Phosphate additive modification of C–S–H and C–A–S–H crystallization pathways

Abstract

Phosphate-based additives are commonly used in concrete technology as hydration retarders and dispersion enhancers, playing a critical role in controlling cement hydration kinetics. This study examines how the molecular structure of three simple polyphosphate molecules affects the early formation of calcium silicate hydrate (C–S–H) and calcium aluminate silicate hydrate (C–A–S–H), the critical binding phases in modern cements that incorporate supplementary cementitious materials. We systematically compared two cyclic phosphate additives (phytic acid and hexametaphosphate) against one chain-like additive (tripolyphosphate), revealing structure-dependent effects on C–S–H and C–A–S–H formation. Our findings reveal that retardation occurs through dual pathways: calcium ion sequestration and formation of calcium–phosphate intermediate phases. The additives exhibit system-dependent behaviours—effectively retarding C–S–H formation while showing reduced retardation in C–A–S–H systems. The aluminium-containing system shows additional effects, with additives capable of stabilizing the C–A–S–H amorphous precursor phase. These distinct effects demonstrate that phosphate additives can produce multiple impacts beyond their intended retardation function depending on the composition of the cement. Understanding additive-hydrate interactions at the molecular level is therefore critical and may provide new avenues for controlling hydrate characteristics.