A MAS-NMR analysis of 29Si, 27Al and 1H on the temperature's impact on geopolymer grouts derived from argillite and metakaolin

This study investigates how the curing temperature can enhance the reactivity of poorly reactive aluminosilicate source for geopolymer synthesis using a structural approach. The studied geopolymers are based on different aluminosilicate sources, a metakaolin and argillite, under varying curing temperatures (20 and 80°C). The impact of curing temperature on the reactivity of the two aluminosilicate sources and on the resulting consolidated materials was highlighted using pH value measurements, X-ray diffraction (XRD), and MAS-NMR for silicon (29Si), aluminum (27Al), and hydrogen (1H) nucleus. Metakaolin-based samples exhibited stable pH values, unvaried amorphous content, and structural consistency whatever the curing temperature. In contrast, argillite-based samples showed a significant drop in pH values (from 13 to 12) and a rise of amorphous content when cured at 80°C, indicating a reactivity change with temperatures. This was corroborated with MAS-NMR analysis, that highlighted a dissolution of the associated minerals such as illite and smectite. It was evidenced the presence of the geopolymer phase Q4(2Al) and Q4(3Al), in addition to two other networks based on silicon (Q4(1Al) and Q4(0Al)), and on aluminum Q4(4Al). The 1H MAS-NMR corroborated this finding by an increase of the bounded water (the band at 4.8 ppm) in the geopolymer network. All these data underline how temperature influences the reactivity of geopolymer materials, especially argillite-based samples by enhancing the dissolution of associated minerals (illite muscovite and smectite).