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.

The proper control of the rheological performance of silicate-based alkali-activated slag (AAS) mixtures is problematic, as the conventional superplasticizers become less effective in alkaline media. Nevertheless, several methods have been proposed to improve the workability of silicate-activated AAS, such as by extending the mixing time, and replacing sodium silicate with sodium carbonate activators. However, the underlying fluidizing mechanism is not yet well understood in the literature, which is crucial knowledge to achieve proper rheology control of silicate-activated AAS. In this study, the effects of mixing conditions and activator anionic species on the rheology of silicate-activated AAS concrete have been assessed. The reaction products, particle size and interparticle interactions, as well as the reaction kinetics in AAS, have been further investigated to understand the distinct fluidizing mechanisms. By using a longer mixing time, it was found that the solid particles formed at early ages are broken down into smaller particles, accompanied by a slight increase in the amount of reaction products to improve the fluidity. With the sodium carbonate substitution, the calcium ions dissolved from slag particles are entrapped into calcium carbonate precipitates to slow down the accumulation of C-(A)-S-H phases, leading to a better dynamic flow. However, the interparticle interactions are intensified due to the formation of larger particles and the declined dispersing effect induced by silicate activators.

Although the application of blast furnace slag and fly ash-based alkali-activated concrete (BFS/FA-AAC) has both economic and environmental benefits, it is limited by the lack of a straightforward mix design method. In this paper, an experiment was conducted to investigate the effect of control factors, including the Na₂O/binder ratio, the SiO₂/Na₂O ratio, the BFS/binder ratio, the water/binder ratio, and the water content on the workability (slump and rheology) of BFS/FA-AAC, and the effect of control factors include the Na₂O/binder ratio, the SiO₂/Na₂O ratio, the BFS/binder ratio, the water/binder ratio, and the curing time on the compressive strength of BFS/FA-AAC. As a result, the influence degree and mechanism of each control factor on the performance of BFS/FA-AAC were quantitively explored and the accuracy of an empirical compressive strength formula was validated. Based on that, a practical mix design method of BFS/FA-AAC was eventually established. It is found that the mixture composition and content of paste can significantly influence the workability of BFS/FA-AAC. The compressive strength of BFS/FA-AAC is determined by control factors when the water content is within 160–195 kg/m³. The mechanical predictive method of BFS/FA-AAC is proven of high accuracy. The mixture designed by this methodology exhibits satisfied fresh and hardened performance as well as high environmental benefits.

In this paper, 871 data were collected from literature and trained by the 4 representative machine learning methods, in order to build a robust compressive strength predictive model for slag and fly ash based alkali activated concretes. The optimum models of each machine learning method were verified by 4 validation metrics and further compared with an empirical formula and experimental results. Besides, a literature study was carried out to investigate the connection between compressive strength and other mechanical characteristics. As a result, the gradient boosting regression trees model and several predictive formulas were eventually proposed for the prediction of the mechanical behavior including compressive strength, elastic modulus, splitting tensile strength, flexural strength, and Poisson's ratio of BFS/FA-AACs. The importance index of each parameter on the strength of BFS/FA-AACs was elaborated as well.

Alkali-activated concrete (AAC) is regarded as a promising alternative construction material to reduce the CO₂ emission induced by Portland cement (PC) concrete. Due to the diversity in raw materials and complexity of reaction mechanisms, a commonly applied design code is still absent to date. This study attempts to directly correlate the AAC mix design parameters to their performances through an artificial intelligence approach. To be specific, 145 fresh property data and 193 mechanical strength data were collected from laboratory tests on 52 AAC mixtures, which were used as inputs for the machine learning algorithm. Five independent random forest (RF) models were established, which are able to predict fresh and hardened properties (in terms of compressive strength, slump values, static/dynamic yield stress, and plastic viscosity) of AAC with equivalent accuracy reported in the literature. Moreover, an inverse optimization was performed on the RF model obtained to reduce the sodium silicate dosages, which may further mitigate the environmental impact of producing AAC. The present RF model gives practical information on AAC mix design cases.

