Si-29, Al-27, (31)p an a B-11 magic angle spinning nuclear magnetic resonance study of the structural evolutions induced by the use of phosphor- and boron-based additives in geopolymer mixtures

摘要

This study aims to develop a geopolymer binder, which requires a control of the reactive mixture setting time, viscosity and pH value. A previous study underlines the possibility of controlling the setting time, viscosity and pH value of geopolymer binders by the addition of phosphate-or boron-based compounds. To understand the structural evolutions responsible for these operating property changes, magic angle spinning (MAS) nuclear magnetic resonance (NMR) analysis were performed on the different binders. A batch of ten specific samples was studied by B-11 or P-31 (depending on the additive nature), Al-27 and Si-29 MAS NMR. The NMR spectral decomposition results were correlated with the operating properties of the different binders. The setting time increase was associated with the formation of a secondary phase based on boron or phosphorous. The pH variation is connected with both the modified Si/Al ratio and the asymmetric B-III species associated with one non-bridging oxygen (NBO) atom. Thus, determination of the various NMR environments is a useful tool to predict the setting time and pH value evolutions.