Rheological properties and shootability of sprayable geopolymer mortar

Geopolymer presents a promising alternative to Portland cement for shotcrete, offering reduced environmental impacts, favorable mechanical performance, and improved durability. At present, small-scale applications have been implemented, but the fresh performances of spayed geopolymer are still rarely studied, restricting its further large application. This research investigates the rheological properties and shootability of sprayable fly ash/slag/silica fume based geopolymers considering the effects of different mix parameters. The water to binder (W/B) ratio has the greatest effect on the yield stress, and the dynamic yield stress is increased from 64.47 Pa to 388.64 Pa as the W/B ratio is reduced from 0.32 to 0.28. The change in dynamic yield stress is mainly related to either the formation of hydration products or the solid volume fraction. Mixtures with the highest thixotropic indexes are ideal for achieving both pumpability and shootability, while also exhibiting relatively low characteristic time. A modified equation for pumpability assessment is presented, which is more reasonable and consistent with the experimental results. The static yield stress is recommended for evaluating the build-up thickness of the designed fresh mixture, as there is a recognized strong correlation between them. In addition, the build-up thickness decreases with the increase of flowability, which could be used as a potential indicator. With the increasing sand to binder ratio, the mechanical strength exhibits a valley, which could be linked to the critical volume fraction of aggregate that corresponds to an effective interacting network, below which the sand-induced defects cause damage to strength and above which the interacting skeleton could contribute to strengthening. The strong environment savings of the current material are demonstrated, with the reduction in carbon emissions more than 50%. This study provides valuable information towards regulating and controlling the shootability of sprayable geopolymer and expands its broader application.