Performance evolution of alkali-activated phosphorus slag paste filling material: Effect of hemihydrate phosphogypsum content

Abstract

The stockpiling issues of phosphorus slag (PS) and hemihydrate phosphogypsum (HPG) have become vital technical obstacles limiting the economic development of enterprises. The synergistic substitution of PS and HPG for ordinary Portland cement in preparing alkali-activated phosphorus slag paste filling material (AAPS-PM) is essential to realize the high-value utilization of PS and HPG and reduce carbon emissions. HPG was selected as a sulfate activator for this research due to its metastable qualities and chemical composition benefits. The effect of HPG on the properties of AAPS-PM was systematically investigated by setting time, fluidity, strength development, water resistance, and pore solution pH experiments. The evolution of hydration products and pore size characteristics was elaborated employing X-ray diffractometer (XRD), scanning electron microscopy (SEM) and low-field nuclear magnetic resonance (NMR). The results demonstrated that the addition of HPG displayed remarkable procoagulant thickening characteristics on AAPS-PM. Additionally, HPG was conducive to developing early uniaxial compressive strength (UCS), whereas 90% content of HPG had a negative impact on the enhancement of later UCS. The softening coefficient and water absorption exhibited duality with 80% HPG content as the critical point, obtaining a maximum value of 0.73 and a minimum value of 6.63%, respectively. It is noteworthy that the decrease in pore solution pH was primarily correlated with the acidity carried by HPG. Herein, the alkali activator accelerated the depolymerization of PS vitreous and generated H3SiO4- and H3AlO42- and Al(OH)63-. Furthermore, the active ions combined with Ca2+ and SO42- supplied by HPG, which contributed to the formation of dihydrate gypsum, ettringite and C-(A)-S-H gels. Encouragingly, microscopic analysis confirmed the macroscopic performance changes of AAPS-PM. The gel network created by the interweaving of hydration products refined the pore structure and improved the microstructure morphology, which is the primary source of mechanical strength. Nevertheless, the high content of HPG provided a driving force for the development of capillary pores, resulting in a rapid reduction in later strength. Overall, this research can contribute new ideas to the large-scale integrated disposal of solid waste.