Abstract: Unlike traditional crushed stone concrete,the interface of phosphogypsum-based lightweight aggregated concrete presents a dual interaction mechanism such as interlocking structure and chemical bonding.Understanding the interface effects of phosphogypsum-based lightweight aggregated concrete subjected to different pretreatments can facilitate practical engineering optimization.In this study,phosphogypsum-based lightweight aggregated concrete was pretreated by three different methods,i.e.,drying (L),saturation (H),and saturation followed by surface drying (M) methods.A series of macroscopic and microscopic tests,including unconfined compressive strength (UCS),scanning electron microscopy (SEM),backscattered electron microscopy (BSE),and nanoindentation (NI),were conducted to obtain strength,hydration product content,porosity,interface zone thickness,and micro-nano elastic modulus subjected to three pretreated methods.Results indicate that phosphogypsum-based lightweight aggregated concrete pretreated by method M exhibits the highest strength (with a strength of 24.3MPa after 180 days of curing),the reason is that pretreatment M can lead to the reduction in thickness and porosity of interface transition zone,and the increase in the micro-nano elastic modulus,thereby contributing on the mechanical properties of the interface transition zone.Furthermore,the swelling properties of phosphogypsum slag cement can effectively mitigate shrinkage induced by the hydration in the interface transition zone,thereby synergistically reinforcing the bonding performance of the interface transition zone and ultimately improving the mechanical properties of lightweight aggregated concrete.