Accelerators for normal concrete: A critical review on hydration, microstructure and properties of cement-based materials

Accelerators for normal concrete play an important role in some engineering constructions. This paper reviews different types of accelerators and discusses their effects on hydration, microstructure and properties of cement-based materials. Generally, calcium and alkali chlorides are the main kinds of chloride-based accelerators, while nitrates, nitrites, thiocyanates, formates and alkanolamines are the chemicals commonly used in non-chloride accelerators. Acceleration of tricalcium oxy silicate (C3S) hydration and corresponding calcium silicate hydrate (C–S–H) formation is regarded as the main reason for rapid setting and/or hardening according to existing researches. Although some accelerators present anti-freezing or corrosion inhibiting properties, most of them have negative effects on the durability of concrete and high dosages of accelerators are particularly harmful for durability. Incorporation of supplementary cementitious materials and/or chemical admixtures presents a feasible approach to overcome these negative effects. This review also gives some perspectives for future development in the field of accelerators for normal concrete.