Intrinsic microstructure of C-A-S-H and N(-C)-A-S-H gels at low Ca/Si and Si/Al ratios

Abstract

Understanding the microstructure of low calcium aluminosilicate hydrate (C-A-S-H) and calcium containing sodium aluminosilicate hydrate (N(-C)-A-S-H) is essential for the performance prediction and therefore manufacturing of geopolymers, or more broadly, chemically activated aluminosilicate cements. This study aims to reveal their atomic ordering and molecular structure using synthetic C-A-S-H/N(-C)-A-S-H gels at low Ca/Si (0-1.0) and Si/Al (1-2) ratios. By high energy XRD with pair distribution function (PDF) analysis, TEM, and 29Si/27Al NMR analysis, it discovers that Ca concentration is a more governing factor than Al to the overall Si/Al evolution of gels. The formation of distinct Q1 and Q2 structural units is at the conditions of Si/Al > 1.75 and Ca/Si ≥ ~0.2-0.5. Ca and Al exhibit synergistic/antagonistic effect on Si tetrahedral units: Al increases Q4 (enhancing polymerization), while Ca promotes Q2 (improving ordering) but reduces Q3 (decreasing connectivity). An estimation calculation formula linking the proportion of Q4 sites to Si/Al and measured Ca/Si ratios of products is established to simplify the determination of C-A-S-H and N(-C)-A-S-H gel proportions in alkali-activated systems (including low-Ca hybrid alkaline cement). The new insights and discovery of C-A-S-H and N-A-S-(H) gel microstructure enables future work of better understanding the relationship between phase composition and geopolymer performances. The findings elucidate the interactions and mechanisms of elemental composition governing gel evolution, with implication for geopolymer manufacturing and application.