was 531 kg/m³, and the density was 2.59 g/cm³. The main chemical composition and mineral phase composition of CFA is shown in Table S1 and Figures S1 and S2. The chemical composition of CFA is very complex, including various substances such as silicates, aluminates, ferrites, etc. The presence of these substances has an important impact on the performance of CFA, causing it to have a certain degree of activity and hydration performance. The chemical composition of CFA is of great significance for its applications in fields such as building materials, cement manufacturing, and road construction. XRF results showed that the main components of CFA, SiO₂, Al₂O₃, and CaO, were 48.10 wt%, 23.68 wt%, and 12.01 wt%, respectively. According to the XRD diffraction pattern of CFA, the main mineral phases of CFA were quartz and tricalcium silicate, as well as a small amount of iron oxide, which could react quickly with CO₂.

2.2. Methods

2.2.1. Carbon Mixing Equipment

The CO₂ mixing reaction device used in the cement hydration process is shown in Figure 1. The CO₂ mixing equipment consisted of a mixing pot and a CO₂ gas cylinder. The carbon dioxide used in the cement hydration process was supplied by industrial cylinders containing CO₂ gas of 99.9% purity, which were equipped with pressure-reducing valves to control the CO₂ output rate. The mixing pot consisted of a CO₂ concentration meter and a hand-held mixer.

2.2.2. Experimental Scheme

Table S2 outlines the experimental design aimed at investigating the effects of carbon content and carbon curing on cementitious material systems. The experimental plan is divided into a pure cement test group and a control group containing 30% CFA (for example, 1 kg of CFA cementitious material includes 700 g of cement and 300 g of CFA ash). The mixing method is as follows: before mixing, the container should be filled with CO₂, and then the dry materials are poured into the container and dry-mixed for 30 s in the CO₂-filled container. Subsequently, water is added for mixing, and CO₂ is continuously introduced during the mixing process. The mixture is stirred for 60 s, allowed to stand for 60 s, and then stirred again for 60 s before being placed into molds. The molds are then vibrated on a vibration table for 60 s, and the surface is leveled before being placed in a curing chamber for 24 h. After 24 h, the samples are demolded and subsequently cured. After mixing according to the procedure, the mixture is placed into molds. The carbon curing regime is as follows: CO₂ concentration is 20%, with fluctuations in CO₂ concentration not exceeding 1% during the curing process. The pressure is standard atmospheric pressure, the temperature is 20 ± 1 °C, and the humidity is 95% ± 1%. In the standard curing system, except for the absence of CO₂ concentration, the temperature and humidity curing parameters are consistent with the carbon curing. In the experiment, the water–cement ratio is 0.3, and the size of the formed sample blocks is 40 mm × 40 mm × 40 mm. Table S3 presents the experimental plan for the effects of CFA content, water–cement ratio, and CO₂ concentration on carbon sequestration. Through a three-factor, two-level orthogonal experimental design, combined with the results of 7 days of compressive strength and carbon sequestration, the appropriate CO₂ curing concentration for cementitious materials is optimized.

2.3. Characterization of Carbon Sequestration

The theoretical carbon fixation amount of the sample is calculated according to Steinour’s formula, as shown in Formula (1) [23]. The actual carbon fixation amount of the cementitious material is calculated based on the TG data of the sample [24]. The decomposition temperature of Ca(OH)₂ is about 450 °C, and it is generally believed that the weight loss at 500~1000 °C is the decomposition amount of CaCO₃. Thus, the carbon fixation amount of the sample can be calculated, as shown in the Formula (2).

$${\mathrm{CO}}_2\mathrm{uptake}(\%)=0.785(\mathrm{CaO}-0{.7\mathrm{SO}}_3)+1{.09\mathrm{Na}}_2\mathrm{O}+0{.93\mathrm{K}}_2\mathrm{O}$$

(1)

$${\mathrm{CO}}_2\mathrm{uptake}(\%)={\displaystyle \frac{\mathrm{weight}\mathrm{loss}\mathrm{of}\mathrm{decarbonation}}{\mathrm{sample}\mathrm{weight}-\mathrm{total}\mathrm{weight}\mathrm{loss}}}\times 100\%$$

(2)

To ascertain the optimal age for representing the actual carbon fixation capacity of cementitious materials, specimens measuring 40 mm × 40 mm, with a Z7 mix ratio, were molded. The hydration process of these materials at 1 day, 3 days, 7 days, and 14 days was analyzed through thermogravimetric curves, as shown in Figure S3. The weight loss rate between 500 and 1000 °C was calculated, with the results presented in Table S4. Figure S3 and Table S4 indicate a rapid increase in carbon fixation in the early stages, stabilizing from the seventh day onwards [25]. Thermogravimetric testing at 7 days was measured by weight loss.

