Phosphate-based geopolymer: Influence of municipal solid waste fly ash introduction on structure and compressive strength

Materials resulting from incorporation of solid waste incineration fly ash into phosphate-based geopolymers, to partially replace metakaolin (up to 50% wt), were studied. X-ray diffraction, scanning electron microscopy, solid-state nuclear magnetic resonance spectroscopy and infrared spectroscopy were adopted to describe the miner-alogical changes and the structural modifications of the geopolymer networks which impacted on the mechanical performance (compressive strength) of the materials. The results indicated that fly ash displays a different reactivity compared with metakaolin, behaving preferentially as a source of alkali that compete with the aluminosilicate metakaolin fraction by precipitating crystalline and amorphous phosphates. At 10 wt% of metakaolin substitution with fly ash, the extent and reticulation of the amorphous geopolymer matrix is pre-served, and the mechanical properties are retained. At higher waste content (30-50% wt), the fast kinetics of the acid-base reactions involving the fly ash reactive phases prevail over the metakaolin dealumination, and the nature of the material shifts to an alkali-phosphate cement/phosphate-geopolymer composite. This behaviour, together with the development of porosity and presence of low-strength phases in the ash, led to a decline in the mechanical performance with increasing amount of substitution. All in all, this work provides fundamental in-formation in the direction of a sustainable employment of phosphate-based geopolymers, which is limited by the relatively high cost of both metakaolin and phosphoric acid. Moreover, it indicates a recycling opportunity for this type of fly ash.