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

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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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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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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Alkali-activated fly ash (AAFA) is being increasingly acknowledged as an eco-friendly binder free of Portland cement, valued for its low carbon footprint and promising engineering properties. However, the application of AAFA has been limited due to its unstable and uncontrollable engineering properties, which are closely tied to its microstructure. The microstructure of AAFA can be affected by several factors, including the intrinsic properties of fly ash, the types of alkaline activators, the mixture and curing regime. These factors can lead to various reactions, resulting in diverse microstructures, and thus a wide range of engineering properties. A deep understanding of the relationship between these influencing factors and the resulting microstructure is essential to bridge the gap between the mixtures and their corresponding engineering properties. Although the effects of these factors on the microstructure of AAFA have been extensively investigated experimentally, there is currently no numerical model capable of simulating the chemical reactions and microstructural development of AAFA. As a result, the reactions and microstructure, and thus the engineering properties of AAFA, remain unpredictable for a given mixture. Simulating the reactions and microstructural development would enable the customization of mix designs to achieve desired engineering properties, thereby promoting the application of AAFA. Therefore, the aim of this research is to simulate the reaction process and development of the microstructure of AAFA.
Thermodynamic modeling is a robust approach to simulate chemical reactions. However, the main challenge in thermodynamic modeling of AAFA lies in the lack of a thermodynamic database of its primary reaction product, N-(C-)A-S-H gel, which varies in Si/Al and Ca/Al ratios. Developing such a database requires accurate determination of the chemical compositions of N-(C-)A-S-H gels, which is difficult to achieve with conventional experimental techniques. Therefore, this research addresses this challenge by utilizing molecular dynamics simulations to determine the chemical compositions of N-(C-)A-S-H gels. By simulating the polymerization process that mimics actual reactions, the atomic structures of N-(C-)A-S-H gels with various Si/Al and Ca/Al ratios were constructed. According to the simulation results, it is proposed that N-(C-)A-S-H gels with a Si/Al ratio of 1-3 and a Ca/Al ratio of 0-0.5 can represent the chemical compositions of N-(C-)A-S-H gel in a mature AAFA paste.
After determining chemical compositions, synthesis of pure N-(C-)A-S-H gels is the second step to determine their thermodynamic data. However, synthesizing N-(C-)A-S-H gel with a Si/Al≥2 at a high pH (corresponding to the alkalinity range of pore solutions in AAFA paste), posed a double challenge. To address this issue, using a concentrated solution with an initial Si/Al ratio higher than the target is the key. Following this approach, N-(C-)A-S-H gels with a Si/Al ratio of 1-3 and a Ca/Al ratio of 0-0.5 were synthesized successfully and characterized by using XRF, XRD, FTIR, and TGA techniques. Subsequently, the solubility of the synthesized N-(C-)A-S-H gels was measured through a dissolution test. A thermodynamic database of N-(C-)A-S-H gels with various Si/Al and Ca/Al ratios was established for the first time, encompassing not only the solubility, but also the Gibbs free energy, heat capacity, entropy, enthalpy, and molar volume. This established thermodynamic database is the key to performing thermodynamic modeling to simulate the reactions of AAFA.
Coupled with the reaction kinetics determined by isothermal calorimetry and SEM-EDS analysis, the thermodynamic modeling of AAFA was performed for the first time to investigate the formation of reaction products and the phase assemblage of AAFA over time in GEMS software. The sodium hydroxide-activated system showed a close consistency between the modeling and experimental data regarding phase assemblage and pore solution chemistry, while for the sodium silicate-activated system, the simulated ion concentrations in the pore solution showed discrepancies compared to the experimental results. This discrepancy may be attributed to the high ionic strength in the sodium silicate-activated system, limitations in thermodynamic data of N-(C-)A-S-H gel and thermodynamic modeling approach itself.
To simulate the microstructure of AAFA, GeoMicro3D model, originally designed for alkali-activated slag, was extended to adapt to AAFA. To achieve this, first, the dissolution of fly ash in an alkaline solution was investigated experimentally, from which prediction functions were developed to describe the dissolution rate of Si and Al, accounting for the intrinsic characteristics of fly ash, solution pH, and temperature. The developed functions can accurately predict the dissolution behavior of fly ash, aligning well with the experimental results. Then, the GeoMicro3D model was extended by equipping with the thermodynamic database of N-(C-)A-S-H gels and the prediction functions for the dissolution of fly ash. GeoMicro3D was employed to simulate the reaction process and the 3D microstructural development over time of the sodium hydroxide-activated fly ash paste. The distribution of various phases in a 3D microstructure of AAFA can be captured and visualized over time. The simulated degree of reaction of fly ash and the porosity of AAFA were in good agreement with the corresponding experimental data. Furthermore, GeoMicro3D can well simulate the pore solution chemistry over time, consistent with the experimental results.
To sum up, the reaction and microstructure evolution of AAFA were investigated using multiple simulation techniques in this work. The extended GeoMicro3D model developed in this research paves the way for simulating the microstructure and the pore solution chemistry for any given AAFA mixture. This advancement contributes to a deeper understanding of the relationship between the AAFA mixture and the resulting microstructure. Furthermore, the mechanical properties, transport properties and durability of AAFA can be further evaluated based on the simulated microstructure constructed by using the extended GeoMicro3D. This model enables industries to effectively manage fly ashes of varying qualities and customize AAFA to meet specific engineering requirements. This not only improves the utilization of fly ash but also promotes the sustainability of construction practices.@en

