Influence of alkali activators on the early hydration of cement-based binders under steam curing condition

Abstract

The effects of water glass, NaOH, Na2SO4, and Na2CO3 on the early hydration of plain cement and two composite binders incorporating 40 mass% ground granulated blast furnace slag (GGBS) and 40 mass% fly ash under the steam curing temperature of 60 °C were investigated. Meanwhile, a 20 °C curing condition was set as a reference. The results showed that water glass, NaOH, and Na2CO3 can improve the exothermic rates of the binders at the acceleration period and promote the reaction of GGBS and fly ash. However, the alkali activators inhibited the hydration of Portland cement at the deceleration period, which was more significant at 60 °C. Thus, the alkali activators cannot efficiently increase the cumulative hydration heat within 16 h under steam curing or even appear to decrease. Na2SO4 shows a better performance than the other three alkalis in the early hydration of the composite binders due to its promotion on the formation of AFt, but it cannot efficiently increase the cumulative hydration heat too. The results also showed that all of the alkali activators significantly decreased the form-removal strength of cement–GGBS composite binder. For cement–fly ash composite binder, the form-removal strengths of the samples activated by a low concentration of water glass and high concentration of Na2SO4 were close to that reacted with water, while the form-removal strengths of the samples activated by other alkaline solutions were lower. Overall, the four alkali activators cannot improve the form-removal strength of steam-cured mortar containing a large volume of mineral admixtures.