Alkali-activated material (AAM) is developed as a green alternative binder to replace Portland cement (PC) in the construction field. However, the large-scale application with AAM concrete is limited so far, with the insufficient knowledge of rheological behavior being a major obstruct. Thixotropy of concrete is of great interest, which can be helpful to predict various early-age performances. The current study dedicates to evaluating the thixotropy of alkali-activated slag (AAS) concrete mixtures with different silicate and water content in activator solutions. In specific, the silicate modulus (Ms) and water to binder (w/b) ratio have been varied. The thixotropic index calculated by the initial and equilibrium shear stress from the stress growth test, as well as the breakdown area obtained by applying different shear speeds were used to evaluate the thixotropy of AAM concrete. Results indicate a good correlation between different approaches. It was found that an increase in Ms led to more pronounced thixotropic behaviors in AAS concrete due to the rapid nucleation and accumulation of early hydration products, resulting in significant increases in peak torque values and slight reductions of torque at equilibrium. Besides, the concrete thixotropy gradually declined by applying a higher w/b ratio.

To better understand early stiffening of AAS pastes, distinctive microstructural features by varying the silicate modulus (Ms) have been visualized with in-situ microscopy. In addition, the activation reaction was monitored with multiple approaches, while solid and liquid phases in hydrating AAS were characterized separately. In silicate-activated AAS, it was found fine granules of reaction products are intensively dispersed in the activator solution, leading to a less flocculated system. Compared to hydroxide-activated AAS, the development of interparticle connections was limited at early ages, whereas reaction products were detected with much smaller grain size, less crystalline phase, and higher Al incorporation. Results indicate that the stiffening of hydroxide-activated AAS is attributed to the formation of a well-percolated network through solid reaction products. Instead, massive fine granules of reaction products dispersed in the pore solution continuously develop, which may intensify the interparticle interactions and macroscopically results in the stiffening of a silicate-activated AAS.

Cementitious materials exposed to marine and saline environments are commonly threatened by a combined attack of sulfate and chloride ions. This study developed a numerical framework to investigate two combined coupling mechanisms of 1) coupled solid-liquid chemical reactions for competitive chloride-sulfate attack and 2) electrostatic multi-ion coupling effect on reactive-transport mechanisms. Various chemical reactions including sulfate attack with anhydrous calcium aluminates, secondary precipitation of expansive minerals, competitive binding, and calcium leaching have been quantified. The electrostatic potential caused by multi-ions coupling was solved according to constitutive electrochemical laws. After model validation, the chemical coupling mechanisms for solid-liquid reactions during competitive chloride-sulfate binding were investigated. On this foundation, the influence of electrostatic multi-ionic coupling effects on ionic transport and its interaction with chemical coupling were disclosed. It was found that neglecting multi-ions coupling effect would result in an underestimated chemical coupling strength in competitive chloride-sulfate binding.

In our former paper, based on a published 3D reactive transport model at microscale with the capability of simulating the chemical reactions involved in ASR, the location of expansive ASR gel related to the reactivity of aggregate, temperature, aggregate porosity and silica content in aggregate, is clarified. Based on the simulation results, in this paper, the cracking process at mesoscale in concrete induced by ASR in the early stage is investigated. The results show that the cracking process can be divided into four stages and three cracking routes are generalized with the behind chemical exposed environments specified. The cracking routes are found to be comparable with the experimental observed routes. For the first time, the cracking patterns induced by ASR in concrete at mesoscale is linked with the chemical reactions at microscale, which is the first step towards building a complete computational tool to predict ASR as realistic as possible.

