Thermodynamic model for ternary OPC/CAC/Calcium Sulfate binders

Abstract

Ternary binders combining Portland (OPC) and calcium aluminate (CAC) cements with calcium sulfate (CS-) capitalise on differential setting and hardening properties to compensate for expansion or shrinkage.This study explored hydration over curing times ranging from 1 h to 1 year in nine cements, combinations of OPC, CAC and CS- prepared with a w/c ratio of 0.4. The experimental findings were subsequently applied to corroborate the results of a thermodynamic model for the same compositions.According to the model findings, under thermodynamic equilibrium conditions, in binary blends with a high OPC content, the addition of CS- induces the formation of more ettringite (AFt), but no calcium monosulfoaluminate hydrate (AFm), whereas a higher CAC replacement ratio prompts AFm phase formation and retards OPC hydration. Those two effects were observed to compete in the ternary system, with phase development depending on the initial proportions of the two binders.A thermodynamic hydration model was developed for the OPC/CAC/CS- ternary system in keeping with the experimental data and factoring in cement composition, clinker phase hydration kinetics and geochemical speciation-based thermodynamic equilibrium calculations. The model predictions for phase development correlated well with the experimental findings where hydration was assumed to be ongoing in the 1 year materials.