Hydration mechanism and phase assemblage of ternary blended cements based on sewage sludge ash and limestone: Modified by Na2SO4

Abstract

This study developed an eco-friendly ternary system based on sewage sludge ash and brought in Na2SO4 as a modifier to increase the mechanical property. The changes of the hydration kinetics, microstructure and phase assemblage of the ternary system with the addition of Na2SO4 were investigated. The results show that Na2SO4 accelerates the hydration kinetics of the ternary system and promotes the reaction of both clinker and sewage sludge ash, leading to the increase in compressive strength. The formation of the aluminate phases is promoted by the addition of Na2SO4, which contributes to porosity refinement and strength development. Moreover, Na2SO4 modifies the composition and microstructure of the aluminate phases in the ternary system. The addition of Na2SO4 promotes the formation of carboaluminate phases due to the increase in the alkalinity of the pore solution, which reflects on the appearance of the third hydration peak in the heat flow curves. However, with the increase in the addition level of Na2SO4, the formation of carboaluminate phases is inhibited due to the fact that the aluminates react preferentially with sulfate rather than limestone.