Utilization of waste phosphogypsum to prepare non-fired bricks by a novel Hydration–Recrystallization process

Phosphogypsum is a solid waste generated in phosphoric acid production. Currently, a large number of phosphogypsum are discarded without any treatment, leading to considerable land occupation and serious environmental contamination. In this study, a novel process of preparing non-fired bricks from waste phosphogypsum, named as “the Hydration–Recrystallization process”, is proposed and tested. In this process, the press-formed green bricks are hot-dried at 180 °C to dehydrate gypsum (CaSO4·2H2O) into semi-hydrated gypsum (CaSO4·0.5H2O), and then water-immersed to in situ recrystallize gypsum (CaSO4·2H2O) crystals, and finally air-dried naturally to obtain the non-fired brick products. A series of the experiments were conducted according to the novel process, and results showed that the optimal mix designation composes of 75.0% phosphogypsum, 19.5% river sand, 4.0% Portland cement and 1.5% hydrated lime, and the corresponding compressive strength, water-saturated compressive strength and bending strength of the as-prepared bricks are 21.8 MPa, 13.7 MPa and 5.2 MPa, respectively. Microstructural characteristics of the bricks were investigated by XRD and ESEM, and the results indicated that the in situ recrystallized gypsum (CaSO4·2H2O) crystals presents regular, dense and interlock crystalline microstructure, which contributes to the high mechanical strength of the non-fired bricks prepared from the waste phosphogypsum. The industrial application of the new process can help to significantly reduce the environmental impacts of waste phosphogypsum.

► We propose a novel process to recycle waste phosphogypsum to prepare non-fired bricks.
► The phosphogypsum content of the optimal mix designation is 75.0%.
► The properties of non-fired bricks can meet the Chinese standard requirement.
► The mechanical strength of bricks is attributed to the recrystallized gypsum crystals.