Cementitious materials can achieve desirable strength development and reduced cracking potential under moist or immersed conditions. However, in this work, we found that alkali-activated slag (AAS) pastes can crack underwater, with a higher silicate modulus showing more pronounced cracking. Chemically, the C-(N-)A-S-H gel in the paste with a higher silicate modulus showed a higher Na/Si ratio and a higher leaching loss of Na, which led to more significant structural changes and gel deterioration underwater. This triggered the propagation of cracks initially present in the material. Physically, the paste with a higher silicate modulus featured a denser microstructure, lower water permeability and higher pore pressure, which resulted in a steeper gradient of pore pressure in the matrix. Consequently, the concentration of tensile stress was simulated at the centre and the corner of the cross-section of the sample. As this simulated concentrated stress exceeded the flexural strength of AAS pastes, significant fractures at the centre and spalling at the corner occurred, consistent with the experimental observation. This work not only elucidated the cracking mechanisms of AAS materials underwater but also provided new insights into mixture designs for these materials under high-humidity conditions.@en

This study focuses on the numerical modeling of the reaction and microstructure development of a one-part granite-based geopolymer, which is often used for carbon capture and storage (CCS) applications. This work extends the capabilities of GeoMicro3D to model one-part geopolymers containing different precursors and activators (solid and in solution). The model considers the particle size distribution of different solids and the real shape of particles to prepare the initial simulation domain. Further, the dissolution rates of different solids estimated from the experiments were used to model the dissolution of different elements in the pore solution. Subsequently, the model utilizes classical nucleation probability modeling coupled with thermodynamic modeling to estimate the precipitation of products in the microstructure. Experiments were performed to study the pore solution, reaction degree, and amount of products in the microstructure, which were further compared with the simulation results to check the rationality of the model.@en

Compared with blast furnace slag (BFS), the less reactive MSWI bottom ash (MBA) plays a minor role in alkali-activated blends. This research optimized the use of MBA as a precursor by enhancing its contribution to strength and microstructure development. The proposed strategy combines pre-treatment with pre-activation processes, enabling MBA to react before BFS addition. The NaOH-based pre-treatment led to the oxidation of metallic aluminum and the partial dissolution of the amorphous phase in MBA. The subsequent pre-activation resulted in the generation of C-A-S-H gel, which promoted later-stage gel formation in the paste. The reacted bottom ash particles exhibited distinct features in alkali-activated pastes. Compared with 100 % slag-based system, blending slag with MBA accelerated the slag reaction at late ages and facilitated the formation of a more polymerized C-(N-)A-S-H gel. The compressive strength results indicate that MBA is a promising alternative to Class F coal fly ash in BFS-based alkali-activated blends.

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Alkali and alkali earth metal ions are normally present in the gels of alkali-activated materials as well as blended PC-based materials. Previous studies have revealed that the leaching of these cations can trigger the change in gel structure and even the gel decomposition. However, the dissolution of cations was rarely known and the underlying mechanisms remained unclear. To address this issue, five calcium-(sodium, potassium-)aluminum-silicate hydrates (C-(N,K-)A-S-H gels) with different Ca/Si ratios (0.8–1.2) and Al/Si ratios (0.1–0.3) were synthesized to investigate the leaching behaviour of Ca, Na and K. For the first time, the dissolution free energies of Ca, Na and K in C-(N,K-)A-S-H gels were calculated using molecular dynamics simulations with the metadynamics method. Experimental results showed that Na showed the highest leaching ratio, followed by K and Ca, attributed to the lowest dissolution free energy of Na. The gel with a higher Ca/Si ratio or a lower Al/Si ratio showed higher charge positivity on the surface, resulting in reduced leaching of the three cations. Additionally, the presence of K was found to promote the dissolution of Na in gels.