2.4. Preparation of Powder Samples

For the thermogravimetric curve measurement, mineral phase analysis, and pH value determination of hydrated cementitious materials, samples were selected and prepared. Test blocks were maintained until the corresponding age, crushed, and core samples were randomly selected and immersed in absolute ethanol for at least 24 h to halt hydration. Prior to testing, the hydrated samples were dried in a vacuum oven at 65 °C, ground to D50 = 20 ± 5 µm, and stored in sealed plastic bags for subsequent analysis.

2.5. Characterization

Using the NETZSCH STA2500 thermal analysis instrument manufactured in Germany to perform thermal analysis on the hydration products powder of gel materials. The temperature range was room temperature up to ~1000 °C, the heating rate was 20 °C/min, and the atmosphere was N₂. Mineral phase analysis (Bruker D8 Advance, Berlin, Gremany) was performed on the cementitious material hydration product powder. The powder sample was scanned with Cu-α radiation (λ = 1.5406 Å) at a scanning rate of 10°/min and a scanning range of 5~90°.

3. Results and Discussion

3.1. CFA Mineralogy Analysis and Adsorption Properties

SEM images revealed that CFA, except for crystallized quartz, predominantly exhibits an amorphous porous morphology conducive to CO₂ adsorption. As shown in Figure 2, the CO₂ adsorption–desorption curve of CFA indicates a porosity of 73%, compared to approximately 50% for Portland cement. The maximum adsorption capacity of CFA was found to be 1.12 cm³/g.

3.2. Effect of CO₂ Mixing and Curing System on the Properties of Cementitious Materials

According to the test plan in Table S1, 12 mixing and curing mixing plans for the two materials were tested for the cementitious material compounded with 70% cement and 30% CFA (referred to as the CFA cementitious material group). Finally, an optimal mixing and curing system was determined based on the strength and carbon fixation amount. Since the centralized curing method is used in the production process of precast concrete, long-term curing energy consumption is high, so this test curing system only studied the curing system for 3 days.

Comparison of the 3-day (3d), 7-day (7d), and 28-day (28d) strengths of the cement group, as shown in Figure 3A, reveals that the inclusion of CO₂ during mixing enhanced the 3d strength for both curing systems 2 and 4 and improved the strengths of curing systems 1, 2, 3, 4, and 6. The 7d strength increased for groups 3 and 4, while the 28d strength remained largely unaffected in the other groups. This suggests that CO₂ mixing enhances the early strength of cement without significantly impacting its long-term strength. Moreover, the compressive strength of cement improves as the carbon curing time extends. Further analysis of the 3d, 7d, and 28d strengths of the CFA group, depicted in Figure 3B, reveals that the inclusion of CO₂ during mixing enhances the 3d strength for groups 2, 5, and 6, as well as the 7d strength for groups 2, 3, 4, 5, and 6. Additionally, the strength per day increases the 28d strength for groups 2, 3, 4, 5, and 6. Consequently, it can be concluded that CO₂ mixing combined with carbon curing significantly improves the strength of the CFA group. During the 3-day curing cycle, the highest strength is observed in the CFA group subjected to standard curing for 2 days followed by carbon curing for 1, 2, and 3 days, respectively. The benchmark cement group had a 28d strength of 65.2 MPa, while under these three curing methods, the 28d strengths are 76.2 MPa, 78.4 MPa, and 77.2 MPa, respectively.

3.3. Study of Carbon Sequestration under Different Mixing and Curing Systems

Figure 4a presents the thermogravimetric spectrum of cementitious materials from the 5 groups with superior strength performance (

Table 1. Carbon fixation amount of cementitious materials under different curing systems.

Group	a1	a6	a11	a22	a66	b1	b6	b11	b33	b66
Carbon sequestration/%	9.34	11.88	11.81	11.54	11.96	11.45	11.24	11.95	11.66	12.45

), particularly after 7 days of hydration. The carbon fixation amount of sample a1 in the reference group was calculated at 9.34%, while the maximum carbon fixation was noted in group a66 at 11.96%. The highest strength at 3d and 7d was found in the a22 group, with a carbon fixation capacity of 11.54%. Thus, for an optimal balance of strength and carbon fixation, a curing system for the cement group involving carbon mixing, followed by a day of carbon curing and subsequent standard curing, is advisable. The 3d and 7d strength observations suggest that extending carbon curing duration does not benefit strength enhancement, as early carbonation reaction forms a protective carbonate layer on the cement surface, impeding normal hydration.

