Experimental study on the pH for activating ground granulated blast-furnace slag activity at different temperatures

In order to explore the initial pH values of activator solution needed for early activation of slag at different temperatures, the effects of curing temperatures (5, 20 and 35°C) and pH (12.10, 12.55, 13.02 and 13.58) on the activation and hydration characteristics of ground granulated blast-furnace slag (GGBFS) were investigated. Sodium hydroxide (NaOH) was used as the alkaline activator. The compressive strength and non-evaporable water content of GGBFS paste cured for 1, 3, 7, 14 days were determined. The hydration characteristics of slag pastes at different temperatures and pH were analyzed by XRD and SEM. The results showed that the pH value required for slag activation decreased with increasing temperature. Based on the 3 days strength of slag paste, the pH values required for activation of slag activity at 5, 20 and 35°C were 13.58, 13.02 and 12.10, respectively. With the high temperature and high pH of solution, a dense calcium-rich product formed on the surface of slag particles, which suppressed the further slag reaction. With the low temperature and high pH of solution, a layer of network product was found on the surface of the slag particles, and the later strength developed rapidly as the further hydration was not prevented. This study provides a reference for the application of alkali-activated slag.