This study presents an extended numerical approach based on GeoMicro3D to simulate the reaction kinetics and three-dimensional (3D) microstructure evolution of alkali-activated fly ash (AAFA). Dissolution experiments were conducted under varying NaOH concentrations and temperatures to formulate predictive rate functions for Si and Al release. These experimentally derived kinetic functions, alongside a thermodynamic dataset for N-(C-)A-S-H gels, were incorporated into the GeoMicro3D model to capture the chemical reactions and 3D microstructure evolution of AAFA. The model well captured reaction degree of fly ash, formation of solid products, evolution of pore solution compositions, and porosity over time. Notably, it is the first to predict the time-dependent spatial distribution of phases within the 3D AAFA microstructure by integrating kinetic and microstructural modeling. Dual validation using both dissolution data and microstructural metrics demonstrates the model's reliability and robustness. This integrated framework provides new insights into the coupled chemical–microstructural evolution of alkali-activated materials.

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.

A dissolution numerical model was proposed in this study to capture the real dissolution kinetics of slag in alkaline solution. It consists of three modules, i.e. (i) simulation of the initial particle parking structure of slag in alkaline solution using real-shape particles of slag, (ii) simulation of the chemical reactions between slag and solution based on the transition state theory, and (iii) simulation of the physical transport of aqueous ions using the lattice Boltzmann method. This dissolution numerical model was verified using experimental results, showing reasonable accuracy. After verification, the dissolution numerical model was implemented to study the influences of temperature and particle shape using a proper recipe of slag in alkaline solution. This recipe was designed to avoid solid phase precipitation or gel formation via thermodynamic analysis. The simulation results showed faster dissolution kinetics of slag when using higher temperatures and more irregular particle shapes.

For the first time, this study developed a novel model, named GeoMicro3D, to simulate the reaction process and microstructure formation of alkali-activated slag. The GeoMicro3D model consists of four modules that are designed to simulate, respectively: (i) the initial spatial distribution of real-shape slag particles in alkaline activator, (ii) the dissolution of slag and diffusion of ions via the transition state theory and lattice Boltzmann method, respectively, (iii) the spatial distribution of reaction products using a nucleation probability theory, and (iv) the chemical reactions with thermodynamic modelling. Afterwards the GeoMicro3D model was implemented and verified. The simulation results were discussed and compared with the relevant experimental data and thermodynamic calculation results using GEMS. A good agreement was found in the comparisons, showing the strong simulation capability of GeoMicro3D.

Many standardised durability testing methods have been developed for Portland cement-based concretes, but require validation to determine whether they are also applicable to alkali-activated materials. To address this question, RILEM TC 247-DTA ‘Durability Testing of Alkali-Activated Materials’ carried out round robin testing of carbonation and chloride penetration test methods, applied to five different alkali-activated concretes based on fly ash, blast furnace slag or metakaolin. The methods appeared overall to demonstrate an intrinsic precision comparable to their precision when applied to conventional concretes. The ranking of test outcomes for pairs of concretes of similar binder chemistry was satisfactory, but rankings were not always reliable when comparing alkali-activated concretes based on different precursors. Accelerated carbonation testing gave similar results for fly ash-based and blast furnace slag-based alkali-activated concretes, whereas natural carbonation testing did not. Carbonation of concrete specimens was observed to have occurred already during curing, which has implications for extrapolation of carbonation testing results to longer service life periods. Accelerated chloride penetration testing according to NT BUILD 443 ranked the tested concretes consistently, while this was not the case for the rapid chloride migration test. Both of these chloride penetration testing methods exhibited comparatively low precision when applied to blast furnace slag-based concretes which are more resistant to chloride ingress than the other materials tested.

To find materials with an appropriate response to THz radiation is key for the incoming THz technology revolution. Unfortunately, this region of the electromagnetic spectra remains largely unexplored in most materials. The present work aims at unveiling the most significant THz fingerprints of cement-based materials. To this end transmission experiments have been carried out over Ordinary Portland Cement (OPC) and geopolymer (GEO) binder cement pastes in combination with atomistic simulations. These simulations have calculated for the first time, the dielectric response of C-S-H and N-A-S-H gels, the most important hydration products of OPC and GEO cement pastes respectively. Interestingly both the experiments and simulations reveal that both varieties of cement pastes exhibit three main characteristic peaks at frequencies around ~0.6 THz, ~1.05 THz and ~1.35 THz, whose origin is governed by the complex dynamic of their water content, and two extra signals at ~1.95 THz and ~2.75 THz which are likely related to modes involving floppy parts of the dried skeleton.

