Plaster composites modified morphology with enhanced compressive strength and water resistance characteristics

•Gypsum-slag composite increased the compressive strength with 30% after the water immersion.•Additives increased water resistance.•3 wt% of either CaCO3 or PVA improved the compressive strength by about 14 and 19% of water-cured samples, respectively.•Generally changing in gypsum morphology increased the water resistance of the produced gypsum composite.