A binder of phosphogypsum-ground granulated blast furnace slag-ordinary portland cement

A new hydraulic cementitious binder was developed by mainly utilizing industrial byproducts phosphogypsum (PG) and ground granulated blast furnace slag (GGBFS) with small addition of ordinary portland cement (OPC). The hydration process and microstructure were studied by X-ray diffraction (XRD) and scanning electronic microscopy (SEM). OPC hydrated first at early age to form primarily C-S-H gel, ettringite and calcium hydroxide (CH). GGBFS activated by CH and sulfate ions hydrated continuously at later age, producing more and more hydration products, C-S-H gel and ettringite. Thus the paste developed a denser microstructure and its strength increased. The 28 d compressive strength of the mixture of 50% PG, 46% GGBFS and 4% OPC exceeded 45 MPa. The setting time was faster and 3 d and 7 d strength were higher when the proportion of OPC increased. But the 28 d strength decreased when OPC exceeded 4% due to large amount of ettringite formed at late hydration age which damaged the microstructure.