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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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.

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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.

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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.

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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.

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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.

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The pore solutions of a series of hardened alkali-activated slag/fly ash pastes were extracted by the steel-die method, and analyzed using ICP-OES analysis technique. According to the saturation index from thermodynamic calculations, the pore solutions of alkali-activated slag pastes kept oversaturated with respect to solid reaction products with time. In the pore solutions of alkali-activated fly ash pastes, an increase of temperature (from 40 °C to 60 °C) led to decreases of the concentrations of Si, Al, Ca, Na, OH−, K, Fe and Mg, while the soluble silicate in the alkaline activator resulted in increases of the concentrations of these elements. Compared to the alkali-activated slag paste with the same alkaline activator, 50% replacement of slag by fly ash did not result in a substantial change of the pore solution composition. Based on the experimental results, conceptual models were proposed to describe the elemental concentrations in the pore solutions.@en

Alkali-activated slag/fly ash (AASF) as an environmental-friendly binder system for construction materials has recently attracted great attention from both academic and industrial communities. Towards its wider engineering application, it is crucial to have a better understanding of the temperature induced effects by different curing regimes and the temperature sensitivity on the thermal properties of this system, for instance the apparent activation energy (Ea). However, the available information on Ea of AASF system is still quite limited. The present study is aimed at investigating the role of alkaline activator chemistry on the reaction kinetics of AASF at early age. The binder is made of 50 wt.% blast furnace slag and 50 wt.% fly ash. Four alkaline activator silicate moduli (SiO2/Na2O ratio = 0.8, 1.0, 1.2 and 1.5) were used for the mixture preparation. The effect of activator modulus on the heat evolution was studied by performing isothermal calorimetry test up to 160 h at both 20°C and 40 °C. The cumulative heat release and ultimate total heat were studied through curve fitting using exponential model. Furthermore, the Ea of AASF pastes was determined using incremental methods and its variation over wide range of early age reaction was studied. It was found that the activator modulus evidently influences the heat evolution of AASF. The cumulative heat release reached the maximum value at activator modulus of 1.0, followed by at 0.8, 1.2 and 1.5. This trend is inversely related to the changes of Ea of AASF mixtures. In addition, it was confirmed that the Ea of AASF was not only related to the chemistry of reactants but also reaction-stage dependent. Particularly it varied significantly at the very early age of reaction. @en