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As the main waste product of iron and steel industry, steel slag possesses considerable cementitious activity, making it a promising alternative to cement in Cement Stabilized Macadam (CSM). However, CSM was inevitably exposed to groundwater and rainwater when served as the pavement base course, leading to concerns over poor early strength and potential pollutant leakage, which are the main factors that may hinder the widespread utilization of steel slag in CSM base. To address these issues, this study investigated the feasibility of using steel slag powder to produce CSM. Steel Slag Powder-Cement Stabilized Macadam (SSCSM) samples were prepared and the hydration products, microstructure, mechanical properties, water damage resistance and heavy metal ions leaching behavior were investigated. The results show that the nucleation effect of steel slag powder accelerates the early hydration, but the C-S-H produced by hydration is not sufficient to form a stable hydration product network, so the microstructure of SSCSM is looser than that of CSM. The addition of steel slag powder improved the shrinkage performance of SSCSM, and the mechanical properties and heavy metal ion leaching concentration of SSCSM meet the engineering application requirements when the steel slag powder replacement level does not exceed 30 %. It was also found that the addition of steel slag powder promoted the development of pores in SSCSM samples after dry-wet cycles, resulting in the reduction of water damage resistance. Compared with conventional CSM, the use of SSCSM can not only reduce 4 % of raw material cost and 23.5 % of equivalent CO₂ emission, but also mitigate the heavy metal ions leaching risk associated with steel slag, making it an effective and sustainable solution for steel slag recycling.

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Recent studies have been focusing on the carbon sink potential of carbonatable binders as an attempt to reduce CO2 levels. Air lime is a carbonatable binder that fully relies on CO2 absorption to harden and, thus, offers great carbon sink potential. Yet, CO2 absorption is favoured only after the evaporation of the excess water. Therefore, this study investigated the behaviour of air lime-containing mortars regarding water retention and evaporation. Four groups (with different contents of air lime) were monitored for up to 91 days after curing. Results showed that higher contents of air lime yielded greater water retention capacity. Yet, water retention did not prevent the carbonation front from further advancing – especially within lime-cement groups. In this case, greater porosity proved to be an open door for the simultaneous evaporation and ingress of CO2. Thus, hero or villain? It depends on the mixture.@en

Microbial induced calcium carbonate precipitation (MICP) technology has been successfully used to enhance the properties of recycled concrete aggregates. However, the complex source and varied physical and chemical properties of recycled aggregates may have influence on the modification efficiency since microbes are often sensitive to the surroundings. In this study, two representative types of recycled aggregates, recycled concrete aggregates (RCA) and recycled brick aggregates (RBA), were subjected to two kinds of MICP treatments, basic MICP treatment and sodium alginate (SA) aided MICP treatment. The absorption and desorption of bacteria in/on aggregates during MICP treatments were quantified. The physical and chemical properties of aggregates after the bio-treatments were tested, and the influence of alkalinity and pore structure of aggregates under various MICP treatment methods on treatment efficiency were detailed investigated. Results indicated that, when the aggregates were subjected to the basic MICP treatment, the treatment efficiency was more remarkable in RBA, because of its high porosity and moderate pH (around 8–9), which facilitated the absorption of bacteria in/on aggregates and urease activity respectively. While under SA-aided MICP treatment, the influence of pore structure and alkalinity of aggregates on the treatment efficiency was not significant compared with that under the basic MICP treatment, especially the mass of CaCO₃ on the aggregates. The biogenic CaCO₃ generated by SA-aided MICP treatment not only plugged micropores, but also distributed all over the entire surface of the aggregates, resulting in a sufficient repair of microcracks. Meanwhile, the surface repair may reduce the influence of pore structure and pH of aggregates on the precipitation process, thereby reducing the impact of varied physical and chemical properties of aggregates on the treatment efficiency, which was conducive to the widely unified application of SA-aided MICP treatment in the modification of recycled aggregate based on construction solid waste.

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To properly control the reaction kinetics and fresh properties evolution in conventional alkali-activated materials (AAMs), a conceptual design of two-stream AAMs has been proposed in this study. This is achieved by dividing the solid and liquid components in AAMs, including blast furnace slag (BFS) and electric arc furnace slag (EFS) precursors, as well as aqueous sodium hydroxide and silicate activators into two separate streams A and B, where a very limited reactivity is expected in individual streams to ensure sufficient workability retention. Moreover, a final-stage intermixing is required to combine individual stream mixtures and trigger the major activation reaction. Fresh and hardened properties of combined mixtures were checked at different stages. The microstructure and reaction products were investigated to understand the strength development. Low dynamic rheological parameters and good workability retention have been detected in all individual stream mixtures, accompanied by limited exothermic heat flows after the initial dissolution confirmed by calorimetry tests. Further, Portland cement (PC) is partially blended into stream A to alter the early stiffening process in combined mixtures and meet various setting demands after intermixing. However, this might lead to a reduction in mechanical properties, associated with the formation of porous microstructures and an increase in the Ca/Si ratio in reaction products. Eventually, the conceptual design is validated in different scenarios including self-compacting and 3D-printing concrete applications.

