Al 2 O 3 ·2SiO 2 powders synthesized via sol–gel as pure raw material in geopolymer preparation

Geopolymers are inorganic aluminosilicates mainly proposed as environmentally friendly building materials, which are obtained by alkali activation of natural minerals, calcined clay (e.g., metakaolin) and other aluminosilicate sources. The wide range of chemical and mineralogical compositions of these raw materials influences several properties of the obtained geopolymers. In the present work, pure Al 2 O 3 ·2SiO 2 powders were synthesized via the sol–gel technique and proposed as pure aluminosilicate sources to prepare alkali activated geopolymers. Samples differing in the ratio between the SiO 2 precursor and the H 2 O used in the sol–gel process were prepared, in order to study the effect of water content on the material structure and reactivity. The chemical structure of all the obtained Al 2 O 3 ·2SiO 2 powders were characterized by Fourier transform infrared ( FT ‐ IR ) and solid‐state nuclear magnetic resonance ( 27 Al and 29 Si MAS NMR ) spectroscopies and compared to that of a reference metakaolin. Moreover, material reactivity was evaluated by alkali activation of the samples. After 28 days of ageing, 27 Al and 29 Si MAS NMR and FT ‐ IR spectra ascertained the formation of a geopolymeric network in the activated samples. The results showed that lower water content allows obtaining a homogeneous Al‐rich geopolymer similar to that obtained, using metakaolin as raw material.