Structural and physicochemical impacts of the curing temperature and Li2B4O7 addition on geopolymers: MAS NMR 29Si, 27Al, 1H and 11B analysis

• Lithium tetraborate induces more structural changes while improving water removal. • The addition of lithium tetraborate involves various geopolymer networks. • The consolidation temperature (20, 90 and 120 °C) leads to the pore coalescence. This study investigates the concomitant effect of lithium and boron from lithium tetraborate on the geopolymer network at different consolidation temperatures. The local network in relation with pore size and water content could allow new fields of applications such as thermal conductivity and electromagnetic properties. The impact of the curing temperature and the addition of lithium tetraborate on the consolidated materials was investigated using, mercury intrusion porosimetry, thermal analysis, MAS-NMR for silicon ( 29 Si), aluminum ( 27 Al), and hydrogen ( 1 H) and boron ( 11 B) nucleus, and X-ray diffraction (XRD). The results showed that the reference sample, based exclusively on metakaolin, exhibited a coalescence of pores going from 0.01 µm to 0.1 µm, in addition to a stable geopolymer network whatever the curing temperature. In contrast, lithium tetraborate based samples showed distinct behaviors depending on the temperature. The results showed a significant increase in pore size reaching 2 µm at 90 and 120 °C. These microstructural evolutions were induced by local network’s modifications upon to the incorporation of boron the geopolymer network. This was evidenced by the presence of Si-O-B bond using MAS-NMR and FTIR spectroscopy.