Study on the curing conditions on the physico-mechanical and environmental performance of phosphogypsum-based artificial aggregates

The considerable accumulation of phosphogypsum (PG) not only occupies land space but also causes environmental pollution, leading to threatening human health. Under the background of the current massive demand for construction concrete, the problem of natural aggregate shortage urgently needs to be solved. In this research, the raw materials of PG, slag, and cement were adopted to prepare PG-based artificial aggregates (PBAAs) by compression methods, which aimed to develop the applications of PG to construction concrete materials. The different curing temperatures and solutions (Na2SO4 and CaSO4·2 H2O) were investigated to explore the effects of curing conditions on the physical properties and ion leaching of PBAAs. The performance of PBAAs from the aspects of compressive strength, pH value, bulk density, softening coefficient, water absorption, and carbonization degree was evaluated. The microscopic morphology analysis and phase, as well as quantitative analysis were conducted by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively, to study the hydration mechanism of PBAAs under different curing conditions. The Avrami model was used to simulate the PG dissolution rate of PBAAs. The results showed that 40 °C was the most favorable temperature for PBAAs and that the dissolution of PG during immersion curing was mainly controlled by ion reactions. The PBAAs cured in saturated Na2SO4 solution raised the pH value of the curing solution. The curing conditions of CaSO4·2 H2O at 40 °C inhibited the dissolution of PG in raw materials, and provided Ca2+ for the formation of ettringite and C-S-H gel, thereby increasing the bulk density, compressive strength, carbonization resistance, and reducing the water absorption rate of PBAAs. Moreover, PBAAs cured in saturated CaSO4·2 H2O and Na2SO4 solutions at 40 °C effectively solidified heavy metals and P elements in PG. This study contributes to developing the replacement of natural aggregates by PBAAs in construction concrete, which can reduce the consumption of natural aggregates and improve the utilization rate of PG.