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Alkali-activated materials (AAMs) are one of green cementitious materials in building materials industry and beneficial to the goals of carbon peaking and carbon neutrality. Compared with ordinary Portland cement (PC) based materials, however, the raw material composition, reaction products and pore solution composition of AAMs are complex and thus their reaction mechanisms and performance evolutions still need to be further clarified. Thermodynamic modelling is an effective method in analyzing AAMs. It can predict the phase assemblage and pore solution composition based on the raw material composition and given reaction conditions, which is of great significance to profoundly investigate the reaction mechanisms and performance evolutions of AAMs. The existing thermodynamic modelling is increasingly applied in AAMs and the related results are achieved. However, the corresponding comprehensive review on the state-of-art in thermodynamic modelling of AAMs is lack. A clear and systematic knowledge of the principles, thermodynamic databases, methods, challenges and gaps remains implicit for thermodynamic modelling of AAMs. In this context, this review summarized recent progress on thermodynamic modelling of AAMs, pointed out the deficiency gaps of current thermodynamic modelling research work and put forward the relevant prospects. This review could provide a theoretical guidance for thermodynamic modelling of AAMs. Chemical reactions in AAMs follow the laws of thermodynamics. There exists two thermodynamic equilibriums in AAMs, i.e., one is between the precursor and aqueous solution and another is between the reaction products and aqueous solution. Thermodynamic modelling can be performed to predict the phase assemblage and pore solution composition of AAMs by assuming the thermodynamic equilobriume. The accuracy and reliability of results by thermodynamic modelling largely depend on the quality of thermodynamic database that consist of solubility products (K_sp), heat capacity (C^Θ_p ), entropy (S^Θ ), Gibbs free energy(Δ_f G_m^Θ ), enthalpy (Δ_f H^Θ ) and molar volume (V ^Θ ) for all solid, liquid and gas phases involved in the system. The thermodynamic database of AAMs is usually established based on the thermodynamic database of PC via introducing the unique reaction products of AAMs. The unique reaction products and their thermodynamic parameters are available for alkali-activated high-Ca and alkali-activated low-Ca systems. Thermodynamic modelling of alkali-activated slag was initially conducted via the thermodynamic database of PC. Although the modelling results can predict the phase composition evolution, it still needs the corresponding experimental measurements to calibrate. with the established CNASH_ss model for describing C-(N-)A-S-H gel, thermodynamic modelling is increasingly used to investigate the phase assemblage evolution of alkali-activated slag cements. Besides the phase evolution, thermodynamic modelling is also applied to predict the phase diagram, providing a theoretical basis for the refined design of chemical properties of alkali-activated slag cement. In recent years, thermodynamic modelling tends to be used to investigate the durability of alkali-activated slag cements under single factor action such as carbonation, chloride attack and sulfate attack, as well as under multi-factors action, i.e., the combined attack by chloride and sulfate salts. Thermodynamic modelling is also applied to predict the phase assemblage evolution of alkali-activated low- and medium-Ca systems. However, it is less applied to those for alkali-activated high-Ca system. This is mainly due to the less developed thermodynamic database for alkali-activated low- and medium-Ca systems. In addition, thermodynamic modelling is also coupled with other simulation techniques to numerically analyze AAMs. For instance, a novel numerical model GeoMicro3D was proposed by coupling thermodynamic modelling and lattice Boltzmann method to simulate the reaction process and microstructure formation of alkali-activated slag cement, clarifing the interaction mechanisms between chemical reaction, multi-ions transport and microstructure formation. However, the numerical studies by coupling thermodynamic modelling and other simulation techniques are still limited for AAMs when compared to those for PC based materials. Summary and prospects Thermodynamic modelling has a robustness in studying the phase evolution and durability performance of AAMs induced by chemical reactions. Firstly, thermodynamic modelling can predict the reaction products assemblage and pore solution composition of AAMs. Secondly, thermodynamic modelling can calculate the phase evolution of AAMs under the action of aggressive media, and then study the deteriation mechanism of AAMs. Finally, thermodynamic modelling can be combined with other numerical simulation techniques to investigate AAMs. At present, however, there are still some issues that need to be further studied as follows: 1) The incomplete thermodynamic database for alkali-activated low-Ca system is an important reason for the limited thermodynamic modelling studies on alkali activated low and medium calcium systems. It is expected that a thermodynamic model describing the N-A-S-H gel can be established by ab-initio calculations and molecular dynamics simulations with the development of atomic- and molecular-scale simulation techniques. 2) It is generally assumed that the amorphous phases in precursors are dissolved synchronously in current thermodynamic modelling of AAMs. However, the heterogeneous distribution of composition and structure of precursor makes this assumption in doubt. The non-uniformity of the dissolution of amorphous phases in precursor is an issue to be further considered in future thermodynamic modelling studies. 3) The phase evolution of AAMs is actually a process coupling thermodynamics and kinetics. However, most of the thermodynamic modelling studies only focus on the phase assemblage in the equilibrium state, ignoring the kinetic issues before reaching the equilibrium. Considering the kinetic parameters (i.e., dissolution rate and reaction rate, etc.) in thermodynamic modelling should be a focus of current and future thermodynamic modelling studies. 4) The phase evolution, microstructure damage and ions transport are three inter-dependent aspects for studying the durability performance of AAMs. However, the current thermodynamic modelling studies mainly focus on the phase evolution under the chemical attacks, while ignoring the interaction between the phase evolution, microstructure damage and ions transport. In future studies, it is necessary to consider the interaction and establish a chemical-damage-transport model to numerically analyze the durability performance of AAMs.

