How does aluminum sulfate-based alkali-free accelerator affect calcium silicate hydrate (C-S-H) formation in accelerated cement pastes?

Alkali-free aluminum sulfate (AS)-based accelerators have become a widely adopted and indispensable admixture for shotcrete applications. However, their effects on the formation kinetics and properties of calcium silicate hydrate (C-S-H) – the primary hydrate in shotcrete – remain insufficiently understood. To bridge this knowledge gap, a combination of isothermal calorimetry, QXRD, and TGA was employed to uncover the temporal evolution of C-S-H. In addition, the chemical compositions and silicate chain structure were comprehensively examined using BSE-EDS, 27 Al NMR, coupled with 29 Si NMR techniques. Our results demonstrate that AS addition accelerated C 3 S hydration, promoting the C-S-H nucleation and growth rate. This acceleration effect can be ascribed to (1) the accelerated dissolution of silicate phases at early stages, and (2) the formation of divergent C-S-H needles, providing more nucleation sites while mitigating impingement between adjacent C-(A)-S-H needles. Multiple characterizations confirmed the enhanced incorporation of aluminum into C-S-H, predominately at the bridging site. Notably, increasing AS content decreased the Si/Ca of the C-(A)-S-H and concurrently enhanced the aluminum incorporation. This compositional shift led to the formation of longer aluminosilicate chains and concomitant improvements in the hardness and elastic modulus of C-(A)-S-H. These findings offer valuable insights into AS-induced alterations in C-(A)-S-H within accelerated cement pastes, facilitating the deeper understanding of the role of C-(A)-S-H in strength development and long-term durability of shotcrete.