Predictive tools to control the structure and the properties ofmetakaolin based geopolymer materials

Geopolymers are innovative mineral binders. These materials are still under research development in order to better understand the formation mechanism, the local structure and the final working properties. The aim of this study is to provide predictive tools permitting to control the geopolymerisation reaction. At first, it is important to determine the parameters responsible of raw materials reactivity. For alkaline solutions, the siliceous species distribution and connectivity control the reactivity. Concerning metakaolins, the Si/Al molar ratio, the wettability value, the amorphous phase content and the amount of reactive tetrahedral aluminum are responsible of metakaolin reactivity. Moreover, the reactivity of raw materials was proven to determine the geopolymer existence domains in the Si–Al–M/O ternary diagram. Thermal analysis during and after curing give informations about the amount of water consumed during the reaction and trapped in the final structure as well as the energy required for oligomer formation which seem to be directly related to raw materials reactivity. The pores distribution and size are also influenced by raw materials reactivity. Reactive precursors, especially reactive alkaline solutions, induce higher densification rate and, therefore, lower porosity and larger pore size. Furthermore, reactive precursors favor the formation of geopolymer network. This fact was evident by 29Si NMR. As a consequence, the increase of geopolymer phase in the structure improves the mechanical strength while the competition of different networks is source of weakness