Carbonation Reaction Research Articles

Heat storage and release characteristics of a prototype CaCO3/CaO thermochemical energy storage system based on a novel fluidized bed solar reactor

CaCO3/CaO thermochemical energy storage (TCES) system has a high heat storage density (1780 kJ/kg) along with high heat storage and release temperature (650–850 °C), which can be applied to concentrated solar power (CSP) technology utilizing CO2 Brayton cycles to improve power generation efficiency. There are several problems to be urgently resolved in current CaCO3/CaO solar calciners, such as poor heat and mass transfer and low thermochemical efficiency. This paper proposed a prototype CaCO3/CaO TCES system based on a novel fluidized bed solar reactor, which has a serrated arc surface in alignment with the direction of the incident solar rays to receive concentrated solar energy. The natural limestone particles were used as the reactive particles. The effects of operating modes (intermittent/continuous), initial mass of reactive particles, and initial carbonation temperature Tcar,initial on the heat storage and release characteristics of the TCES system were experimentally explored. During the carbonation reaction stage, the effective carbonation degrees in the continuous mode were generally higher than in the intermittent mode under the experimental conditions and declined at a slower rate. The reactive particles were recovered from the reactor for microstructural characterization after completing the experiments, high-temperature particle agglomeration and pore structure clogging were directly responsible for the decrease of the effective carbonation degree. In the continuous mode, the average thermochemical and total thermal efficiencies of 24.7 % and 53.5 % were obtained in the 1st calcination reaction, respectively, which represents significant advantages over other CaCO3/CaO solar calciners. The novel fluidized bed reactor demonstrated excellent fluidization in the experiments and did not exhibit mechanical deformation after multiple experiments under non-uniform concentrated radiation provided by an 84 kWe high-flux solar simulator. Overall, this paper provides a successful gas-solid reactor design to be coupled with CSP technology.

In-situ investigation on the carbonation behaviors of various mineral phases in steel slag: The role of RO phase

Exploring the mineral compositions of steel slag and the microstructure evolutions of its carbonation products is very important for deeply understanding the carbonation mechanism of steel slag, which can promote the application of steel slag in carbon capture and storage. In this paper, the carbonation behaviors of various mineral phases in steel slag were investigated by using an ingenious in-situ observation method through combing laser scanning confocal microscopy (LSCM), backscattered scanning electron microscopy (BSEM) and focused ion beam-transmission electron microscope (FIB-TEM). It is shown that dicalcium silicate (C2S) and free lime (f-CaO) had high carbonation reactivity, while the calcium aluminoferrite with different Al/Fe ratios and mayenite (C12A7) exhibited extremely poor carbonation reactivity. The carbonation reactivity of RO phase was positively correlated with its Mg/(Fe + Mn) ratio. RO phase with a Mg/(Fe + Mn) ratio below 0.5 owned almost no carbonation reactivity, while the Mg2+ would leach from RO phase with a Mg/(Fe + Mn) ratio higher than 2.0 during the carbonation process, inhibiting the precipitation of CaCO3, changing the micro morphology of CaCO3, and generating spheroidal aragonite. The ingenious in-situ observation method developed in this study provides a very visual information, which can be further extended to more fundamental research.

Semi-dry and aqueous carbonation of steel slag: Characteristics and properties of steel slag as supplementary cementitious materials

Accelerated carbonation of steel slag presents a viable method for CO2 sequestration and the valorized utilization of steel slag. This study aimed to investigate the impact of semi-dry and aqueous carbonation, as well as the carbonation degree, on the microstructure and properties of steel slag. Additionally, it evaluated the feasibility of utilizing carbonated steel slag as supplementary cementitious materials (SCMs). Steel slag was subjected to CO2 (20% volume percentage) at 2 bar pressure for both semi-dry and aqueous carbonation process at room temperature with water/solid ratio of 0.08 and 5, respectively. The carbonation duration was varied (10 min, 30 min, 60 min, and 120 min) to adjust the degree of carbonation. Mortars and pastes with 30 wt% carbonated steel slag were formulated and their compressive strength and volume expansion values were quantified. The findings revealed that the CO2 uptake values for steel slag after 2 hours of semi-dry and aqueous carbonation were only 3.75% and 1.50%, respectively. Compounds such as Ca(OH)2, Mg(OH)2, and Ca2SiO5 demonstrated high carbonation reactivity during the semi-dry carbonation process, whereas Ca(OH)2 and Mg(OH)2 exhibited only a marginal increase in aqueous carbonated steel slag. The specific surface area of steel slag increased with an increase in the carbonation degree, attributed to the formation of silica gel or amorphous C-S-H during carbonation. However, the hydration activity of carbonated steel slag diminished with an increased degree of carbonation due to the consumption of alkaline minerals. Conversely, pozzolanic activity was enhanced with a higher degree of carbonation, owing to the formation of silica gel. The cumulative heat release of cement containing SDC30 (semi-dry carbonation for 30 min) within 24 h and 72 hours showed increases of 16.0% and 3.5%, respectively, aligning with the compressive strength results. Semi-dry carbonation treatment significantly enhanced the early-stage cementitious properties of steel slag/cement composites. Specifically, the 3-day compressive strength of mortar containing 30 wt% SDC30 increased by 22.7% relative to the reference mortar, composed of 30 wt% uncarbonated steel slag and 70 wt% Portland cement. Carbonation markedly improved the volumetric stability of steel slag with the extent of improvement was positively correlated with the carbonation degree for both semi-dry and aqueous carbonated steel slag. Aqueous carbonation was more effective in promoting the hydration process of free CaO and MgO more effectively than semi-dry carbonation.