Many calorimetric studies have been carried out to investigate the reaction process of alkali-activated slag paste. However, the origin of the induction period and action mechanism of soluble Si in the dissolution of slag are still not clear. Moreover, the mechanisms behind different reaction periods are not well described. In this study, the reaction kinetics of alkali-activated slag paste was monitored by isothermal calorimetry and the effect of soluble Si was investigated through a dissolution test. The results showed that occurrence of the induction period in hydration of alkali-activated slag paste depended on the presence of soluble Si in alkaline activator and the soluble Si slowed down the dissolution of slag. A dissolution theory-based mechanism was introduced and applied to the dissolution of slag, showing good interpretation of the action mechanism of soluble Si. With this dissolution theory-based mechanism, origin of the induction period in hydration of alkali-activated slag was explicitly interpreted.

The aim of this paper was to investigate the effect of natural carbonation on the pore structure, and elastic modulus (E_m) of alkali-activated fly ash (FA) and ground granulated blast furnace slag (GBFS) pastes after one year of exposure in the natural laboratory conditions. The chemical changes due to carbonation were examined by X-ray diffraction (XRD), scanning electron microscope/energy-dispersive X-ray (SEM−EDX) and attenuated total reflectance Fourier transformed infrared spectroscopy (ATR-FTIR). Subsequently, the pore structure and E_m of the degraded material were tested by mercury intrusion porosimetry (MIP), nitrogen (N₂) adsorption, and nanoindentation. The chemical degradation of alkali-activated pastes due to natural carbonation is showed to be dependent on the GBFS content and their pore structure development. It was found that the pure alkali-activated GBFS paste was not carbonated at all within the tested period due to fine gel pore structure. On the other hand, carbonation of the gel in the pastes consisting FA and GBFS generated significant mineralogical and microstructural changes. The extensive decalcification of the gel was reflected in the increase of nanoporosity. Consequently, the E_m of the carbonated pastes decreased. This study suggests that the degradation of alkali-activated FA and GBFS pastes due to carbonation may be accurately evaluated through micromechanical properties measurements rather than only by testing alkalinity of the pore solution and corrosion of reinforcement such as commonly studied carbonation effect in the ordinary Portland cement (OPC)-based materials.

In previous researches, the thermodynamic modelling of alkali-activated slag was conducted as a function of the degree of reaction of slag, which makes it difficult to compare the modelling results with the experimental results in a time scale. In this study, the reaction kinetics of sodium hydroxide activated slag was studied using isothermal calorimetry and quantified using the Ginstling-Brounshtein equation. With the quantified reaction kinetics, the hydration of slag was thermodynamically modelled in a time scale. Based on the thermodynamically modelled phase assemblage, chemical shrinkage and phase evolution were derived as a function of time. Besides the isothermal calorimetry, a series of experimental techniques were used to evaluate the thermodynamic modelling results. Inductively coupled plasma-optical emission spectroscopy (ICP-OES) was used to investigate the pore solution composition. Thermogravimetric analysis (TGA) and X-ray diffraction (XRD) were used to study the reaction products. Energy-dispersive X-ray spectroscopy (EDX) was used to examine the elemental composition of reaction products. The experimental results were presented, discussed, and used to evaluate the thermodynamic modelling results in terms of pore solution composition and reaction products. The modelled pore solution composition matched the experimentally measured data within ± 1 order of magnitude. The thermodynamic modelling and experimental results were in agreement regarding bound water, type and amounts of reaction products.

Mercury intrusion porosimetry (MIP) measurements are widely used to determine pore throat size distribution (PSD) curves of porous materials. The pore throat size of porous materials has been used to estimate their compressive strength and air permeability. However, the effect of sample size on the determined PSD curves is often overlooked. In pursuit of a better understanding of the effect of sample size on mercury intrusion into porous materials, a combined experimental and numerical approach was applied. Quartz sand and epoxy resin were mixed to form artificial sandstone. Digital microstructures of the sandstone were obtained by using X-ray computed tomography (CT scan) technique. PSD curves of the artificial sandstone with different sample sizes were determined both by MIP measurement and by simulation of mercury intrusion (i.e., MIP simulation). Percolation analysis was performed on mercury-intruded pores in the digital microstructures. The PSD curves determined both by MIP measurements and by MIP simulations show that there was a significant effect of sample size on mercury intrusion before percolation of mercury-intruded pores. The effect of sample size decreased with the increasing pressure. After the mercury-intruded pores percolated through the samples, the effect of sample size on mercury intrusion became minor. The pore throat size of the artificial sandstone was used to estimate the air permeability using the relation proposed in the literature. The calculated air permeability of the smaller sandstone sample was higher. However, in principle, the air permeability of sandstone samples should be independent of the sample size. Two main conclusions can be drawn: (1) a fixed sample size should be used in MIP measurements or MIP simulation so that the PSD curves of different samples can be properly compared, (2) sample size needs to be considered when the pore throat size determined by MIP measurement is used for estimating air permeability.