A novel Two-step Hydration Process of preparing cement-free non-fired bricks from waste phosphogypsum

Phosphogypsum (PG) is a solid waste mainly containing calcium sulfate dihydrate (CaSO4·2H2O). Preparing the non-fired wall bricks/blocks from PG is a prospective strategy for utilizing the waste. However, there are several shortages in the current technologies of PG brick production, such as, indispensable using of cement, high pressure in press-forming, high energy consumption in autoclaving-curing, and/or poor mechanical properties of bricks. This study proposes a novel “Two-step Hydration Process” for preparing PG non-fired bricks of a high 7-day compressive strength of 29 MPa, using no cement and a low pressure of 10 MPa in press-forming. The optimal formulation comprises 75.0% of PG, 23.47% of river sand and 1.53% of hydrated lime, and the water incorporation is 22% of all the above solids. The corresponding water absorption, weight loss and compressive strength after 15 freezing-thawing cycles of as-prepared 7-day bricks are 10.19%, 1.1% and 23 MPa, respectively, which fully meet the quality requirements of the highest MU25 grade in the Chinese standard (JC/T422-2007). This creative process has the potential to cost-effectively recycle the waste PG and solve its environmental pollution.