Strengthening mechanism of red mud with calcium oxide

Currently, the utilization rate of red mud is very low, and the problems of occupying land and polluting the environment are becoming increasingly prominent. This study utilized CaO to prepare Red Mud-based cementitious material (RMC) to achieve clean and efficient utilization of red mud. The mechanism of Ca2+ enhancing the strength of RM and the carbonation reaction in RMC were studied using X-ray diffraction (XRD), Fourier transform infrared spectrum (FTIR), Thermogravimetric Analysis (TGA), Scanning electron microscopy (SEM) and other analysis methods. The results showed that the carbonization of CaO in RMC bonded the red mud particles and provided early strength. CaO activated the components of gibbsite, diaspore, and silica in RM, and hydration reaction generated gel-like Al-substituted tobermorite (Ca5Si5Al(OH)O17·5 H2O), ensuring the increase of later strength. At the age of 28 days, the RMC with a CaO content of 25% had the highest unconfined compressive strength, reaching 2.19 MPa. When the CaO content exceeded 25%, the compressive strength of RMC gradually decreased. This is due to the reaction of calcium carbonate and aluminate to form a large number of sheets of calcium aluminate monocarbonate (Ca4Al2O6CO3·11 H2O), which weakened the carbonation binding effect and competitively consumed aluminate, resulting in an increase in structural cracks and pores, and a decrease in the compressive strength of RMC.

Elucidating hardening mechanism of carbonatable binders prepared with steel slag based on distribution of microhardness

To ascertain the hardening process of carbonatable binders prepared with steel slag, this study examined the distribution of carbonation products on the surface of steel slag particle and the microhardness distribution of both the particles and the compact. The experimental results indicated that carbonation initiates in the active area rich in calcium silicate phase, surrounded by the RO phase (a solid solution formed from FeO, MgO, MnO, and other divalent metal oxides). In this region, calcium carbonate gradually grows, condenses, and spreads outward, eventually covering the surface of steel slag particle. Therefore, the microhardness of the carbonated steel slag particle shows a significant statistical distribution, corresponding to the calcium silicate phase, RO phase, and carbonation products, respectively. After carbonation, the carbonation matrix of compact forms progressively, including the voids between particles (microhardness below 100 HV 0.05), the carbonated area (microhardness between 100 and 200 HV 0.05), the transition zone (microhardness between 200 and 350 HV 0.05), and the uncarbonated area (microhardness above 350 HV 0.05). Moreover, through the statistical analysis of microhardness distribution of various region, the gradient change of carbonation reaction degree of compact is accurately obtained, which demonstrates the hardening process from the outside inward of carbonatable binders compact. This research sheds new light on the elucidation of hardening process of carbonatable binders.