Alkali-activated materials (AAMs) show promising potentials for use as building materials. Before the utilization, AAMs must satisfy the properties required from the construction sector, such as good durability and long-term service life. These properties mainly depend on the chemical and physical properties of the microstructure of AAMs. In the literature many studies have been presented about the reaction process and microstructure formation of AAMs, but still some aspects are not given due attention, such as the pore solution composition of AAMs and the origin of the induction period during the reaction of AAMs. Furthermore no computer-based simulation models have been developed so far for simulating the reaction process and microstructure formation of AAMs. It is still a big issue and challenge today to numerically obtain the microstructure of AAMs.This research adopted two routes to study the reaction process and microstructure formation of AAMs, i.e. the experimental study route and numerical simulation route. Ground granulated blast furnace slag and fly ash were used as the aluminosilicate precursors and sodium hydroxide and sodium silicate were used as the alkaline activators. The experimental study route provided new results for better understanding the reaction process and microstructure formation of AAMs. Then the new insights from the experimental study route helped to develop the GeoMicro3D model for simulating the reaction process and microstructure formation of AAMs.(1)Experimental study routeFirstly, the pore solutions of alkali-activated slag, alkali-activated fly ash and alkali-activated slag/fly ash pastes with different activators and reaction conditions were studied by means of ICP OES. It was found the pore solution composition of AAMs depended on the activation conditions, such as the type and concentration of alkaline activator and curing temperature. Then, the reaction kinetics of alkali activated slag, alkali activated fly ash and alkali activated slag/fly ash pastes were investigated by using isothermal calorimetry. The sodium content, silica content and curing temperature affected the reaction kinetics of AAMs. The origin of the induction period was different for alkali-activated slag systems and alkali-activated fly ash systems. For alkali activated slag systems, the presence of soluble Si in the activator slowed down the dissolution of slag and caused an induction period. In contrast, the fact that an induction period occurred in alkali-activated fly ash systems was mainly attributed to the passivation of the leached surface layer caused by the absorbed Al.Finally, the microstructure development of alkali-activated slag, alkali-activated fly ash and alkali-activated slag/fly ash pastes was studied using SEM and MIP. It was found that the type and concentration of alkaline activator affected the microstructure formation of AAMs. An increase of Na2O content led to a reduction of the total porosity and a refinement of the microstructure for alkali-activated slag systems. In contrast, an increase of Na2O content did not affect the total porosity of alkali activated fly ash systems very much. Instead, it altered the microstructure by increasing the amount of large pores and decreasing the amount of small pores. The SiO2 content seriously affected the microstructure formation of AAMs. In sodium silicate activated systems, the soluble silicate in the activator led to a relatively dense microstructure with separated small capillary pores. This was different from the relatively coarse microstructure with connected capillary pores in sodium hydroxide activated systems.(2)Numerical simulation routeFirstly, the initial particle parking structure of AAMs, as the starting point for simulating the reactions and microstructure formation, was simulated using real-shape particles of slag and fly ash. In comparison with the spherical particles, using real-shape particles increased the total surface area (up to 23 %) and bulk specific surface area (at least 12 %) of the simulated initial particle parking structures. At low liquid/binder ratios (≤ 0.47), using real shape particles led to a significant shift of the pore size distribution to small pores as compared to using the spherical particles in the simulated initial particle parking structures.Secondly, a dissolution model was developed for simulating the dissolution of aluminosilicate precursors in alkaline solution. The influences of temperature, reactivity of precursors, alkalinity of solution and inhibiting effect of aqueous Al etc. on the dissolution of precursors were taken into account in this model. The simulation results of the dissolution of slag and fly ash in alkaline solution were in agreement with the experimental data.Then, the reactions in AAMs were thermodynamically simulated. A thermodynamic model, i.e. N(C)ASH_ss, was developed to describe the N A S H gel. With this model and the C(N)ASH_ss model in the literature it is possible to perform thermodynamic modelling of the reactions in alkali-activated fly ash systems and alkali-activated slag/fly ash systems. The simulated pore solution compositions of AAMs were in line with the experimental results.Finally, the GeoMicro3D model was built up based on the numerical modules of initial particle parking structure, dissolution of aluminosilicate precursor, thermodynamic modelling and nucleation & growth of reaction products. As a case study GeoMicro3D was implemented to simulate the reaction process and microstructure formation of alkali-activated slag with three different alkaline activators. The simulated reaction kinetics (degree of reaction of slag) and pore solution chemistry (element concentration) were in agreement with the experimental results. The simulated volume evolution of solid phases by GeoMicro3D was consistent with the results calculated by GEMS with regard to the primary reaction products (C (N )A S H) and some crystalline reaction products, such as the hydrotalcite-like phase and katoite. Besides the volume evolution of solid phases, GeoMicro3D also provided the volumes of adsorbed water and gel pore water that were retained by the C (N )A S H gel.To sum up, the reaction process and microstructure formation of AAMs were studied experimentally and numerically. Based on the insights obtained from the experimental study numerical models were developed and validated. With these models it is possible to simulate the initial particle parking structure of slag/fly ash in alkaline activator, dissolution of slag/fly ash, chemical reactions and microstructure formation of AAMs with reasonable accuracy.@en

The aim of RILEM TC 247-DTA ‘Durability Testing of Alkali-Activated Materials’ is to identify and validate methodologies for testing the durability of alkali-activated concretes. To underpin the durability testing work of this committee, five alkali-activated concrete mixes were developed based on blast furnace slag, fly ash, and flash-calcined metakaolin. The concretes were designed with different intended performance levels, aiming to assess the capability of test methods to discriminate between concretes on this basis. A total of fifteen laboratories worldwide participated in this round robin test programme, where all concretes were produced with the same mix designs, from single-source aluminosilicate precursors and locally available aggregates. This paper reports the mix designs tested, and the compressive strength results obtained, including critical insight into reasons for the observed variability in strength within and between laboratories.@en