This study experimentally investigated the effects of surfactants and water-repelling agents on the hydration process, relative humidity, and mechanical properties of Portland cement pastes. Based on the measurement results, the degree of hydration, degree of saturation, capillary tension of autogenous shrinkage, and magnitude of autogenous shrinkage were simulated using a numerical model. In the numerical model, the elastic and creep components of autogenous shrinkage were calculated separately, and the creep component was simulated based on the solidification theory. The simulation results indicated that adding admixtures led to lower degrees of hydration and saturation. The capillary tension of the pure Portland cement was larger than that of the other mixtures. This can be attributed to several factors, including the smaller surface tension of mixtures with surfactants, larger contact angle of mixtures with water-repelling agents, and a lower degree of hydration of mixtures with both admixtures. Analyses of the simulated and measured results for different mixtures also show that creep plays an indispensable role in autogenous shrinkage. Adding a surfactant and a water-repelling agent can effectively mitigate autogenous shrinkage. However, when an excessive amount of water-repelling agent was added, its influence on the mitigation of autogenous shrinkage was insignificant.

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In this paper, the atomic structures of sodium aluminosilicate hydrate (N–A–S–H) gels with different Si/Al ratios are studied by molecular dynamics simulation. An N–A–S–H gel model was obtained from the polymerization of Si(OH)₄ and Al(OH)₃ monomers with the use of a reactive force field (ReaxFF). The simulated atomic structural features, such as the bond length, bond angle, and simulated X-ray diffraction pattern of the gel structure are in good accordance with the experimental results in the literature. Si–O–Al is found to be preferred over Si–O–Si in the N–A–S–H gel structure according to the amount of T–O–T bond angles and distribution of Si⁴(mAl). Pentacoordinate Al is identified in all simulated N–A–S–H models. It provides strong support to current knowledge that pentacoordinate Al in geopolymer does not only come from raw material. Furthermore, the structural analysis results also show that N–A–S–H gel with lower Si/Al ratios has a more cross-linked and compacted structure.

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Emergency evacuation is viewed as a common strategy adopted during the disaster preparedness stage of evacuation to ensure the safety of potentially affected populations. In emergency evacuation studies, soft computing approaches and methodologies have been widely used to support effective decision-making, providing robust and low-cost solutions. To understand the current status and trends of research on soft computing applications for emergency evacuation studies, 778 related studies published in the core database of Web of Science from 2000 to 2020 were considered in this study. A scientometric analysis and a comprehensive review were performed using a scientific mapping of the knowledge domain. This paper presents a set of analyses with the following primary objectives: (1) to explore and visualize the bibliometric characteristics and contents of the academic field concerned with the soft computing approaches for emergency evacuation; and (2) to review and analyze the knowledge, hotspots, and future outlooks related to soft computing approaches for emergency evacuation. The results provide some important insights regarding the existing soft computing methods that have been used in the emergency evacuation field over the past 20 years. Based on the conducted review, this paper proposes that future studies should concentrate on exploring the potential of innovative soft computing approaches for crowd modelling and enabling more accurate evacuation simulation and optimization.@en

