Accelerating effect of calcined hydrotalcite-Na2SO4 binary system on hydration of high volume fly ash cement

This work aims to propose a new method to overcome the shortcomings of slow setting and low early strength occurring in cement-based composites with high-volume fly ash (HVFA). The effects of binary system containing calcined hydrotalcite (CHT) and Na2SO4 (NS) on early strength, hydration process, pore structure and micro-structural morphology of HVFA composites were investigated. Specifically, the setting time, the compressive strength and pore size distribution of HVFA cement doped with CHT and NS were measured. Isothermal calorimetry, quantitative analysis of Ca(OH)(2) and chemically bound water contents, XRD, TGA and SEM were employed to characterize the hydration process, the assemblage of hydration products and microstructural morphology. Results indicated that higher contents of hydration products such as AFt and C-S-H gel in HVFA paste incorporating with CHT and NS were found in XRD and TGA analyses. Consequently, regarding improving mechanical properties and porosity of HVFA cement, the CHT and NS co-doped system illustrated a more outstanding performance than the CHT or NS single system. This improvement was attributed to the accelerated hydration process, increased hydration product content and refined pore structure. These findings are expected to provide guidance on enhancing the early strength of HVFA concrete in practical engineering.