Recycling of phosphogypsum for low-carbon cement: Clinker production, mechanical properties and hydration mechanism

Abstract

The emission of solid waste phosphogypsum (PG) imposes significant environmental stress. Utilizing PG to produce sulfoaluminate cement (SAC) offers a dual benefit of consuming PG and enhancing resource recycling. This study focuses on preparing a low-carbon phosphogypsum-based SAC (PGSAC) using PG. PGSAC clinker was successfully synthesized with 19.7 % calcined PG as a raw material at a calcination temperature of 1200 °C. Notably, the production temperature of PGSAC clinker was 70 °C lower than that of conventional SAC clinker, resulting in a 10 % reduction in fuel consumption and a 15.5 % reduction in electricity usage. Incorporating PG into the clinker to prepare PGSAC significantly enhanced the 3-day compressive strength of PGSAC mortar by 24.8 % and 18.3 % at 10 % and 20 % PG levels, respectively. Additionally, the setting time for PGSAC was notably extended by 30.77 % and 80.77 % at the respective PG contents. Compared to dihydrate gypsum specimens, the setting time of PGSAC paste with added PG was prolonged by 7.7 % (10 % PG) and 30.77 % (20 % PG). The introduction of PG resulted in a decrease in the pH of PGSAC, an advancement in dissolution exothermic reactions, and an increase in AFt formation during hydration. PG influenced cement hydration by accelerating the hydrolysis of clinker minerals, promoting AFt generation, and forming insoluble phosphate. In summary, PG can be a retarder for PGSAC and enhance cementitious strength, with an optimal dosage range of 10%–20 %. For each ton of R·SAC 42.5 grade PGSAC, 208 kg of calcined PG and 200 kg of PG were utilized, leading to a 19 % reduction in CO2 emissions. The findings of this research advocate for the application of PG in SAC, thereby promoting increased utilization of PG and contributing to environmental sustainability.