Effect of curing regime on the distribution of Al3+ coordination in hardened cement pastes

The effect of curing regime on the distribution of Al³⁺ coordination in hardened cement pastes within 28 d were investigated by ²⁹Si and ²⁷Al magic angle spinning (MAS) nuclear magnetic resonance(NMR) with deconvolution technique. The results indicate that the tetrahedral coordination Al³⁺ incorporated in C-S-H structure mainly originate from the Al³⁺ incorporated in the alite and belite phases in the Portland cement. The curing regime of constant temperature of 20 °C is beneficial to the octahedral coordination Al³⁺ transforming to tetrahedral coordination Al³⁺ incorporated in C-S-H structure. However, at curing regime of variable temperature, the temperature rising process is more advantageous to the transformation from ettringite to monosulphate, substitution of Al³⁺ for Si⁴⁺ in the C-S-H structure and the formation of the third aluminate hydrate (TAH) than that at constant temperature of 20 °C. The high temperature of 60 °C in the holding temperature process promotes the decomposition of ettringite, and enhances the consumption of the Al³⁺ incorporated in C-S-H phases and the Al³⁺ in TAH for the monosulphate forming. The temperature decreasing promotes the transformation from monosulphate to ettringite, and increases the consumption of the Al³⁺ incorporated in C-S-H phases, and then increases the quantity of the TAH.