Hydration and Properties of Slag Cement Activated by Alkali and Sulfate

In this paper, the results of a study on the influence of sodium hydroxide, calcium hydroxide, aluminum hydroxide, sodium sulfate, calcium sulfate, and calcium sulfate dihydrate as activators on the setting time and mechanical properties of slag–cement mixtures are presented. The results obtained show that the sulfate is more effective in activating the activity of the slag cement than the other chemical activators. Specimens prepared from calcium sulfate, blast-furnace slag, and ordinary portland cement exhibit better mechanical performance compared to admixtures prepared from other mix ratios. The sulfate ions from sulfates would react with Ca2+ from slag cement and form the reaction product ettringite. However, the addition of sodium hydroxide seems ineffective in activating the slag cement. Successive addition of sodium hydroxide deteriorates the mechanical strength of the slag cement. The reaction kinetics are also studied by examining the hydration products of slag-blended cement paste through the technologies of X-ray diffraction (XRD), scanning electron microscopy (SEM), and nuclear magnetic resonance spectroscopy (NMR). Those experimental results provide detailed insights into the understanding of how the activators influence the hydration products of slag-blended cement and structural models of calcium silicate hydrates (C-S-H).