Study on the mechanical properties, microstructure and hydration mechanism of phosphogypsum-based geopolymer cement with added micron-sized silica particles

Abstract

Phosphogypsum (PG) is a solid waste product that is generated during the manufacture of phosphoric acid. In this study, beta-hemihydrate phosphogypsum (β-HPG), slag, sodium metasilicate (SM), citric acid (CA) and micron-sized silica particles (MSSP) were mixed to form a phosphogypsum-based geopolymer (PBG) cement. The results revealed that by adding 3 % MSSP, the 28-day unconfined compressive strength (UCS) reached its highest level of 59.2 MPa. Furthermore, adding 2 % MSSP resulted in the greatest water resistance, with a softening coefficient of 0.92. Additionally, X-ray diffractometry (XRD) Rietveld analysis, scanning electron microscopy-energy dispersive spectrometry (SEM-EDS) imaging, thermogravimetry-differential scanning calorimetry (TG-DSC) analysis, high-temperature XRD analysis, and mercury intrusion porosimetry (MIP) testing were employed to analyse the PBG cement. Results indicated that dihydrate gypsum (CaSO4·2H2O), ettringite (Ca6(Al(OH)6)2(SO4)3(H2O)26), and polymers (Nan{−(SiO2)zAlO2}n·ωH2O) were the principal hydration products in the PBG cement system. Also, the addition of MSSP led to reduced porosity, lower pore-specific surface area, and fewer cracks in the cement structure. After exceeding temperatures of 990.9 °C, these polymers gradually decomposed into Na6(AlSiO4)6, Al2O3, and SiO2. PBG cement is characterized by its easy processing, high strength, eco-friendly nature, and high PG incorporation. It shows promise as a potential substitute for traditional Portland cement in certain sectors, thereby facilitating the consumption of waste gypsum.