Figure 4b exhibits the thermogravimetric spectrum of the 5 CFA groups showing enhanced strength after 7 days of hydration. Analysis reveals that cementitious materials containing 30% CFA achieved higher carbon fixation than pure cement. The b66 group exhibited the highest carbon fixation capacity at 12.45%. Additionally, strengths at 3d, 7d, and 28d were all superior in the b66 group compared to both the pure cement group and the CFA group with standard curing. Consequently, the recommended mixing and curing system for CFA cementitious materials involves beginning with two days of standard curing post-carbon mixing, followed by a day of carbon curing, and then continuing with standard curing until the desired strength is attained. Immediate carbon curing after mixing hampers the hydration of low-activity CFA cementitious materials. Hence, initial standard curing post-mixing is recommended to allow cement hydration and CFA activation. Subsequent carbon curing then aids the carbonation reaction between CO₂ and calcium hydroxide, tricalcium silicate, and dicalcium silicate in both cement and CFA.

3.4. Mineral Phase Analysis of Cementitious Materials with Different Mixing and Curing Systems

Figure 5a–c indicate that the primary hydration products in the CFA gelling materials were Ca(OH)₂, C₃S, C2S, and CaCO₃. Ca(OH)₂, C₃S, and C₂S all reacted with CO₂ to form CaCO₃. Due to the instability of Ca(OH)₂. C₃S was primarily present during early hydration. At 28 days, the main carbonation product was CaCO₃. The XRD patterns, as displayed in Figure 5d, show a progressive decrease in the Ca(OH)₂ peak and an increase in the CaCO₃ peak as hydration advanced. The CFA group, particularly with CO₂ mixing and carbon curing, exhibited more pronounced CaCO₃ peaks, indicating these processes favored the carbon fixation reaction in CFA cementitious materials. Notably, the CaCO₃ peak values in groups b66 and b33 surpassed those in other test groups.

3.5. Micromorphology of Cementitious Materials during Hydration Process

Figure 6 presents SEM images of two CFA hydration groups (3d and 28d) with superior carbon fixation and strength performance. Under identical magnification, both groups, subjected to CO₂ mixing and carbon curing, exhibited significant calcite presence in the microscopic topography. At the early hydration stage, the carbonation reaction precedes cement hydration, with calcium carbonate as the primary product. In the 28d spectrum, the calcium carbonate content was markedly lower than in hydrated calcium silicate, the hydration product of cement minerals. The abundant early-stage calcium carbonate contributed to the higher strength of the CFA system compared to the cement benchmark group. In later stages, calcium silicate in CFA and minerals like tricalcium silicate in cement, forming calcium silicate hydrate, became the main products, resulting in comparable strengths at 28 days between the CFA and cement groups. In some experimental groups, the 28-day strength under carbon mixing and curing conditions was lower than under standard curing. This is attributed to early-formed calcium carbonate encasing part of the cementitious material, thereby affecting normal hydration and the production of calcium silicate hydrate. Circled in the picture are calcite crystals.The reaction diagram is illustrated in Figure 7, leading to reduced system strength in later stages.

3.6. Study on Factors Affecting Carbon Sequestration

An investigation into the impact of water-to-binder ratio, CFA dosage, and CO₂ curing concentration on the carbon fixation amount in CFA cementitious materials identified several suitable application methods.

Table 2. Orthogonal test factors and test results.

Test No.	Fly Ash Content	Water-Cement Ratio	Carbon Concentration	3-Day Compressive Strength	7-Day Compressive Strength	28-Day Compressive Strength	Fixed Carbon Content
1	25%	0.4	20%	34	43.2	52.5	11.07
2	25%	0.45	25%	22.4	32.6	42.6	10.85
3	25%	0.5	30%	24.5	29.3	33.3	13.92
4	30%	0.4	25%	40.2	46.3	60.8	8.88
5	30%	0.45	30%	26	35.2	40.8	14.14
6	30%	0.5	20%	23.3	28.1	42.7	9.16
7	35%	0.4	30%	24.5	31.6	43.2	16.27
8	35%	0.45	20%	28.5	40.5	49.6	11.4
9	35%	0.5	25%	17.7	25	31.2	13.98

outlines the three influencing factor schemes and test results. Optimal parameters were determined based on the balance of carbon fixation amount and strength.

