The effect of alkaline activators and sand ratio on the physico-mechanical properties of blast furnace slag based mortars

Abstract

Alkali activation of blast furnace slag with sodium silicate (SS) and sodium hydroxide (NaOH) or potassium hydroxide (KOH) has been extensively studied, however a comprehensive and comparative study between NaOH and KOH activated blast furnace slag without SS is limited, especially in terms of drying shrinkage. Therefore, the present study focused on the effect of using NaOH or KOH as sole alkaline activators to produce alkali-activated slag mortars (AASMs) and to comparatively evaluate their effects on the rheological properties, setting time, drying shrinkage, compressive strength and microstructural properties. Furthermore, statistical analysis was per-formed to establish relationship between selected parameters. The results showed that using KOH as the sole activator provided advantages such as enhanced fluidity and compressive strength and NaOH provided benefits such as reduced setting time, water absorption and drying shrinkage. In addition, the increase in the molarity of either alkaline solution decreased the yield stress and plastic viscosity of the paste. The X-ray diffraction analysis revealed major peaks of calcium silicate hydrate, calcium aluminosilicate hydrate, and hydrotalcite as reaction products in both NaOH and KOH activated mixes. The higher molarity of NaOH or KOH solution led to a decrease in the total porosity of the paste mixes but increased macro-pores as identified by mercury intrusion porosimetry analysis. The use of higher amount of sand reduced the permeable pore volume and the drying shrinkage of the mortar samples. The development of capillary pore volume and the drying shrinkage were well correlated irrespective of the alkaline activator types and sand ratio. Statistically significant relationships were found to predict compressive strength and drying shrinkage of AASM depending on mix design parameters and curing time.