The composition range of aluminosilicate geopolymers

Aluminosilicate geopolymers with SiO 2/Al 2O 3 ratios ranging from 0.5 to 300 have been prepared from mixtures of dehydroxylated kaolinite with either ρ-Al 2O 3 or fine Aerosil SiO 2, with the ratios Na 2O/SiO 2 and H 2O/SiO 2 kept constant throughout the series. All the compositions hardened at ambient temperature, but the high-alumina compositions were of low strength and did not display typical XRD and NMR geopolymer characteristics, by contrast with the compositions of S/ A between 2 and 300 which showed typical amorphous geopolymer XRD traces and 27Al, 29Si and 23Na NMR spectra. The samples with increasing SiO 2 content ( S/ A > 24) showed increasingly elastic behaviour, deforming rather than crushing in brittle fashion, and upon heating at 100–250 °C, their hydration water was expelled as bubbles, forming stable foamed materials at about 300 °C.