Mechanical performance, durability, qualitative and quantitative analysis of microstructure of fly ash and Metakaolin mortar at elevated temperatures

► Performance of FA and Metakaolin mortar at elevated temperatrues was evaluated. ► Using gray scale segmentation, different hydration phases were found. ► For all mixes, major strength and durability loss occurred at 400 °C. ► Major drop in hydrated paste area and rise in pore area fraction occurred at 400 °C.