Hydration products and strength development of calcium hydroxide-based alkali-activated slag mortars

To strengthen the practical application of the Ca(OH)2-based alkali-activated (AA) ground granulated blast-furnace slag (GGBS) system, the hydration products, microstructural characteristics and compressive strength development of the AA GGBS mortars were examined for 3 pastes and 21 mortar mixes according to the different auxiliary activators and curing conditions. 7.5% Ca(OH)2 was used for the main activator and either 1% Na2SiO3 or 2% Na2CO3 was added for an auxiliary activator. Test results revealed that the diffraction peaks and molar Si/Ca ratio of the calcium silicate hydrate (CSH) gels were higher for water curing than for air-dried curing and with the addition of auxiliary activators than with the use of the single Ca(OH)2. Furthermore, the addition of auxiliary activators contributed to the stabilization of CSH gels at a long-term age, showing that the molar Si/Ca ratio in CSH gels increased with the hydration time. These growth and intensity of CSH gels were consistent with higher strength development. The strength gain ratio of water-cured mortars was lower at the early ages but higher at the long-term ages than that of the companion air-cured mortars. This trend was independent of the mixing proportions of the mortars and the type of auxiliary activators. Simple equations were also proposed to predict the strength development of Ca(OH)2-based AA GGBS mortars considering the influencing parameters including the curing condition, the type of auxiliary activators and the mixing proportions. The predicted compressive strength of Ca(OH)2-based AA GGBS mortars at different ages was in good agreement with the test results, though the 3-day compressive strength was underestimated.

▸ This study examines the auxiliary activators in the Ca(OH)2-based AA GGBS binder. ▸ Simple equations to evaluate the strength development of AA GGBS mortar are proposed. ▸ We find that the addition of auxiliary activators contributes to the stabilization of CSH gels at a long-term age. ▸ The molar Si/Ca ratio in CSH gels increases with the addition of auxiliary activators. ▸ These growth and intensity of CSH gels are consistent with higher strength development.