Technical feasibility for use of FGD gypsum as an additive setting time retarder for Portland cement

Flue gas desulfurization (FGD) gypsum was evaluated as a setting time retarder to replace the natural gypsum in the production of Portland cement (CP II E-32). The results of physical-chemistry and morphological characterization of both products showed a material of high purity, calcium sulfate dehydrate for natural gypsum, and higher percentages of bassanite (CaSO4·0.6H2O) and hannebachite (CaSO3·0.5H2O) with low concentrations of impurities for FGD gypsum. Based on the results, the FGD gypsum is a suitable alternative to replace natural gypsum. The setting time with FGD gypsum showed about 1 h delay compared with natural gypsum and its effect on compressive strength depend on composition of the mixtures, reaching a maximum value for the mixture of 1.4 wt% natural gypsum and 2.1 wt% FGD gypsum. According to the results of leaching and solubilization tests FGD gypsum residue was classified as a non-hazardous and non-inert.