Effects of Phosphogypsum on Strength Properties of Lime-Fly Ash Stabilized Red Clay and Its Microscopic Mechanism Analysis

In order to solve the problems of low early strength and poor water stability in lime-fly ash soil （a mixture of lime， fly ash， and red clay）， while considering the recycling of solid waste phosphogypsum， the study investigated the effects of phosphogypsum on the mechanical properties of lime-fly ash soil by indoor tests， and the microstructural characteristics and compositional changes of lime-fly ash soil before and after improvement were analyzed by scanning electron microscope tests and X-ray diffraction tests. The results revealed that phosphogypsum significantly enhanced the soil's early strength and water stability. With increasing phosphogypsum dosage， the soil’s unconfined compressive strength （UCS） initially increased and then decreased. The optimal mix（lime∶fly ash∶phosphogypsum=1∶2∶0.5） achieved peak strength at 8% lime addition， with UCS increasing by 67.2% at 7 days and tripling at 28 days. After saturation， the 7-day cured specimens showed no significant visual changes but demonstrated improved disintegration resistance. The water stability coefficient was elevated by 43%， reaching 0.91 at 28 days. SEM analysis demonstrated that phosphogypsum addition promoted the formation of new hydration products， transforming the soil structure from discrete granular particles into a denser cementitious network. XRD analysis identified these products as ettringite， which contributed to the soil’s strength enhancement.