Sustainable fly ash-based geopolymer composites: The influence of RAP aggregates and silica fume on strength, durability, and microstructural properties

This study investigates the effects of reclaimed asphalt pavement (RAP) aggregates and silica fume (SF) on the mechanical, durability, and microstructural properties of fly ash (FA)-based geopolymer composites (GC) under varying curing conditions. GCs were prepared with RAP contents of 0%, 15%, 30%, and 60% as replacements for river aggregate (RA) and with partial replacement of FA by 15% SF. Samples were cured at 60 degrees C and 90 degrees C, and their performance was evaluated through compressive and flexural strength tests, freeze-thaw (F-T) resistance, high-temperature exposure, sorptivity, porosity, and microstructural analysis. The results reveal that substituting RA with 15% RAP demonstrated the optimal content for enhancing compressive strength, with increases of 33.64% at 60 degrees C and 8.04% at 90 degrees C. At 30% RAP, the improvements were smaller (7.36% at 60 degrees C and 7.37% at 90 degrees C), while 60% RAP led to strength reductions of 0.97% and 8.12% at 60 degrees C and 90 degrees C, respectively. The mixture containing 100% FA and 15% RAP as a replacement for RA demonstrated the best high-temperature performance, exhibiting the lowest strength loss at 750 degrees C. The lowest strength loss after 50 F-T cycles was observed in the mixture cured at 90 degrees C with 30% RAP and 100% FA. Mixtures with 100% FA and 30% RAP cured at 90 degrees C exhibited the lowest strength loss after 50 F-T cycles, demonstrating enhanced durability. Mixtures containing 100% FA exhibited superior F-T performance with the incorporation of RAP compared with the reference mixture without RAP, irrespective of curing temperature. Mixtures with 100% FA and 15% RAP demonstrated the highest resistance to elevated temperatures, with minimal strength loss at 750 degrees C. However, increasing RAP content to 30% and 60% led to greater deterioration due to thermal degradation of bitumen-coated aggregates. Replacing FA with 15% SF significantly reduced high-temperature resistance, causing over 90% strength loss at 750 degrees C.