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

Abstract

Understanding the microstructures 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. High energy XRD with pair distribution function (PDF) analysis, TEM, and ²⁹Si/²⁷Al NMR analysis show that Ca concentration is a more governing factor than Al in governing the overall Si/Al evolution of gels. Distinct Q¹ and Q² structural units are formed under the conditions of Si/Al > 1.75 and Ca/Si ≥ ∼0.2–0.5. Ca and Al exhibit synergistic/antagonistic effects 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 into C-A-S-H and N-A-S-(H) gel microstructures enable future work to better understand the relationship between phase composition and geopolymer performance. The findings elucidate the interactions and mechanisms of elemental composition governing gel evolution, with implications for geopolymer manufacturing and application.