High value-added utilization of desulfurized building gypsum as self-leveling mortar: the comprehensive effect of cement.

The utilization of desulfurized building gypsum as raw material for gypsum-based self-leveling mortar (GSL) is limited by its low strength and poor water resistance. The objective of this study is to enhance comprehensive properties of GSL and prepare qualified desulfurized building gypsum-based self-leveling mortar that can be effectively applied in practical engineering projects. The influence of cement on water consumption rate of initial fluidity (W/M ratio), fluidity, setting time, mechanical strength, and water resistance of GSL were evaluated. Additionally, rheological parameter, heat of hydration, crystal morphology, and pore structure were also analyzed. Cement significantly improved the fluidity of slurry. Moreover, the compressive strength and softening coefficient of GSL reached 20.6 MPa and 0.56 at 10% cement, respectively. Furthermore, cement reduced the 30-min-fluidity loss and improved fludity by reducing the yield stress and increasing the plastic viscosity of screed. The transformation of hydration kinetics of GSL could be due to Ca and OH released by cement, thus resulting in the shortening of initial setting time and the prolongation of the interval between initial and final setting time. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) showed that CSH gel and AFt crystal would generate on the surface of CaSO·2HO crystal, making the structure more compact. Mercury intrusion porosimetry (MIP) indicated that cement greatly reduced the porosity through the water reduction effect in the early stage and continuous hydration in the later stage. The continuous hydration of cement also increased the shrinkage rate. This work was expected to provide reference for promoting the application of desulfurized building gypsum as the high value-added screed.