Chemical deformation (chemical shrinkage/expansion), the absolute volume change during reactions, is a key parameter influencing the volume stability, especially the autogenous deformation of a binder material. This work, for the first time, reports an in-depth investigation on the chemical deformation of metakaolin based geopolymer (MKG). Unlike ordinary Portland cement-based binders with monotonic chemical shrinkage, MKG experiences three stages of chemical deformations: chemical shrinkage in the first stage, chemical expansion afterward and chemical shrinkage again in the final stage. Various experimental techniques (XRD, FTIR and NMR) plus theoretical calculations are applied to explore the mechanisms behind the chemical deformation of MKG. Clear correlations are found between the chemical deformations and the reaction processes during geopolymerization. A conceptual chemical deformation model for geopolymer is summarised. The insights into the chemical deformation provided by this study will play a fundamental role in further understanding, controlling and even utilizing the deformation behaviours of geopolymers.@en

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.

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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.

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Many particle-based numerical models have been used to simulate the hydration process of cementitious materials. Most of those models employ regular shape particles, like the commonly used spheres, to represent cement, slag, or fly ash, which neglects the influence of particle shape. To deal with this issue, this study extended the Anm material model and used irregular shape particles to simulate the initial particle parking structures of cement/geopolymer pastes. The irregular shapes of cement, slag and fly ash particles were characterized by spherical harmonic series. Compared to the initial particle structures simulated using spherical particles, those using irregular shape particles had total surface areas and bulk specific surface areas with up to 37.40% and 36.84% larger, respectively. However, the pore size distributions of the simulated initial particle structures did not show significant influence of particle shape. As a demonstration to illustrate the influence of particle shape on dissolution, the initial particle parking structure of amorphous silica in alkaline solution was generated using irregular shape particles, and was used as input to simulate the dissolution of silica particles. The Lattice Boltzmann method was used to simulate the transport process of aqueous ions and thermodynamics was employed to consider the rate of dissolution of silica. The dissolved fractions of silica at different temperatures in the simulations agreed well with experimental measurements. The influences of continuous stirring, concentration of alkali and particle shape on the dissolution kinetics of silica were investigated numerically.@en

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.

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The carbonation-induced degradation in alkali-activated materials (AAMs) is unknown. This is due to the current level of understanding of the carbonation mechanism in accelerated conditions, whereas the results from the long-term performance of AAMs in service are not available.

In this paper, the natural laboratory carbonation of alkali-activated fly ash (FA) and blast furnace slag (BFS)-based pastes was studied. The aim of experiments was to investigate the influence of the binder composition on the carbonation mechanism. The microscope techniques, i.e. polarization, fluorescent and Environmental Scanning Electron Microscope (ESEM) enabled the identification of the carbonation front and local defects due to carbonation, such as microcracks, and pore structure changes. The pH of the simulated pore solution in carbonated and noncarbonated samples was analyzed. The pore structures after carbonation were characterized in terms of capillary and gel pores by Mercury Intrusion Porosimetry (MIP) and nitrogen sorption. The effect of the change in the porosity on the mechanical properties was examined by Nanoindentation tests.

Results show that the natural carbonation did not reduce the alkalinity of the pore solution to below the pH 9. The pH was kept above 10.5 in all the mixtures. The samples with 50 wt% and less BFS content, were uniformly carbonated. The increase of the total porosity in those samples has been attributed to the simultaneous effect of the carbonation and drying shrinkage of the pastes. The change in the porosity for a consequence has a reduction in the modulus of elasticity. The samples containing more than 50 wt% of BFS were fully resistant to carbonation or carbonation was induced along the cracks as observed from the microscopic analysis.@en

