Hydration kinetics in hybrid binders: Early reaction stages

Activated blends of Portland cement and fly ash with a high ash content (>70%) are a new alternative to traditional OPCs. A number of papers have been published on C–S–H and N–A–S–H, the two gels that constitute the main cementitious products generated by the alkaline activation of these cements, and the elements that may be taken up into their structure. Very little is known about the kinetics of these systems, however, particularly during the early stages of the reaction. The present study used isothermal conduction calorimetry to explore hydration kinetics during the first 72 h in a cement containing 30% OPC and 70% fly ash. Two activating solutions were used: a mix of NaOH + Na₂SiO₃ and a Na₂CO₃ solution. The findings showed that hydration kinetics were substantially modified by the type of alkaline activator used, particularly with respect to the secondary phases generated. In both cases the main reaction product appeared to be a mix of C–A–S–H and (N,C)–A–S–H gels, whose proportions were clearly impacted by the type of activator used.