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Wood biomass fly ash (WBFA) has emerged as one of the most dominant by-products in the biomass energy sector. Circulating WBFA for construction practice can mitigate the secondary pollution caused by improper ash management, and provide a new material source to compensate for the scarcity of raw materials in the construction industry. This paper reviewed the current research progress on recycling WBFA in cementitious materials. The physicochemical properties of WBFA were summarized based on the literature. Further, the implementations of WBFA for the development of cementitious materials were categorized into three binder systems: clinker, blended cement, and alkali-activated materials (AAMs). Owing to the large variation in chemical compositions of WBFA and strict requirements in clinkering parameters, employing WBFA in blended cement and AAMs seems to be more promising. A new classification approach for WBFA was proposed to divide WBFA into two categories. This helps to provide simple guidance for ash recycling in construction practice. Finally, the current research gaps in WBFA valorization in cementitious materials were summarized, outlining the research for further exploration.

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Municipal solid waste incineration (MSWI) bottom ash, due to its high mineral content, presents great potential as supplementary cementitious material (SCM). Weathering, also known as aging, is a treatment process commonly employed in waste management to minimize the risk of heavy metal leaching from MSWI bottom ash. Using weathered MSWI bottom ash to produce blended cement pastes is considered as a high-value-added and sustainable waste disposal solution. However, a critical challenge arises from the metallic aluminum (Al) in weathered MSWI bottom ash, which is known to induce detrimental effects such as volume expansion and strength loss of blended cement pastes. While most metallic Al in weathered MSWI bottom ash can be removed with eddy current separators in metal recovery plants, the residual metallic Al, owing to its small particle size, cannot be removed with the same technique. This study is dedicated to addressing this issue. An in-depth analysis was conducted on residual metallic Al embedded in weathered MSWI bottom ash particles, aiming to guide the removal of this metal. This analysis revealed that mechanical removal was the most suitable method for extracting metallic Al. The specific processes and mechanisms underlying this method were elucidated. After reducing metallic Al content in weathered MSWI bottom ash by 77 %, a significant improvement in the quality of blended cement pastes was observed. This work contributes to the broader adoption of mechanical treatments for removing residual metallic Al from weathered MSWI bottom ash and facilitates the application of treated ash as SCM.

