Abstract: In order to reduce the water absorption of phosphogypsum-based ceramsite and provide support for its practical application in engineering，dehydrated phosphogypsum，red mud，blast furnace slag powder and type II anhydrous gypsum are used as the main raw materials，and sodium bicarbonate foaming agent is added. The effects of hydrophobic agent type，addition method，dosage gradient and steam curing temperature on the water absorption performance of ceramsite are systematically investigated. The preparation process is optimized by orthogonal test，and the microstructure evolution mechanism of ce⁃ ramsite is characterized by XRD and SEM/EDS analysis.The test results show that the effect significance of each factor on the 24 h water absorption of ceramsite is steam curing temperature > hydrophobic agent type > dosage gradient > addition method. Under the optimal process parameters of internal mixing 0.3% anti-permeability microcrystalline waterproofing agent and steam curing temperature of 100 °C，the water absorption of ceramsite in 24 h decreases to 19.8%，and the bulk density reaches 679 kg/m3 . XRD analysis shows that the hydrophobic agent can promote the reaction of phosphogypsum with the active components in mineral powder and red mud，generate dense phases such as ettringite (AFt) and hydrated calcium sili⁃ cate (C-S-H) gel，fill pores and reduce connectivity.SEM/EDS analysis confirms that the ceramsite forms a “hydrophilic-hydrophobic gradient pore structure”，the micron-sized macropores maintains the water ab⁃ sorption channel，and the submicron-sized micropores forms a Mo/Ti oxide hydrophobic film layer due to the modification of the hydrophobic agent，which synergistically inhibits the water retention. The phase composition and pore structure are synergistically regulated by the hydrophobic agent，and the effective control of the water absorption rate of the phosphogypsum-based ceramsiteis achieved.

Key words: phosphogypsum, water absorption, hydrophobic agent, lightweight ceramsite