Structural evolution of calcium sodium aluminosilicate hydrate (C-(N-)A-S-H) gels induced by water exposure: The impact of Na leaching

Calcium sodium aluminosilicate hydrate C-(N-)A-S-H gels, formed through the alkali-activation of calcium silicate-based materials, may exhibit greater susceptibility to aqueous environments when compared to traditional C-(A-)S-H phases formed by hydration of blended Portland cements. This study investigates structural changes in synthesized C-(N-)A-S-H gels triggered by water immersion. Three gels have been examined, each with stoichiometrically controlled ratios of Ca/Si (0.8 and 1.2), Al/Si (0.1 and 0.3), and Na/Si (0.1, 0.2, and 0.3). The gel with a higher Ca/Si ratio demonstrated enhanced resistance to water leaching and only experienced marginal decalcification whereas the gels with lower Ca/Si ratios exhibited more pronounced effects including leaching losses of Si. Notably, all gels displayed rapid and substantial sodium leaching, contributing to an increased degree of polymerization for the aluminosilicate tetrahedra in the gels. A plausible mechanism for this change is that Na leaches out from the interlayer and Ca ions progressively take over the role of charge compensators in the interlayer of the C-(N-)A-S-H structure.