Study on Controlled Synthesis of Mesoporous Pseudoboehmite via Carbonation Reaction

Study on Controlled Synthesis of Mesoporous Pseudoboehmite via Carbonation Reaction

Study on the carbonation properties of BOFS with γ-C2S blending

β-C2S is the main active mineral component in Basic Oxygen Furnace slag (BOFS), while the content of β-C2S in BOFS is usually limited and fluctuated, which hindered the improvement of CO2 capture during carbonation. To enhance the carbonation reactivity of BOFS products, different ratio of γ-C2S was blended with BOFS powder and carbonated, followed by hydration curing. XRD and TGA analysis were used to determine the mineral composition of each blending group after carbonation. SEM analysis was applied to observe the microstructure, and the MIP method was used to analyze the pore size structure. The results show that the carbonation degree and compressive strength of BOFS cubes were increased by the addition of γ-C2S. With 20 wt% blending of γ-C2S, the carbonated BOFS obtained the optimum increasement of carbonation degree and compressive strength, reaching 39.3% and 42.4 MPa, respectively. However, the subsequent hydration reactivity of BOFS was inhibited when blended with 20 wt% of γ-C2S. Despite this, the increased poorly crystalline CaCO3 filled the system and densified the matrix, contributing to a 17.9% increase in compressive strength of SH20 after 56 days of hydration curing.

The synergistic effect of microorganisms and multiple admixtures on improving the self-healing of cracks in biogenic mortar exposed to different marine environments

Existing research has inadequately addressed the synergistic effects of microorganisms and admixtures in promoting self-healing behavior and mechanisms of cracks in marine environments, including the atmospheric zone, tidal-splash zone, and submerged zone. This study primarily involved the incorporation of multiple admixtures and microorganisms into cement to prepare two types of biogenic mortars: Bacillus pasteurii-based mortar (BPM) and Bacillus subtilis-based mortar (BSM), with ordinary mortar (OM) serving as the control group. The mechanical properties, electrical flux, and pore structure of the mortars were investigated. Additionally, the self-healing behavior and mechanisms of cracks were studied under atmospheric, tidal-splash, and submerged conditions. The results indicate that with increasing curing age, the synergistic effect of admixtures and microorganisms continuously refined the pore structure of the specimens and enhanced their flexural strength, compressive strength, and resistance to chloride ion penetration. The atmospheric environment was found to be unfavorable for the self-healing of cracks in the specimens. However, in the submerged and tidal-splash zones, the combined action of microorganisms and admixtures led to more pronounced self-healing phenomena in the biogenic mortars compared to OM. Particularly, the self-healing effect of BSM cracks was more pronounced in the tidal-splash zone, with a 100% self-healing efficiency observed after 53 days. This was attributed to the ability of microorganisms in the tidal-splash zone to induce calcium carbonate precipitation on the surface of cracks in admixture-modified cementitious materials, thereby reducing the penetration of seawater, CO2, and O2 into the cracks, lowering the carbonation reaction of Ca(OH)2 crystals, and reducing the formation of brucite crystals inside the cracks, consequently enhancing the water permeability resistance of the specimens. These research findings hold significant engineering implications for improving the durability and service life of coastal self-healing concrete structures.

TiO2/MnFe2O4 co-modified alkaline papermaking waste for CaO-CaCO3 thermochemical energy storage

CaO-CaCO3 thermochemical energy storage is a promising technology for solar energy utilization and storage. Alkaline papermaking waste (APW) from paper mills, which is mainly composed of CaCO3. Herein, TiO2/MnFe2O4 co-modified APW was synthesized. The energy storage capacity of TiO2/MnFe2O4 co-modified APW was studied during CaO-CaCO3 thermochemical energy storage cycles in a duel fixed bed reactor. TiO2/MnFe2O4 co-modified APW mainly consists of CaO, MgO, CaTiO3, CaMnO3, and Ca2Fe2O5. The introductions of TiO2 and MnFe2O4 significantly improve the cyclic stability and optical absorption property of APW in 30 cycles. The co-modified APW (the mass ratio for CaO:TiO2:MnFe2O4 = 100:5:5) possesses high sintering resistance, energy storage, and optical absorption capacities in 30 cycles. The energy storage conversion and capacity of the co-modified APW are 0.72 and 2288.57 kJ/kg after 30 cycles, respectively. The optical absorptance of the co-modified APW reaches 49%, which is 9.25 times as high as that of APW. CaTiO3 and CaMnO3 effectively mitigate the sintering of CaO grains. CaMnO3 and Ca2Fe2O5 significantly increase the optical absorptance of APW. The co-modified APW possesses stronger basicity and more oxygen vacancies, which is favourable for the carbonation reaction. Thus, TiO2/MnFe2O4 co-modified APW is a good energy storage material.

