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.

Automated summary

This study investigates the incorporation of solid waste incineration fly ash into phosphate-based geopolymers as a substitute for metakaolin. Analytical techniques such as X-ray diffraction, scanning electron microscopy, solid-state nuclear magnetic resonance spectroscopy, and infrared spectroscopy were used to analyze the changes in mineralogy and structural modifications of the geopolymer networks, which affected the mechanical performance of the materials. The results show that fly ash behaves differently from metakaolin, primarily acting as an alkali source and competing with the aluminosilicate metakaolin fraction by precipitating crystalline and amorphous phosphates. The substitution of 10% metakaolin with fly ash preserves the amorphous geopolymer matrix and retains the mechanical properties. However, at higher waste content (30-50% wt), the fast acid-base reactions involving the fly ash reactive phases dominate, leading to a shift in the material nature towards an alkali-phosphate cement/phosphate-geopolymer composite. This change, along with the development of porosity and the presence of low-strength phases in the ash, results in a decline in mechanical performance. Overall, this work provides valuable information for the sustainable utilization of phosphate-based geopolymers, while highlighting the recycling potential for this type of fly ash.