Delayed setting time for alkali-activated slag composites using activator containing SiO₂ and Na₂O

This study examines the influence of SiO₂ and Na₂O mix proportions on the properties of alkali-activated slag (AAS) pastes. In order to solve the rapid setting problem associated with AAS, phosphoric or silicic acid is commonly added to the alkaline activator. The retarding effect of these additions suggests a close relationship between pH and setting time. In this study, AAS pastes with various SiO₂ and Na₂O dosages were analyzed. Regression analysis was carried out using pH value and setting time to identify the main parameters affecting the setting time. Results showed that the alkali modulus (i.e., the weight ratio of SiO₂ over Na₂O) is correlated with the pH value. Specifically, the initial and final setting times were significantly reduced under lower SiO₂ and Na₂O dosages. Therefore, a higher alkali modulus prolonged the setting time.