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Given the crucial role of carbonation in the hardening of lime-based binders, accurate measurements of carbonation depths are essential for analysing both carbonation kinetics and carbon sequestration capabilities. This study employed both the conventional phenolphthalein spray method and the profile-based method to determine carbonation depths in four types of binders. Unlike the distinct carbonation front observed in cementitious materials, lime-based binders displayed a transition zone between fully carbonated and uncarbonated areas. Meanwhile, the remaining portlandite content in some specimens did not necessarily increase with depth, and typical Liesegang patterns were observed. Compared to phenolphthalein spray method, profile-based methods provide more quantitative evidence for further analyses, but inevitable slice interval can also lead to errors in carbonation depth estimation. Therefore, developing a more precise and convenient method remains essential for a deeper understanding of the carbonation behaviour in lime-based binders.@en

With the development of waste recovery techniques, previous research has revealed that coarse fractions of municipal solid waste incineration (MSWI) bottom ash (BA) after proper treatment could be applied in the construction sector, while the fines are seldom recovered in practice and normally landfilled. This study explores the potential application of fine MSWI BA (0–2 mm) as a supplementary cementitious material (SCM) in Portland cement (PC) mixtures. Mechanical and chemical pre-treatment approaches have been designed with various conditions to optimize the treating process. The chemical and mineralogical compositions, as well as the metallic Al content in BA were characterized before and after the pre-treatment. It was found that both methods are effective in removing the metallic Al content in BA, Moreover, BA derived from mechanical treatment exhibited more contribution to the hydration reaction in PC mixtures, as revealed by the amount of reaction products and mineral phases formed in hardened trial mixtures. BA obtained was further partially blended in PC mortars to evaluate the performance as compared to SCMs and inert fillers. It was found that treated BA resulted in a slight retarding effect on the reaction kinetics. Treated BA behaved better than the coal fly ash to contribute to the strength development, while the inclusion of BA did not lead to significant influences on the workability.

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While alkali-activated slag (AAS) has emerged as a promising alternative binder in construction engineering, a consensus on the optimal curing condition for this material has not been reached yet. It is well known that AAS can harden at ambient temperatures, but the influence of humidity on its properties remains poorly understood. Herein, we considered five curing conditions with different relative humidities (RH), including ambient/dry condition (RH=55 %), sealed condition (RH=80–95 %), fog condition (RH>95 %), water immersion condition (RH=100 %), and saturated limewater immersion condition (RH=100 %). Various properties have been examined, including flexural and compressive strengths, elastic modulus, shrinkage, pore structure, carbonation resistance, and freeze-thaw resistance of AAS mortars (AASM). Two types of activators, sodium hydroxide and sodium silicate (modulus at 1) solutions were used. The experimental results indicate that drying at early ages is detrimental to almost all the properties investigated. Sealed curing can deliver desirable mechanical properties and durability, but considerable shrinkage. Fog and water curings are highly effective at mitigating early shrinkage in AASM, but the problem of leaching adversely affects its long-term properties. Generally, limewater curing offers limited benefits compared to other high-humidity curing methods.

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This research explored the microstructure formation and strength development of blended cement pastes prepared with municipal solid waste incineration (MSWI) bottom ash. A new sample preparation approach involving water treatment of MSWI bottom ash was developed to prevent sample cracking caused by the presence of metallic aluminum (Al) in bottom ash. The result showed that ions released during water treatment of MSWI bottom ash delayed cement hydration but promoted ettringite formation in blended cement pastes during the first day. Due to water treatment, the compressive strength of MSWI bottom ash blended cement paste increased to a level similar to that of Class F coal fly blended cement paste after 28 days. Blending water-treated MSWI bottom ash (WMBA) with cement promoted clinker hydration at later stages. The reaction products of WMBA in blended cement system were C-S-H gel and sodicgedrite, which contributed to strength development by filling the capillary pores.

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Alkali-activated materials (AAMs) are a class of potentially eco-friendly construction materials that can contribute to reduce the environmental impact of the construction sector by offering an alternative to Portland cement (PC). With the rapid development of both computational capabilities and theoretical insights into alkali-activation reaction processes, there has been a surge in research activities worldwide, leading to a growing demand for computational methods that can describe different characteristics of AAMs. This review summarizes the collective efforts made in the past two decades on this topic, and highlights the most relevant results and advances in the aspects of atomistic simulation, thermodynamic modeling, microstructure/−based simulation, and multi-scale modeling. The gaps and challenges in current numerical research on AAMs are pointed out and discussed in comparison with PC-based materials. This review aims to provide a critical overview of the state-of-the-art in modeling and simulating AAMs, while also outlining potential avenues for future development.

