Engineering Performance of Expansive Soil Stabilized with Cement and Montmorillonite Adsorption Modifier

To enhance the strength and water stability of stabilized expansive soil, this study investigates the use of cement, montmorillonite adsorption modifier (MAM), and their composite system. Laboratory tests evaluated compaction characteristics, swell–shrink behavior, and mechanical performance. The results show that MAM more effectively regulates compaction by reducing optimum water content and increasing maximum dry density; 6% MAM increases maximum dry density by ≈0.04 g/cm3 and reduces optimum water content by ≈2%. In terms of swell–shrink behavior, MAM reduces both swelling and linear shrinkage more effectively than cement. The incorporation of 5% MAM reduces the free swelling ratio by 40% and the equilibrium moisture absorption by 2.7%, lowering the swelling classification to non-expansive. Furthermore, 5% MAM decreases the unloaded and loaded swelling ratio by 14.7% and 5%, respectively, while increasing MAM from 2% to 6% further reduces linear shrinkage by 1.12%. Cement significantly enhances compressive strength, with 7–28 d values reaching 2.2–2.7 times those of untreated soil at 9% content; however, its water stability under wet–dry cycles is limited. In contrast, the cement–MAM composite system achieves balanced improvement by simultaneously suppressing swelling and enhancing both strength and water stability. These findings provide a reference for the treatment and engineering application of expansive soils.