CO2 sequestration by ammonia-enhanced phosphogypsum mineral carbonation: Development and optimization of reaction kinetic model

Comparing of existing kinetic models for direct mineral carbonation by solid waste, an optimized kinetic model was developed to depict the relationship between phosphogypsum carbonation ratio and reaction time. Based on the optimized model, the reaction mechanism of CO2 sequestration by ammonia-enhanced phosphogypsum mineral carbonation was analyzed. The experimental results show that, under specific conditions, including an ammonia ratio of 2.3, room temperature (25 °C), a liquid–solid ratio of 5:1, a gas flow rate of 200 mL/min, and a rotational speed of 500 rpm, phosphogypsum achieved its peak carbonation efficiency of 91 % within 30 min. The optimized model fitting curve has a high consistency with the experimental data, and the average R2 value is 0.991. The process of ammonia-enhanced phosphogypsum mineral carbonation comprises three distinct sub-processes. It is consisted of mass transfer in gas–liquid interface and solid–liquid interface, respectively, and product layer diffusion. Among them, the gas–liquid mass transfer is identified as the rate-controlling step in the mineral carbonation process. The present study provides a basis for the operation optimization and large-scale utilization of phosphogypsum mineral carbonation.

Preparation of CaCO3/TiO2@SiO2 pigments by the one-pot method: Surface-coated SiO2 improves the weatherability of pigments

In this paper, a CaCO3/TiO2@SiO2 composite pigment (CTS) was prepared by a carbonation reaction using a one-pot method. CTS can reduce the amount of rutile TiO2 needed and improve the weatherability compared with TiO2 and CaCO3/TiO2 (CT). The performance test results showed that the composite pigment with 60% TiO2 has a coating film contrast ratio of 91.05% of pure TiO2, and the oil absorption and whiteness values were also close to those of pure TiO2, indicating that the CTS has good pigment properties. The XPS and infrared results showed that the bonding of CaCO3, TiO2, and SiO2 occurred at the interface. The tight bonding of the three substances was realized through a dehydroxylation reaction on the surface. In addition, the SiO2 layer weaken the photocurrent response of TiO2 in the CTS and reduced the UV absorption of the composite pigment, which slowed the degradation rate of rhodamine B in the composite pigment coatings; thus, the CTS exhibits excellent weatherability performance in coating applications.

Quantitative characterization of recycled cementitious materials before and after carbonation curing: Carbonation kinetics, phase assemblage, and microstructure

Carbonation curing for cement-based materials is one of the effective means to achieve carbon neutrality in the construction industry. The pure cement clinker was hydrated for 90 d to simulate recycled cementitious materials (RCMs) to further investigate the carbonation reaction kinetics, phase assemblage and microstructure. The results revealed that the carbonation rates of cement clinker’s main components, i.e. C3S, C2S, C3A, and C4AF were 100%, 63.24%, 28.92%, and 14.93%, respectively. As the carbonation reaction proceeded, the calcium hydroxide (CH) would be fully consumed, while C-S-H began to become the main reactant, generating low-Ca C-S-H and even fully polymerized Ca-modified silica gel. At the same time, the unhydrated C3S and C2S were carbonized, forming carbonation products, i.e. CaCO3 and xCaO·SiO2·nH2O. Interestingly, the amount of poorly-crystalline CaCO3 was initially decreased and then increased, possibly attributed to the partial transformation into well-crystalline CaCO3 at an early stage, whereas the remaining amount was produced through the reaction of C-S-H at a later stage. Finally, the carbonation reaction was further explained through two models, the early stage was described by a chemical reaction model while the later stage was represented by a product-layer diffusion model.

Experimental study on eco-friendly one-part alkali-activated slag-fly ash-lime composites under CO2 environment: Reaction mechanism and carbon capture capacity

This work studied the reaction mechanism and carbon capture capacity of novel type eco-friendly one-part alkali-activated slag-fly ash-lime composites (SFL) based on the change in SFL under CO2 environment from the macroscale to the microscale. Experimental results showed that SFL had a higher carbonation rate and led to the formation of carbonates (calcite, vaterite, aragonite), gypsum, amorphous calcium carbonate, and alumino-silicate gel. The carbonation reaction of SFL can be generally divided into three periods. In period 3, Al from the decomposed aluminum-bearing hydrates was gradually incorporated into the amorphous silica phase in the form of tetrahedral Al(-SiO)4 sites, which finally resulted in the formation of an alumino-silicate gel. The mass ratio of carbon capture in the complete carbonation zone of SFL is approximately 13%. Due to a large amount of CO2 binding in hydrates in a wider carbonation zone, the actual total capacity of carbon capture in SFL under CO2 environment was slightly lower than that in Portland cement. Thus, SFL had a better and adjustable capacity for CO2 capture compared to other low carbon binder materials.

