Early Production of High Strength and Improved Water Resistance Gypsum Mortars from Used Plaster Mould and Cullet Waste

Abstract

This research aims at producing high-strength, water-resistant, gypsum-based materials (from used plaster moulds) with the additions of glass cullet and cements and investigating the influence of glass cullet contents on their properties. Block-shaped specimens of gypsum mortars containing 70–90 wt% β-hemihydrate with up to 20 wt% cullet waste powder and a small amount of Portland and calcium aluminate cements, as early strength enhancer and setting time modifier, were cured under damp, ambient air for 7–90 days. Properties including setting times, water absorption, compressive strength, water resistance, volume change, and microstructure were observed. The results exhibited that all specimens could retain their original strength after 10 wet/dry cycles, while the volume shrinkage difference between 7 and 90 days was small (3%–7%), which is consistent with the minimal change of bulk densities at various curing times (densities 1.1–1.20 g/cm3). Prolonged curing times led to lower water resistance due to a combination of hydration and pozzolanic reaction products. The mixture with 10 wt% cullet provided optimum characteristics and retained strength of 5.5 MPa with the smallest volume shrinkage of 7% at 90-day curing, suggesting possible applications for indoor and nonstructural outdoor applications.