The rapid workability loss of alkali-activated materials (AAM) has been a major obstacle limiting its onsite application. In this study, two conventional SPs (made of polynaphthalene sulfonate (PNS) and lignosulfonate (LS) salts), which have been reported to be effective in some specific AAM mixtures were separately applied in alkali-activated slag (AAS) concretes. A comprehensive testing program was performed to study their effect on reaction kinetics, rheology evolution, and strength development. Results showed sodium silicate-activated AAS mixtures exhibited lower yield stress than those activated by sodium hydroxide. In hydroxide media, PNS and LS remained effective to reduce yield stress and increase slump value, while they both failed to improve the rheological behavior of AAS activated by silicate. Moreover, the inclusion of 2% admixtures did not result in much strength reduction in both activators although LS showed a retardation effect and subsequent increase in the setting time in the fresh state.

The present study explores the possibility of replacing blast furnace slag (BFS) with coal fly ash (FA) to produce alkali-activated material (AAM) concrete with hybrid precursors. With an increased FA replacement ratio, the reaction kinetics, fresh and hardened properties of AAM mixtures have been investigated. The retardation effect on the reaction kinetics with an increased FA content has been observed, which not only extended the induction period along with the heat flow evolution but also reduced the cumulative heat release up to 24 h. Spherical FA particles can provide a ball-bearing effect to improve the workability of the hybrid AAM mixtures, while FA also slows down the deterioration of fresh properties since they are less reactive compared to BFS particles. Regarding the strength development, FA results in the reduction at all curing ages in the mixtures with a low silicate modulus (Ms0.25). Similarly, reduction in 1-day compressive strength has been detected in high silicate modulus mixtures (Ms0.5) with FA replacement, while the mixture with 10% FA exhibits the highest compressive strength among Ms0.5 concretes at later curing ages. Bigger capillary pores have been detected in AAM mixtures with an increase in FA content. However, AAM with 10% FA shows the lowest porosity in Ms0.5 mixtures, which is in agreement with the compressive strength results.

Alkali-activated materials (AAM) are known to be environmentally friendly alternatives to cement-based materials because they can potentially reduce greenhouse gas emissions and reutilize industrial by-products/wastes. To study the factors influencing the strength of slag based alkali-activated materials (BFS-AAM), fly ash based alkali-activated materials (FA-AAM), slag, and fly ash-based alkali-activated materials (BFS/FA-AAM), and clarifying their reaction mechanisms, this paper reviews current knowledge about the mechanical properties and the reaction mechanisms of BFS-AAM, FA-AAM, and BFS/FA-AAM. The precursor requirements and the strength control factors are summarized. The control factors for the strength of BFS/FA-AAM are the BFS/binder ratio, the Na₂O/binder ratio, the SiO₂/Na₂O ratio, and the w/binder ratio. Ion concentrations, determined by these control factors, play a decisive role in the development of strength. Generally, the strength is proportional to the BFS/FA ratio. The optimal values of the Na₂O/binder ratio of BFS-AAM and FA-AAM are between 5.5% and 8% and between 7 and 10%, respectively. The optimal values of the SiO₂/Na₂O ratio of BFS-AAM and FA-AAM are between 0.85 and 1.4 and between 0.6 and 1, respectively. Increasing the w/binder ratio will only benefit workability but will affect the strength negatively. A w/binder ratio of around 0.4 may strike a balance between strength and workability.

This study provides a detailed investigation on the reproducibility of two groups of alkali-activated slag (AAS) mixtures, from both fresh properties and strength development perspectives. Three different commercial sodium silicate solutions and one lab-produced silicate activator (made of silica fume and sodium hydroxide) were used to prepare AAS pastes with the same nominal composition in each group. The reaction process of each AAS mixture was monitored by calorimetry and ultrasonic pulse velocity (UPV) measurements. Meanwhile, mini-slump and flow curve tests measured by rheometer were conducted in the first hour to characterize the evolution of fresh properties. The compressive and flexural strength of hardened AAS mortars were measured at different curing ages. The results revealed that AAS pastes prepared with three different sodium silicate solutions exhibited almost identical reaction kinetics, as well as the evolution of fresh properties and strength development. However, the reaction took place rather fast in AAS pastes made of silica fume. These mixtures showed worse rheology and less strength than the corresponding mixtures prepared with sodium silicate solutions. Furthermore, the present study also showed the feasibility of making the same AAS paste through different class commercial sodium silicate solutions.

