Synthesis and thermal behaviour of potassium sialate geopolymers

The geopolymers potassium polysialate (K-PS) and potassium sialate disiloxo (K-PSDS) have been found to possess extremely good thermal stability. K-PS shows little sign of melting up to 1400 °C, its amorphous structure being replaced by the crystalline feldspars leucite and kalsilite at 1000 °C. 27Al and 29Si MAS NMR confirm the ease of this thermal reaction which involves only slight changes to the tetrahedral Al environment. Silica-rich K-PSDS becomes friable and porous at >1200 °C, with less complete crystallisation of K-feldspar, the formation of some cristobalite and the retention of a degree of amorphous geopolymer. 39K NMR suggests that the charge-balancing alkali ions in the K-PS and K-PSDS geopolymer networks behave similarly to those of Na geopolymers, dehydrating on heating and moving into the feldspar lattice >1000 °C.