Phosphogypsum composite cement-based permeable bricks: optimization of mix design, performance evaluation, and environmental impact

Phosphogypsum (PG) is produced in large quantities, and its main resource utilization is in the construction sector. This study investigates the feasibility of using PG to manufacture phosphogypsum composite cement-based permeable bricks (PGCPB), focusing on the effects of aggregate size distribution, water-to-binder ratio, and slag powder (SP) content on their mechanical and durability properties and assesses the potential risks related to heavy metal content in PGCPB. The results indicate that the highest 3-day compressive strength of PGCPB is 21.1 MPa at a water-cement ratio of 0.26. The maximum 3-day compressive strength of 25.78 MPa is achieved when the fine-to-coarse aggregate ratio is 3:2. At 14 days, SEM observations reveal that incorporating 20% SP leads to an optimal crystalline microstructure and a denser matrix, corresponding to flexural and compressive strengths of 4.47 MPa and 15.25 MPa, respectively. The 14-day flexural and compressive strengths of the cementing material are 4.47 MPa and 15.25 MPa, respectively, when the SP content is 20%. With an increase in PG proportion, the 28-day compressive strength of PGCPB declines, the water permeability coefficient first rises and then falls, and its frost resistance progressively deteriorates. When PG content is 20-30%, PGCPB meets the JC/T 945-2005 permeability standard and reduces carbon emissions by 22.91% compared to conventional cement-based bricks. Environmental risk assessments confirm that PGCPB poses no risk to either soil ecology or human health, making it a safe and eco-friendly material for pavement applications.