Effects of Sodium Silicate Alkali Sludge on the Rheological and Mechanical Properties of an Alkali-Activated Slag System

Abstract

The application of alkali sludge (derived from sodium silicate production) as a supplementary material for the cementitious system of alkali-activated slag was investigated through tests of mechanical strength, rheology, heat of hydration, and microscopic analysis. The enhanced alkali sludge dosage and alkali equivalent content increased the yield stress and plastic viscosity of the alkali-activated slag while decreasing the fluidity due to the better water absorption of the alkali sludge, which increased the flocculation of the particles. The compressive strength reached the maximum, with 30% alkali sludge and 7% alkali equivalent content. The addition of the alkali sludge postponed the formation of the hydrated calcium silicate (C-S-H) gel and, therefore, delayed the peak heat of hydration, but ultimately increased the total hydration heat release. The major hydration product of calcium silicate hydrate was found in the alkali-activated slag with a 10–30% blend of alkali sludge. This work provides a reference for the utilization of alkaline solid waste from the chemical industry as an alkali activator in cementitious construction materials.