Influence of the Original Concrete Strength and Initial Moisture Condition on the Properties Improvement of Recycled Coarse Aggregate via Accelerated Carbonation Reactions.

The physical and mechanical properties of recycled coarse aggregate (RCA) are worse than those of natural coarse aggregate (NCA), and the overall performance of recycled concrete prepared from RCA is worse than that of natural aggregate concrete. Treatment of RCA by CO2-accelerated carbonation effectively improves the macroscopic properties of RCA. The degree of influence of raw material factors, i.e., the original concrete strength (OCS) and initial moisture content (IMC) of RCA, on the carbonation of RCAs is very complex. Herein, an accelerated carbonation experiment for RCA with different material factors as variables was carried out to explore the influence of the abovementioned factors on the physical properties of carbonated recycled coarse aggregate (CRCA). By analyzing the microstructure of the RCA with the best modification effect before and after carbonation, the carbonation modification mechanism of the RCA was revealed. The physical performance indexes, including the apparent density, water absorption and carbonation rate, of the dried RCA with an OCS of C40 and C50 were significantly improved. The research results can provide basic data and theoretical support for promoting the popularization and application of RCA and recycled concrete in practical engineering.

Accelerated carbonation of regenerated binding material prepared with waste cement paste powder

An approach for the efficient re-utilisation of waste cement paste powders is proposed, wherein the powders are heated to a certain temperature to prepare regenerated binding materials (RBMs), and then rapidly carbonated by high-concentration carbon dioxide to prepare high-strength products. The structural evolution during the heating process of waste cement paste powder and the carbonation process of RBMs were investigated by X-ray diffraction analysis, scanning electron microscopy and 29Si nuclear magnetic resonance spectrum, and the compressive strength of the carbonated RBM samples was measured. The results show that the differences in carbonation reactivity, carbonation products of the RBMs and properties of carbonated RBMs can mainly be attributed to the differences in the degree of structural order of newly formed β-dicalcium silicate (C2S), the content of β-C2S, free calcium oxide (f-CaO) and incompletely decomposed calcium silicate hydrate (C–S–H) in RBMs. The strength of the carbonated sample of RBM prepared at 1050°C was the highest, followed by that prepared at 950°C, which was significantly higher than those prepared at 650, 750 and 850°C. The content of highly ordered β-C2S in RBMs with high carbonation reactivity and the dense stacking of the carbonation products are beneficial for the mechanical properties.

Microstructural Evolution of Calcium Sulfoaluminate Cement during the Wet-Carbonation Process

Calcium sulfoaluminate (CSA) cement, as a type of low-carbon cement, can contribute to further reduction in carbon emissions with carbonation technologies. However, the detailed microstructure development of CSA cement during the carbonation process has been rarely analyzed. In this paper, wet carbonation was applied to CSA cement to investigate the microstructure evolution of carbonation products and carbon absorption capacity of CSA cement by means of pH measurement, X-ray diffraction (XRD) measurement, thermogravimetric (TG) measurement, Fourier-transformed infrared spectroscopy (FT-IR) measurement and scanning electron microscope measurement. During the carbonation process, the formed ettringite product and the dicalcium silicate clinker were carbonated immediately to generate calcium carbonate crystals, silica gel and aluminum hydroxide (AH3) gel. With the trend of pH increasing first and notably decreasing later, the coupling interaction between the hydration and carbonation reactions of CSA cement was revealed. From the XRD and TG results, three types of calcium carbonate crystal forms (calcite, vaterite and aragonite) were detected, and the content of calcium carbonate increased with the increase in carbonation time. FT-IR analysis further confirmed the existence of calcium carbonate, silica gel and AH3 gel with their characteristic vibrations. Moreover, the microstructure of carbonation products with different morphologies was observed. The application of wet carbonation to CSA cement provides a more comprehensive insight to the carbonation mechanism of this low-carbon cement.

Effect of Glycine on the Wet Carbonation of Steel Slag Used as a Cementitious Material.

