Understanding the roles of activators towards setting and hardening control of alkali-activated slag cement

Sodium silicate-activated slag often gives high early strength but sets too rapidly. Commercial retarders for Portland cement and partial replacement with other cementitious components in alkali-activated slag (AAS) system have been proven ineffective. This study investigated the approach of controlling setting by altering activator ion composition and silica polymer status. The roles of activators during the alkali-activation process were studied via pore solution chemistry analysis and microstructural analysis of hydration products. The addition of Na₂CO₃ does not, but NaOH does alter the polymerization of silicate ion groups in sodium silicate solution due to the increases in pH and Na₂O concentration. The specific effects of Na₂CO₃ on the setting and hardening of AAS depend on the relative contents of NaOH and Na₂O·2SiO₂ in alkaline solution. The reaction degree of AAS is dependent on activator compositions, which govern the kinetics of formation and intrinsic characteristics of the reaction products calcium aluminosilicate hydrates. The setting and strength development of sodium silicate-activated slag can be controlled by manipulating the compositions of Na₂CO₃-NaOH-Na₂O·2SiO₂.