A 3D reactive transport model at microscale is proposed for simulating the chemical reaction process of alkali silica reaction (ASR) thermodynamically and kinetically including the dissolution of reactive silica, the nucleation and growth of ASR products and the dissolution of calcium hydroxide (CH) and calcium silicate hydrate (C-S-H) as a buffer of Ca²⁺ and OH⁻ to ASR. The implementation methodologies are firstly explained. Sensitivity analyses are done to calibrate some important parameters. The model is then applied to investigate the influence of the silica microstructural disorder degree on ASR. The simulation results show that the model is able to simulate successfully two typical patterns of the expansion sites location depending on the silica reactivity (inside the aggregate or in the aggregate-cement paste zone) found in field concrete and laboratory samples. A possible mechanism is provided based on the quantitative data captured by the model. The model can be extended to a multiscale ASR model for physic-chemical simulation to bridge the gap between the fundamental chemical mechanisms and the physical response of concrete.

Prediction of alkali silica reaction is still difficult due to the lack of a comprehensive understanding of its chemical fundamentals. In-site experimentally revealing the fundamentals is not realistic as ASR shows over several years or even decades and is affected by many factors. In this paper, by utilizing a 3D reactive-transport simulation model at microscale, we have numerically explored the fundamentals of ASR in the early stage under the influence of reactive silica fraction, alkali concentration, silica disorder degree and aggregate porosity. Based on the simulation results, the chemical sequences of ASR, the initial location of ASR products, the mechanism behind and the role of calcium under the influence of the above factors are elaborated. Furthermore, a comprehensive mechanism to explain the pessimum reactive aggregate content is derived. The results of this paper give some insights about ASR in the early stage such as the initial expansion locations.

The production of cement and concrete contributes significantly to global greenhouse gas emissions. Alkali‐activated concretes (AACs) are a family of existing alternative construction materials that could reduce the current environmental impact of Portland cement (PC) production and utilisation. Successful applications of AACs can be found in Europe and the former USSR since the 1950s and more recently in Australia, China and North America, proving their potential as construction materials. However, their utilisation is limited presently by the lack of normative and construction guidelines. Raw materials’ non‐uniform global availability and variable intrinsic properties, coupled with the lack of specific testing methods, raise questions regarding reproducibility and reliability. The mechanical and chemical behaviour of AACs has been investigated extensively over the past decades, strengthening its potential as a sustainable substitute for traditional PC‐based concrete. Although a wide amount of studies demonstrated that AACs could meet and even exceed the performance requirements provided by European design standards, a classification of these broad spectra of materials, as well as new analytical models linking the chemistry of the system components to the mechanical behaviour of the material, still need further development. This report gives an overview of the potential of alkali‐activated systems technology, focusing on the limitations and challenges still hindering their standardisation and wider application in the construction field.

This study aims to interpret the early-stage rheology of alkali-activated slag (AAS) paste from microstructure perspectives. The microstructures visualized by cryogenic scanning electron microscopy (cryo-SEM) revealed the essential distinction between hydroxide and silicate-activated slag pastes. The hydroxide-based mixture showed typical suspension features, where slag particles were dispersed in the hydroxide activators. In the hydroxide media, even at very early ages (5 min), the solid grains were attached to each other through rigid connections of reaction products, which resulted in high yield stress. As for the silicate-based mixtures, an emulsion phase has been observed between slag particles, which consists of discontinuous water droplets and continuous silicate gels. Fine emulsions with smaller water droplets were observed as the silicate modulus of activators increased, which dispersed the slag particles but on the other hand improved the viscosity of the paste. With increasing water to binder ratio, both yield stress and viscosity of AAS pastes significantly reduced.

