Properties of Full-volume Fly Ash Geopolymer Mortar incorporating Fly Ash aggregates

The disposal of unutilized fly ash (FA) consumes large area of landfills and/or lagoons occasioning soil and water pollution due to the availability of heavy metals in FA. Meanwhile, the CO2 emission companioned with cement production urges the need of eco-friendly construction materials. To maximize the recycling amount of FA in construction and to generate cement-free concrete, the concept of “full-volume fly ash” (FVFA) geopolymer mortar using alkali-activated FA as a binder and sintered fly ash aggregates (FAAs) as fine aggregates was proposed in this study. The effects of Na2O/FA mass ratio and steam-curing duration on the performance and microstructure development of the FVFA geopolymer mortars were evaluated. The performance of control mortars incorporating conventional river sand was also assessed as a benchmark reference. The chemical reactivity of FAAs in the interface transit zone (ITZ) between geopolymer binders and FAAs was mainly examined. Additionally, the influence of the FAAs on the drying shrinkages of FVFA geopolymer mortars was explored.

According to the experimental results, it was found that both compressive strengths and densities of the FVFA geopolymer mortars were relatively decreased compared to the control mortars depending on the Na2O/FA mass ratio and steam-curing duration. Further, the FVFA geopolymer mortars were found to have much higher cumulative porosity (i.e. higher content of large capillary pores) relative to the control mortars; therefore, FVFA geopolymer mortars has a great potential to be used as lightweight concrete. Besides, the microstructure of the FVFA geopolymer mortars showed denser ITZ between the sintered FAAs and the FA geopolymer binder. After soaking in sodium hydroxide (NaOH) solution and conducting X-ray diffraction (XRD) test, no significant changes were observed in the XRD results before and after treating in NaOH solution; thus, the sintered FAAs were found to have high resistance to alkaline solutions which proves their chemical stability in the proposed system. The FVFA geopolymer mortars showed improved ability to resist drying shrinkage compared with the control mortars due to the higher porosity of the sintered FAAs (i.e. could serve as micro reservoirs for internal curing).