Experimental study on full-volume fly ash geopolymer mortars: Sintered fly ash versus sand as fine aggregates

In this study, the concept of “full-volume fly ash (FVFA) geopolymer mortar” is proposed using FA geopolymer as a binder and sintered fly ash aggregates (FAAs) to fully replace the conventional river sand (by volume), aiming to conserve natural sand resources through further utilizing of FA. The influences of the FAAs, the alkali concentration, and curing regime on the physical, mechanical, microstructure, and mineralogy properties of the FVFA geopolymer mortars were experimentally evaluated. The properties of the conventional river sand control mortars were used as a benchmark reference. The results indicated that both compressive strength and density of the FVFA geopolymer mortars were relatively lower compared to that of the control mortars. The change of alkali concentration and steam-curing duration could generate a wide variant range of the compressive strength and density. Further, the FVFA geopolymer mortars were found to have much higher total porosity relative to the control mortars. The drying shrinkage of the FVFA geopolymer mortars was much lower than that of control mortars due to the internal curing effect of the FAAs. It was challenging to identify the interfacial transition zone (ITZ) between the sintered FAAs and the FA geopolymer binder relative to that between sand and paste in control mortars. It was found that the external layer of the FAAs has reacted with the alkaline solution while the internal core remains relatively stable during the 28 days.