Table 2 reveals that group 7 exhibited the highest carbon fixation, with the water–cement ratio being the principal factor influencing the strength of the cementitious material. The test results indicate that the compressive strength at a water–cement ratio of 0.4 was superior to the other two groups. The cement used in this experiment was P.O. 42.5, and the 28-day compressive strength of the cementitious material exceeded 42.5 MPa, satisfying the basic strength requirements. However, the determinants of carbon sequestration are multifaceted, including the water–cement ratio, high calcium ash content, and carbon dioxide concentration. The orthogonal test results show that the groups with a 30% carbon concentration exhibited the highest carbon fixation, and similarly, those with 35% high calcium ash content had the highest unit test carbon fixation. Considering carbon fixation, compressive strength, CFA usage, and carbon dioxide concentration, the ratio in group 7 was optimal, as it not only achieved the highest carbon sequestration but also met usage requirements and maximized CFA solid waste utilization.

3.7. Effect of Curing Temperature on Carbon Sequestration

The ratio from 3.6 was employed to study the influence of temperature on the carbon fixation of CFA cementitious materials, aiming to ascertain the most suitable curing temperature.

Table 3. Carbon fixation capacity of CFA at different carbon curing temperatures.

Temperature/°C	20	30	40	50	60
Fixed carbon content/%	16.27	19.64	20.70	21.29	19.84

shows that the curing temperature impacted the carbon fixation of CFA materials, with an increase in carbon fixation corresponding to a rise in temperature. However, beyond 50 °C, the carbon fixation began to decline. The fixed carbon content at 40 °C and 50 °C exceeded 20%. The reactions of carbon dioxide with water and calcium hydroxide to form calcium carbonate are exothermic. High temperatures do not promote carbonation, unlike cement hydration. The test results indicate that 40–50 °C is an optimal temperature range. Subsequent strength testing confirmed that this range not only ensures CFA cementitious material strength but also maximizes system carbon fixation. Considering both strength and economy, the most suitable curing temperature for CFA cementitious materials is 40 °C.

4. Conclusions

This study explores the process of CO₂ solidification in coal fly ash (CFA) gelling materials, utilizing thermogravimetric analysis (TG) and a designated reaction vessel. It focuses on how different mixing and curing methods affect the strength and carbon capture capabilities of CFA-based cement materials. Additionally, the study delves into the effects of the water-to-cement ratio, carbon dioxide concentration, CFA ash content, and temperature on these properties. Through examination of the chemical makeup, physical structure, and mineral phase shifts during CO₂ curing, the research sheds light on how carbonation levels vary across different curing conditions. The main findings are as follows: (1) Incorporating carbon into the mix significantly boosts carbon capture in cementitious materials containing coal fly ash (CFA), especially under standard atmospheric pressure. When 30% CFA ash is used instead of pure cement, the carbon capture efficiency jumps by 28-fold. This addition not only enhances environmental benefits by increasing carbon capture but also improves the material’s mechanical properties. Specifically, the process leads to a 6% rise in compressive strength after three days, a 13% increase after seven days, and a 9% boost after 28 days. Moreover, a method involving two days of carbon mixing followed by a day of carbonation further elevates carbon capture by 33% over the baseline while concurrently enhancing the compressive strength of the material. This approach presents the dual advantage of significantly mitigating carbon footprint and strengthening the structural integrity of the cementitious materials. (2) Increasing the concentration of carbon curing and the content of coal fly ash (CFA) improves the carbon capture in cement-based materials. The ratio of water to cement affects the material’s strength, and higher temperatures can decrease the amount of carbon captured. The best conditions for maximizing CO₂ solidification in CFA-based cement materials, while also meeting strength criteria, include a curing temperature between 40–50 °C, a CFA content of 35%, a carbon curing concentration of 30%, and a water-to-cement ratio of 0.4. (3) CFA’s ability to solidify CO₂ through carbonation is notable, particularly in early hydration stages, when calcium carbonate formation can encase the cementitious material, hindering cement hydration. Consequently, the early strength of CFA cementitious materials tends to surpass that of pure cement, but after 28 days, their strengths converge. In contrast, cement maintains consistent strength throughout hydration. Employing carbon mixing and curing together generates more Ca(OH)₂ in early stages, which reacts with CO₂ to form CaCO₃, allowing the system to solidify a greater amount of CO₂.