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In this study, nano- SiO2 was synthesized using the sol-gel method to modify the properties of fly ash-slag geopolymers (FASGs). FASGs with six contents of sol-type nano- SiO2 (0, 0.1%, 0.2%, 0.3%, 0.4%, and 0.5%) and two alkali dosages (5% and 7% of Na2O) were prepared. The influences of sol-type nano- SiO2 and alkali content on the compressive strength, flexural strength, drying shrinkage, and microstructure of FASG were studied. The results demonstrated that the early-age compressive and flexure strength of FASG can be enhanced by sol-type nano- SiO2 due to the improved microstructure and the reaction products. The improvement effect of sol-type nano- SiO2 on mechanical properties was more significant for FASGs with lower alkali dosages. The optimized content of sol-type nano- SiO2 in FASG was 0.2% for the mechanical properties. The addition of the sol-type nano- SiO2 can mitigate the drying shrinkage of the FASG when the content is lower than 0.4%. The sol-type nano- SiO2 can promote the formation of reaction products in FASG and result in a denser microstructure of FASG. [ABSTRACT FROM AUTHOR]

The effects of calcium on the atomic structure of alkali‐activated binder materials were investigated. The alkali‐activated binder samples with varied Ca/Si ratios were first statically examined with both the X‐ray diffraction (XRD) and total scattering function analysis. The added calcium can reorder the atomic structure of the alkali‐activated materials and enhance its crystallization based on the X‐ray pair distribution function (PDF) and XRD analysis. In situ PDF experiments were undertaken to examine the evolution of the atomic structure of calcium‐contained geopolymer. The increase in calcium content accelerated the dissolution of precursor and the formation of the binder. It was shown that the additional Ca(OH)2 could serve as nucleation sites for the precipitation of sodium aluminosilicate hydrate gel to accelerate the reaction. PDF analysis and XRD characterization enable the examination of the phase development of alkali‐activated materials at the atomic scale. [ABSTRACT FROM AUTHOR]

This work aims to study the alkali‐silica reaction (ASR) with incorporated kinetic‐thermodynamic analysis. The model reactant tests were first conducted to study the silica dissolution in the simulated pore solution under different temperatures. Then, the kinetic model for silica dissolution was further improved by considering the influence of effective surface area. The improved kinetic model was also incorporated into the GEMS thermodynamic simulation. The model analysis demonstrated the silica dissolution rate is decreased mainly caused by reduced hydroxyl activity with increased saturation degree. The dissolved silica first reacted with the portlandite phase and formed the Calcium–Silicate–Hydrate (CSH) gel. The ASR gel can only be generated under high silicate ion concentration after the consumption of portlandite. [ABSTRACT FROM AUTHOR]