Effect of Foaming Agent on the Performance of Phosphogypsum-Based Lightweight Ceramsite
Phosphogypsum, a by-product of wet-process phosphoric acid production, poses severe environmental and safety challenges due to its massive annual output and stockpiling. This study addresses the urgent need for resource utilization by employing phosphogypsum as the primary raw material, supplemented with ground granulated blast furnace slag, fly ash, and type II anhydrite. Two foaming agents, sodium bicarbonate (NaHCO3) and aluminum powder, were used to regulate pore structure, and their effects on ceramsite performance were compared. Under identical preparation conditions, aluminum powder yielded higher 7-day cylinder compressive strength than NaHCO3. Optimal formulations achieved a maximum cylinder compressive strength of 6.5 MPa with a bulk density of 1020 kg/m3, meeting lightweight aggregate concrete strength requirements. Aluminum powder produced closed pores, reducing bulk density to as low as 765 kg/m3, while NaHCO3 generated interconnected pores leading to higher water absorption. XRD, SEM, and BET analyses revealed that strength-contributing phases are calcium silicate hydrate and calcium aluminate hydrate; trace heavy metals (Mo, Ti) hinder their formation, causing structural defects. This work demonstrates a green, non-fired route for phosphogypsum valorization, offering environmental and economic benefits and a pathway for large-scale utilization.