Recycling thermal modified phosphogypsum in calcium sulfoaluminate cement: Evolution of engineering properties and micro-mechanism

Abstract

The phosphogypsum (PG) can be used as a substitute for calcium sulphoaluminate cement (CSC) due to its high content of calcium sulfate dehydrate. In this study, the engineering properties including setting time, compressive strength and fluidity index of CSC with addition of thermal modified phosphogysum (MPG) were experimentally investigated. Furthermore, the micro-mechanisms including crystalline phase and microstructure evolutions of CSC incorporating MPG under different thermal treatment temperatures were analyzed via conducting X-ray diffraction (XRD), heat of hydration, thermogravimetry (TG) and scanning electron microscopy (SEM) tests. The results show that the optimum PG content and thermal modification temperature were respectively 10% and 300℃, which achieved the maximum compressive strength of 41.59 MPa. The hydration heat release peak of CSC with 300℃ MPG was advanced, contributing to shorter setting time and poor fluidity of CSC. Whereas after incorporating untreated and 100℃ MPG, the hydration exothermic peak of CSC lagged behind, causing higher retarding effect and better fluidity. The main hydration products of ettringite (AFt) and alumina gel (AH3) in CSC-MPG mixture were respectively produced in the early and later stages, which were similar to that in pure CSC. The amount of AFt was the most under the optimum incorporation of 10% MPG at 300℃, which was mainly responsible for the highest structure density. The key findings in this study can achieve the double benefits of utilizing waste PG and improving the engineering properties of CSC.