Phase Evolution of Hybrid Alkali Sulfate-Activated Ground-Granulated Blast Furnace Slag Cements

In this study, a hybrid alkali-activated ground-granulated cement consisting of 70% blast furnace slag (GGBFS) and 30% Portland cement (PC) activated with sodium sulfate was studied. Results were compared with those of a blended system without an activator. The addition of the activator significantly increased the kinetics and degree of reaction of these cements, particularly at early curing ages (2 days), without leading to significant changes in the phase assemblage. The main reaction product formed was an aluminum-substituted calcium silicate hydrate (C-A-S-H) type gel, with a Ca/Si ratio comparable to that of the activator-free blended cement; however, in the presence of the activator, sorption of sulfur was observed in the C-A-S-H phase. The formation of secondary phases including ettringite and Ca- or Mg-rich layered double hydroxides was also identified in these cements depending on the curing age and activation addition. This study demonstrates the effectiveness of sodium sulfate in accelerating the phase assemblage evolution in high-GGBFS-content PC-blended cements without leading to significant changes in the reaction products formed, particularly at advanced curing ages. This represents a step forward in the development of cements with a reduced clinker factor.