Effect of fluorogypsum and KH2PO4 on physical properties and hydration mechanisms of aluminate cement based grouting materials

This study investigated the influence of fluorogypsum and KH2PO4 on the mechanical properties and hydration mechanisms of aluminate cement based grouting materials. Physical properties including setting time, pH value and the compressive strength of aluminate cement based grouting materials with different mix design were investigated. The influencing factors and hydration mechanisms were quantified and discussed through XRD, TG, SEM and NMR. The water-cement ratio was fixed at 0.40, and the fluorogypsum content accounted for 10 wt% of the cement mass. With increasing KH2PO4 content, the setting time initially decreased and then prolonged, while the compressive strength at 3d and 28d first increased and then decreased. Among these variations, a content of 0.2 wt% KH2PO4 exhibited the optimal performance with initial and final setting times of 264 min and 308 min, respectively. The compressive strengths at 3 days and 28 days were recorded as 26.4 MPa and 60.5 MPa, respectively, exhibiting an increase of 76% and a remarkable enhancement of 228.80% compared to that of pure aluminate cement. The addition of fluorogypsum facilitated the formation of AFt, while KH2PO4 regulated the slurry's pH, concurrently enhancing AFt production, and the morphology of AFt transformed from acicular to a more slender fiber shape, which exhibited improved interlacing and overlapped properties, thereby promoting strength development. The hydration process was analyzed in conjunction with low-field nuclear magnetic resonator analysis. The findings revealed that the samples containing 10 wt% fluorogypsum and 0.2 wt% KH2PO4 in aluminate cement exhibited the highest levels of chemically bound water at both 3d and 28d, followed by samples with 10 wt% fluorogypsum in aluminate cement, while those composed solely of aluminate cement displayed the lowest content. The T2 spectra data were also fitted to a function, revealing a positive correlation between pore evolution and the hydration rate of the samples.