In this paper, a series of experiments were conducted to systematically and quantitively explore the effects of control factors on the early age properties, i.e., workability and strength of slag and fly ash-based alkali-activated paste (BFS/FA-AAP). The control factors on the workability (flowability and setting time) of BFS/FA-AAP include the Na₂O/b ratio, the SiO₂/Na₂O ratio, the w/b ratio, and the BFS/b ratio. The control factors on strength (compressive strength and flexural strength) of BFS/FA-AAP further also include the curing condition and the curing age. The results show that a higher BFS/b ratio and Na₂O/b ratio could increase the strength while decreasing the workability. A higher w/b ratio could increase the workability while slightly decreasing the strength. Higher SiO₂/Na₂O ratio increases both strength and workability. Despite that, higher Na₂O/b ratio and SiO₂/Na₂O ratio could hinder the strength development. Sealed curing condition is proved to be a simple but efficient way to assure the steady strength development of BFS/FA-AAP. Although the strength of BFS/FA-AAP could generally stabilize after 90 days, the strength development rate varies with different mix proportions. In addition, a general methodology have been proposed to predict the compressive strength of BFS/FA-AAP and verified with experiments. Finally, a mix design table is proposed for the preliminary design of BFS/FA-AAP according to the principle of satisfying early age requirements.

This paper investigates the plastic viscosity of cement mortar with manufactured sand (MS) concerning the influences of geometric features and particle size of MS. The geometric features, including overall shape, angularity and roughness, of MS with various particle sizes were evaluated by aspect ratio, convexity area ratio, convexity perimeter ratio and circularity. The plastic viscosity of cement mortar was calculated based on the Bingham model. Results show that the combined effects of overall shape, angularity and roughness provide coarser MS particles with lower circularity. In terms of relative plastic viscosity, Robinson model shows optimal fittings for all mixtures and is thus used to determine the packing fraction of MS under shearing. From the particle packing viewpoint, shear-induced orientation increases the packing fraction of non-spherical MS particles from the random loose packing fraction and the influence is increasingly prominent with the decrease of circularity. The relative volume fraction is an important parameter influencing the relative plastic viscosity of mixtures with MS while the relative paste film thickness (R_PFT), calculated from the real packing fraction and specific surface area (SSA), is found as the dominating factor. The dependence of plastic viscosity of cement mortar on geometric features and particle size of MS can be attributed to their influences on the packing fraction and SSA of particles.

The microstructure of alkali-reactive aggregates, especially the spatial distribution of the pore and reactive silica phase, plays a significant role in the process of the alkali silica reaction (ASR) in concrete, as it determines not only the reaction front of ASR but also the localization of the produced expansive product from where the cracking begins. However, the microstructure of the aggregate was either simplified or neglected in the current ASR simulation models. Due to the various particle sizes and heterogeneous distribution of the reactive silica in the aggregate, it is difficult to obtain a representative microstructure at a desired voxel size by using non-destructive computed tomography (CT) or focused ion beam milling combined with scanning electron microscopy (FIB-SEM). In order to fill this gap, this paper proposed a model that simulates the microstructures of the alkali-reactive aggregate based on 2D images. Five representative 3D microstructures with different pore and quartz fractions were simulated from SEM images. The simulated fraction, scattering density, as well as the autocorrelation function (ACF) of pore and quartz agreed well with the original ones. A mm concrete cube with irregular coarse aggregates was then simulated with the aggregate assembled by the five representative microstructures. The average pore (at microscale m) and quartz fractions of the cube matched well with the X-ray diffraction (XRD) and Mercury intrusion porosimetry (MIP) results. The simulated microstructures can be used as a basis for simulation of the chemical reaction of ASR at a microscale.