Accelerators for shotcrete – Chemical composition and their effects on hydration, microstructure and properties of cement-based materials

Abstract Accelerators are widely used in shotcrete to achieve rapid setting and hardening properties. This paper reviews different chemicals of accelerators and discusses their effects on hydration, microstructure and properties of cement-based materials. Generally, alkali aluminates, silicates, carbonates, etc. are main types of alkali used in alkali-containing accelerators while aluminum salts and alkanolamines are important components in alkali-free accelerators. These accelerators significantly promote the hydration of tricalcium silicate (C3S) and/or tricalcium aluminate (C3A), producing large amounts of calcium silicate hydrate (C-S-H) and/or calcium sulfoaluminates (AFt and AFm) at early hydration ages to shorten the setting times and increase early strength of cement-based materials. However, the accelerated formation of AFt and AFm, especially AFm, results in retardation of hydration of C3S. Most accelerators, especially alkali-containing accelerators, exhibit negative effects on long-term strength, shrinkage and durability of cement-based materials. The incorporation of reinforcement, supplementary cementitious materials, chemical admixtures etc. is a feasible approach to overcome these negative effects. This review also gives some perspectives for future development of accelerators for shotcrete.