Pore solution of hardened alkali-activated fly ash paste was extracted by the steel-die method. The aqueous phase composition of pore solution was analyzed using ICP-OES analysis technique. The results show that the concentrations of Si, Al, Ca, K and OH- decrease with curing time regardless of the curing temperatures (40°C/60°C) and alkaline activators (sodium hydroxide with/without sodium silicate). On the contrary, the concentration of S increases with curing time. A higher temperature curing decreases the solubility of Si, Al, Ca and K in the alkali-activated fly ash, while it doesn’t show much influence on the solubility of S. The plot of the concentration of Al versus the concentration of Si displays a quasi-linear logarithmic relationship. This relationship implies congruent removal of Si and Al from the frameworks of fly ash.@en

The pore structure of alkali-activated slag has a significant influence on its performance. However, the literature shows insufficient studies regarding the suitability of different techniques for characterizing the pore structure and the influences of Na2O and curing age on pore structure development. In pursuit of a better understanding, the pore structure of sodium hydroxide activated slag paste was characterized by multiple techniques, e.g., mercury intrusion porosimetry (MIP), nitrogen (N2) adsorption, and scanning electron microscopy (SEM) image analysis. The sodium hydroxide activated slag pastes were prepared with three different contents of Na2O (Na2O/slag = 4, 6, and 8%) and cured for different times up to 360 days. The microstructure observation reveals that outer C–(N–)A–S–H and inner C–(N–)A–S–H grow successively around the reacting slag grains, along with crystalline reaction products which are formed in the empty coarse pore space. The increase of Na2O content and curing age lead to a finer pore structure. The MIP measurements show that the total porosity drops about 70% within the first day, and that one peak at most, corresponding to gel pores, was identified in the differential curves of all the investigated samples from 1 to 360 days. On the contrary, only one peak, corresponding to capillary pores, was identified by SEM-image analysis. The differential curves derived from N2 adsorption generally reveal two peaks, and the trend that the pore diameters of those two peaks vary with curing age depends on the content of Na2O. Compared to Portland cement, sodium hydroxide activated slag has a higher pore space filling capacity (χ, Vproducts/Vslag-reacted), while the capacity decreases with increasing Na2O content and curing age@en

Carbonation of the pore solution in alkali-activated materials (AAMs) produces alkali and/or alkali-earth carbonates. When the carbonate solubility in the water is very high (case of the most alkali carbonates), it is very hard to determine the carbonation depth in AAMs with the phenolphthalein indicator frequently used in Ordinary Portland Cement (OPC)-based materials. Carbonation gradually decreases the alkalinity of the pore solution, while the color after spraying phenolphthalein changes from colorless to pink when pH< 13 and changes back to colorless when pH< 8.2. The color change with phenolphthalein indicator may still exist in the less alkaline areas where carbonation may have already occurred. Therefore, using the indicator test is likely to underestimate the depth to which carbonation reaction has occurred in AAMs and more complete assessment is required. This study investigates the carbonation front in alkali-activated fly ash (FA) and blast furnace slag (BFS) pastes in natural laboratory conditions. Monitoring carbonation front in the samples after one year of exposure has been carried out under polarized light microscope (PLM), and environmental scanning electron microscope (ESEM). The carbonation products were sharply distinguished from the other constituents of the paste, by their crystallographic and optical characteristics under PLM, and characterized by X-Ray diffraction (XRD). @en

In this paper, carbonation resistance of alkali-activated slag (AAS) pastes exposed to natural and accelerated conditions up to 1 year was investigated. Two aspects of carbonation mechanism were evaluated. The first was the potential carbonation of the main binding phases in finely powdered AAS pastes. The second was the reactivity and diffusivity of CO2 within the bulk AAS paste. From Fourier transform infrared spectroscopy and thermogravimetric analysis coupled with mass spectroscopy time-series measurements, it was found that powdered AAS was largely carbonated within 28 days with a CO2 uptake of 14 wt%. The main carbonation products were calcium carbonates. Nevertheless, the bulk paste samples were highly resistant to carbonation, regardless of the exposure conditions. The findings showed that the pH value (initial pH[12) and strength of the samples did not decrease under accelerated carbonation compared to those of the samples exposed under natural conditions. The mineralogy of the samples in these two carbonation exposures did not alter either, except for outdoor conditions. The gel pores were dominant in the pastes (pore size in range of 2–15 nm). The dense microstructure was the main barrier for CO2 to diffuse and further react with binding phases.@en