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Efflorescence presents not only as a cosmetic concern but also as a structural issue, which impacts the performance of alkali-activated materials (AAMs). In this study, the mechanisms of efflorescence of alkali-activated slag (AAS) pastes are investigated. First, the efflorescence of AAS pastes with different alkali dosages (3 %, 5 % and 7 %), activator types (sodium hydroxide (NH) and sodium silicate (NS)), exposure atmospheres (ambient, N₂ and 0.2 vol% CO₂), and relative humidities (40 %, 60 % and 80 %) was observed. Subsequently, leaching tests were performed and the impacts of efflorescence on AAS pastes at different heights were studied. It was found that a lower relative humidity facilitated more rapid and severe efflorescence. The positioning of efflorescence products was dependent on the porosity of the matrix. Compared to NH pastes, NS pastes subjected to semi-contact water conditions were more vulnerable to cracking problems, which turned out to be exacerbated by the formation of efflorescence products. A new method to quantify efflorescence was developed and it corresponded well with both efflorescence observations and leaching experiments. Furthermore, a competitive reaction between Ca and Na in the presence of carbonate ions was identified. CaCO₃, a representative product of natural carbonation, was rarely found in the regions where efflorescence products (sodium carbonate) formed. Regarding compressive strength, NS pastes were more adversely affected by efflorescence than NH pastes.

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The synthesis of N-A-S-H gel with high Si/Al ratios (>2) has been rarely reported in the literature, leaving the establishment of a reliable synthesis route as an open challenge. This paper aims to synthesize N-A-S-H gels with Si/Al ratios ranging from 1 to 3 and establish their thermodynamic database. The effects of reaction temperature, reaction time, initial Si/Al, concentration of reactants and pH of the matrix on the Si/Al ratios of the synthesized N-A-S-H gel were investigated. Results showed that N-A-S-H gels with target Si/Al ratios can be synthesized by controlling the concentration of reactants, pH and initial Si/Al ratios. The solubility products of the obtained N-A-S-H gels were determined via dissolution tests at different temperatures, to determine thermodynamic data. The development of this experimentally derived thermodynamic database of N-A-S-H gels constitutes a crucial step in the advancement of thermodynamic modeling of geopolymer, providing valuable insight into geopolymer reactions and phase assemblages.

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Carbonation of alkali-activated slag (AAS) materials has been primarily concerned in atmospheres with gaseous CO₂. This study, by contrast, highlights that AAS pastes would also be carbonated under tap water immersion. Calcite is the main CO₂-bear phase in both sodium hydroxide- and sodium silicate-activated AAS pastes, and the paste pre-cured for a longer curing period shows more severe carbonation. Additionally, calcium carbonate can densify the deteriorated microstructure of sodium hydroxide-activated paste caused by long-term leaching. The indentation modulus of pastes subjected to tap water immersion is higher than those under deionized water immersion. The uptake of CO₃^2- by hydrotalcite (Ht) and gels is also detected, resulting in the formation of Ht-CO₃ and decalcification of gels. Due to the synergistic effect of leaching and carbonation, a characteristic layered distribution of pastes close to the exposure front is observed, comprising the carbonated layer, transitional (carbonated + leached) layer, and leached layer, progressing from the outermost to the inner regions. Eventually, the kinetics of underwater carbonation, as well as the discrepancy between dry and underwater carbonation, is revealed.

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Previously, the lack of a thermodynamic database for N-(C-)A-S-H gel limited the application of thermodynamic modeling to alkali-activated fly ash (AAFA). This study pioneers thermodynamic modeling of AAFA using a recently developed thermodynamic dataset for N-(C-)A-S-H gel. The reaction products, pore solutions and reaction kinetics of AAFA pastes were experimentally determined. Based on the reaction kinetics, the composition of the solid phases and the pore solution of AAFA were modeled over time. The results showed that the simulated compositions of the solid reaction products and pore solution match closely with the experimental results, especially for the sodium hydroxide-activated system. Moreover, modeling results point out the potential presence of minor reaction products (e.g., C-(N-)A-S-H gel, microcrystalline ferrihydrite, Mg-containing phases) undetectable by experimental techniques. The study also demonstrated that thermodynamic modeling accurately captured the amount of bound water in reaction products, highlighting its robustness in both qualitative and quantitative analysis.

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