The high autogenous shrinkage of alkali-activated materials made from slag and fly ash is recognised as a major drawback with regard to the use as construction materials. In this study, metakaolin was introduced into the alkali-activated slag-fly ash (AASF) paste to mitigate the autogenous shrinkage. The shrinkage mitigation mechanism of metakaolin was explained by studying the influences of metakaolin on the microstructure, shrinkage related properties, and mechanical properties of AASF paste. It was found that adding metakaolin could significantly reduce the chemical and autogenous shrinkage of AASF paste. This shrinkage mitigation is accompanied by a decrease in the alkalinity of AASF paste pore solution, a reduced drop in internal relative humidity, and an increase in porosity of AASF paste. Moreover, the incorporation of metakaolin does not change the type of the reaction products, but greatly delays the formation of the reaction products of AASF paste. The addition of metakaolin, above 5% of the binder, results in lower 28-day compressive and flexural strength of AASF paste.

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In this study, glass wool waste was utilized by means of alkali-activation with blast furnace slag. Reaction kinetics, workability, mechanical properties and autogenous shrinkage of alkali-activated slag and glass wool were comprehensively studied. Results indicated an optimal modulus (SiO₂/Na₂O) of the activator related to a long enough setting time and a high reaction degree of alkali-activated slag paste. The incorporation of glass wool as partial slag replacement did not necessarily lead to degradation in the performance of the pastes. While the compressive strength was always lower when glass wool was incorporated in the mixture, the flexural strength and workability could be improved with proper glass wool dosages. Autogenous shrinkage of blended pastes was always lower compared to the the mixture without glass wool. The results in this paper suggest that waste glass wool can be used as a precursor in slag-based alkali-activated system, resulting in improvements in the early-age properties of the paste such as a prolonged setting time and reduced shrinkage.

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This study aims to predict the autogenous shrinkage of alkali-activated concrete (AAC) based on slag and fly ash. A variety of analytical and numerical models are available for the prediction of autogenous shrinkage of ordinary Portland cement (OPC) concrete, but these models are found to show dramatic discrepancies when applied for AAC due to the different behaviours of these two systems. In this study, a new numerical approach is developed to predict the autogenous shrinkage of alkali-activated slag (AAS) and alkali-activated slag-fly ash (AASF) concrete from the experimental results on corresponding paste. In this approach, the creep of AAS and AASF and the restraining effect of the aggregate are particularly considered. By this approach, a fairly good prediction is obtained. Moreover, the microcracking in paste caused by restraining aggregates is evaluated. The results indicate that AAC is subjected to high tendency of development of microcracking.

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The understanding of sodium aluminosilicate hydrate (N-A-S-H) gel is still limited due to its complex and amorphous structure. Recently, molecular dynamics simulation has provided a unique opportunity to better understand the structure of N-A-S-H gel from nanoscale. In this work, the N-A-S-H gel structure was obtained by simulating the polymerization of Si and Al monomers by molecular dynamics. The simulated polymerization process is in good agreement with the experimental results especially in terms of the reaction rate of Si and Al species. The atomic structural features of the N-A-S-H gel were analyzed in terms of bond length and bond angle information, simulated X-ray diffraction (XRD) and Qn distribution. A significant finding is the existence of pentacoordinate Al in all simulated N-A-S-H structures, indicating that pentacoordinate Al in geopolymer does not only come from raw material. Besides, the results show that a smaller Si/Al ratio led to a more crosslinked and compacted structure of N-A-S-H gel.@en

Alkali-activated slag and fly ash (AASF) materials are emerging as promising alternatives to conventional Portland cement. Despite the superior mechanical properties of AASF materials, they are known to show large autogenous shrinkage, which hinders the wide application of these eco-friendly materials in infrastructure. To mitigate the autogenous shrinkage of AASF, two innovative autogenous-shrinkage-mitigating admixtures, superabsorbent polymers (SAPs) and metakaolin (MK), are applied in this study. The results show that the incorporation of SAPs and MK significantly mitigates autogenous shrinkage and cracking potential of AASF paste and concrete. Moreover, the AASF concrete with SAPs and MK shows enhanced workability and tensile strength-to-compressive strength ratios. These results indicate that SAPs and MK are promising admixtures to make AASF concrete a high-performance alternative to Portland cement concrete in structural engineering.

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