Effect of Alkali Activation on Swelling Suppression and Microstructural Development in Geopolymer-Stabilized Bentonite

Geopolymers, a class of alkali-activated aluminosilicate binders, have emerged as a sustainable alternative for expansive soil stabilization. In this study, the swelling behavior of geopolymer-treated bentonite was systematically investigated using a Taguchi orthogonal design, complemented by XRD, FTIR, and SEM analyses to elucidate the underlying mechanisms. Specimens were compacted to an initial void ratio of e = 1.1, sealed, and cured under controlled conditions (22 ± 2 °C and 70 ± 2% relative humidity) prior to testing. The free swell ratio (FSR) was determined using a standardized free swelling test in accordance with GB/T 50123-2019, which is technically consistent with ISO 17892-13, under zero vertical surcharge. Each orthogonal condition was tested using a single specimen, and the reported values represent individual measurements. The results show that NaOH concentration is the dominant factor controlling swelling response, with a quantified contribution of 55.04%. The swelling behavior exhibits a distinct two-stage trend, characterized by an initial enhancement at low alkali concentrations followed by a significant suppression beyond a critical threshold of approximately 3 mol/dm3. Microstructural analyses reveal that this transition is governed by a progressive interlayer cation exchange, the structural dissolution of clay minerals, and the formation of geopolymer gel, which densifies the soil matrix and restricts interlayer expansion. These findings provide quantitative and mechanistic insight into the role of alkali activation in expansive clay stabilization and establish a practical concentration threshold for optimizing swelling suppression.