Relation between working properties and structural properties from 27Al, 29Si and 31P NMR and XRD of acid-based geopolymers from 25 to 1000°C.

Acid-based geopolymers exhibit interesting properties, such as higher temperature resistance (up to 1450 °C) and better mechanical properties, as compared to alkali-based geopolymers. The aim of this work was to highlight the origin of water and thermal resistances of acid-based geopolymers, demonstrated in our previous study, by 27Al, 29Si and 31P NMR and XRD experiments. Different networks were indeed identified in consolidated materials, such as Al-O-P, Si-O-P, vitreous silica and Si-O-Al (only for Al/P = 4, due to unreacted metakaolin) depending on processing conditions. The final properties are induced by the proportion of these networks which are controlled by the Al/P ratio and the setting temperature. It was shown that a higher amount of Al-O-P bonds and the absence of unreacted metakaolin conduct to a thermally resistant sample. A more organized structure with a low proportion of Q0(1Al) and a preference for Q1(2Al), as seen by 31P NMR, induces a great water resistance of geopolymers. After heating at 1000 °C, different AlPO 4 polymorphs crystallize, as well as mullite in the case of Al/P = 4. Resistance of acid-based geopolymers with associated structural and working properties. Image 1 • Acid-based geopolymers display different processing-dependent working properties. • Acid-based geopolymers are mostly composed of Al-O-P and Si(OH) 4 networks. • The quantities of Al-O-P and unreacted metakaolin govern the thermal resistance. • The increase of the Young's modulus is related to a higher thermal resistance. • Ordered structures and low amounts of physisobed water lead to water resistance. [ABSTRACT FROM AUTHOR]