The wet carbonation process of steel slag (SS) is envisaged to be an effective way to sequestrate CO2 and improve the properties of SS as a supplementary cementitious material. However, the carbonation process still struggles with having a low carbonation efficiency. This paper studied the effect of glycine on the accelerated carbonation of SS. The phase composition change of carbonated SS was analyzed via XRD, FT-IR, and TG-DTG. The carbonation process of SS is facilitated by the assistance of glycine, with which the carbonation degree is increased. After 60 min of carbonation, SS with glycine obtained a CO2 sequestration rate of 9.42%. Meanwhile, the carbonation reaction could decrease the content of free calcium oxide in SS. This significantly improves the soundness of SS-cement cementitious material, and the compressive strength of cementitious materials that contain carbonated SS with glycine is improved. Additionally, the cycling performance of glycine in the successive wet carbonation process of SS was investigated. Multicycle experiments via solvent recovery demonstrated that although the promotion effect of glycine was reduced after each cycle, compared with the SS-water system, the carbonation process could still be facilitated, demonstrating that successive wet carbonation via solvent recovery has considerable potential. Herein, we provide a new idea to facilitate the wet carbonation process of SS and improve the properties of SS-cement cementitious material.

Performance evaluation of carbonated cement paste derived from hydrated Portland cement based binders as supplementary cementitious material

The production of concrete has gone up from 2.3 billion m3 in 2002 to 14 billion m3 in 2020. At the same time, the amount of construction and demolition waste generated annually has reached levels of 3 billion tons, with concrete rubble making up a major portion of it. This study investigates the performance of carbonated fines derived from hydrated cement paste of different binders. Four cement paste blends, containing Portland cement (PC) and blends of Portland cement with fly ash (FA30), perlite (PL30) and metakaolin (MK30) at a 30 % cement replacement level, were cured for 90 days prior to subjecting them to a carbonation reaction. The R3 test gauged pozzolanic reactivity, while calorimetry, XRD, and TGA studied hydration characteristics. Compressive strength of mortar samples prepared using the same binder composition as paste was also measured at different ages. Carbonated fines exhibited notable pozzolanic activity compared to waste hydrated cement paste. The reactivity of carbonated fines derived from different binders were similar, however slightly higher reactivity was measured for fines comprising of reactive SCM in the precursor material (hydrated paste). Mixes with carbonated fines demonstrated enhanced kinetics, exhibiting over 25 % higher compressive strength at 3 and 7 days compared to the fly ash control mix. Similar compressive strength characteristics were observed in mortar mixes with different carbonated fines, indicating a limited impact of the various binder types from which the fines originated.

Carbonation kinetics of calcium-based solids at the centimeter scale for moving bed applications

Countercurrent moving bed reactors were recently proposed to capture CO2 from dispersed sources using calcium materials. Due to disparities with Calcium Looping systems in particle size and solids residence times, additional investigations are required to identify reaction controlling steps and kinetic parameters. This study examines the carbonation reaction of centimeter-scale CaO and Ca(OH)2-derived-CaO pebbles under conditions relevant to moving bed applications. To this end, the impact of carbonation temperature (450–650 °C) and CO2 concentration in the inlet gas (2.5–7.5 %v) was evaluated in TGA, employing particle sizes from 0.26 to 1.65 cm. The obtained results were fitted to a shrinking core model where the rate-determining step is CO2 diffusion through the gas volume that occupies the pores of the solid CaCO3-rich layer formed during carbonation. The experimental findings can be predicted using the effective porosity of the materials as the sole fitting parameter.

Study on the physical encapsulation mechanism of heavy metals in fly ash carbonation solidification based on synthetic pellets

The physical encapsulation mechanism prevents the leaching of heavy metals by forming calcium carbonate-encapsulated fly ash through the reaction of carbon dioxide and calcium oxide. However, current research in this area primarily involves qualitative analysis, focusing on various methods to enhance the fixation effect of heavy metals. There is a lack of quantitative description regarding how physical encapsulation solidifies heavy metals, and the solidification mechanism of heavy metals is challenging to verify through macroscopic experiments. In this paper, a method for synthesizing small fly ash particles into pellets is proposed to quantitatively study the mechanism of physically encapsulating heavy metals. The data from the carbonation reaction and leaching reaction are fitted using the shrinking core model. The depth of carbonation increases with the lengthening of carbonation time, and the physical encapsulation strengthens the immobilization of heavy metals. The leaching of heavy metals after carbonation can be categorized into three stages based on the shrinking core model: external diffusion, internal diffusion, and chemical reaction control. Synthetic pellets enhance all three stages, especially the external diffusion, which is not detectable with fly ash powder. The fitting results of the experimental data for the carbonation reaction of the enlarged synthetic pellets and the shrinking core model are both above 0.9. This indicates that the primary barrier to leaching of heavy metals is the physical encapsulation formed by the carbonation reaction following the shrinking core model. The physical encapsulation serves as the primary mechanism for the solidification of heavy metals during the carbonation process.