Pozzolanic reaction in the presence of chemical activators: Part I. Reaction kinetics

Abstract

This study has examined the effect of activators (Na 2SO 4 and CaCl 2) on pozzolanic reaction kinetics in a lime-pozzolan blend, which consists of 80% ground volcanic ash and 20% hydrated lime, at 23, 35, 50, and 65°C. The addition of 4% Na 2SO 4 increased both the early and later strengths of lime-pozzolan pastes from 23 to 65°C. The presence of 4% CaCl 2 · 2H 2O decreased the early strength but increased the later strength at 23°C, and increased both the early and later strengths of the paste from 35 to 65°C. Measurement of free Ca(OH) 2 in hardened pastes indicated that the presence of Na 2SO 4 and CaCl 2 accelerated the consumption of Ca(OH) 2. A fair linear correlation could be established between measurable compressive strength and the degree of reaction of Ca(OH) 2 regardless of activators. Kinetic analyses using modified Jander's equation suggest that the pozzolanic reaction process in the lime-pozzolan pastes could be divided into three phases: Phase I, controlled by the pozzolanic reaction rate; Phase II, controlled by the diffusion of reactants through a layer of porous hydration products; and Phase III, controlled by the diffusion of reactants through a layer of dense hydration products