Early strength development and hydration of alkali-activated blast furnace slag/fly ash blends

This study concerns the effect of two types of fly ash and the addition of lime on the strength development and hydration of sodium hydroxide- and sodium silicate-activated slag/fly ash blends which consisted of 50% fly ash and 50% slag by mass. Performance was compared to that of 100% slag cements. When NaOH was used as an activator, the slag replacement with ASTM Type F fly ash did not show a significant effect on either the strength development or hydration; Type C fly ash did not affect the strength development, but affected the hydration process due to the presence of C³A in the fly ash. Both fly ashes had a significant effect on the hydration and strength development when Na²SiO³ was used as an activator. The addition of a small amount of hydrated lime significantly increased the early-age strength but slightly decreased the later-age strength of the activated slag/fly ash blends. Measurement of the heat evolution during hydration indicated that the addition of the hydrated lime had a slight effect on the hydration during the pre-induction period, but accelerated the hydration thereafter.