An investigation of the microstructure and durability of a fluidized bed fly ash–metakaolin geopolymer after heat and acid exposure

•A geopolymer is proposed based on fluidized bed fly ash–metakaolin.•Geopolymer shows relatively lower mass loss and strength loss compared to ordinary Portland cement.•Geopolymer possesses good heat resistance and acid resistance compared to ordinary Portland cement.•A model is proposed for microstructure development.•Durability of geopolymer is demonstrated by monitoring the pore structure.

This study aims to investigate durability and microstructure of fluidized bed fly ash and metakaolin based geopolymer exposed to elevated temperatures and acid attack. Geopolymer specimens were prepared by combination of fly ash and metakaolin activated by sodium silicate and sodium hydroxide solutions and were cured in microwave radiation environment plus a heat curing period. Compressive strength and several key durability parameters for geopolymer and ordinary Portland cement (OPC) were assessed and compared. Microstructure formation and development was characterized in terms of morphology and pore structure as well as simulation.The experimental results reveal a dense microstructure of geopolymer compared to OPC. In terms of resistance to the acidic solution and elevated temperatures, geopolymer is superior to OPC as indicated by the relatively lower strength loss and lower mass change. Compressive strength shows a dramatic drop in OPC while geopolymer shows a strength increase after 400 °C. The mass loss curves of geopolymer are similar to OPC, but it shows relatively lower mass loss compared to OPC. The result of saturated water absorption after 28 days curing indicates less water absorption in geopolymer before and after thermal and acid exposure. Durability of geopolymer is demonstrated by monitoring the pore structure.

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