Longitudinal single-sided NMR study: Silica-to-alumina ratio changes the reaction mechanism of geopolymer

Abstract

Characterizing and understanding the mechanisms underlying geopolymerization are critical in achieving the use of sustainable construction material, geopolymer, for widespread commercial production. Non-destructive 1H NMR relaxometry can provide novel information about geopolymerization as it allows simultaneous detection of where the water goes and how the pore structure changes. Coupled with the development of single-sided NMR devices, NMR measurements are not limited by the specimen size and are therefore able to observe in-situ conditions of geopolymer synthesis. Here, the curing process of metakaolin-based geopolymers was monitored by 1H relaxometry on a single-sided NMR device. The silica-to-alumina ratio (Si/Al) was found to affect reaction stages of the geopolymerization. After the dissolution of aluminosilicate precursor, the low Si/Al of 1 was found to generate three gelation/polymerization stages as well as a water-binding stage, and two gel phases appeared. When Si/Al varied in 1.5–2.5, two gelation/polymerization stages and only one gel phase was observed.