Crystalline swelling of GMZ bentonite under controlled relative humidity: Mineralogical basis and cation effects

This study presents an integrated mineralogical and crystallographic characterization of the montmorillonite in Gaomiaozi (GMZ) bentonite and quantifies its crystalline swelling under controlled relative humidity (RH) conditions. Bulk chemical composition of montmorillonite was determined by X-ray fluorescence (XRF), and its structural formula was calculated on an O 10 (OH) 2 basis, whereas mineralogical components and dioctahedral character were established by X-ray diffraction (XRD). An ex-situ RH-controlled XRD protocol (12%–100%) was developed to resolve discrete hydration states in oriented films of raw GMZ and homoionic forms (Na-, Li-, K-, and Ca-GMZ). The raw GMZ shows d -spacing equals 12.6 Å (1 Å = 1 × 10 -10 m) at RH = 12%, consistent with a single water layer (1W), and d -spacing equals 18.7 Å at RH = 100%, consistent with three water layers (3W). Crystalline swelling behavior of montmorillonite is strongly cation-dependent: Na- and Li-GMZs exhibit clear progression to 2W, while Ca-GMZ displays a broad, stable 2W plateau before entering 3W at very high RH and K-GMZ remains restricted to 1W, which is consistent with limited hydration due to partial fixation of K + in the ditrigonal cavities. The influence of layer charge on d -spacing is most evident at low to intermediate RH (0W–2W) and becomes secondary near saturation, particularly for Ca-saturated interlayers due to strong ion–ion correlations. Modified Laird model reproduces the stepwise swelling of Li-, Na-, and Ca-GMZ with good fidelity but not the K-form. The results establish a reproducible RH-controlled methodology, clarify how interlayer cations and layer charge govern discrete hydration states, and provide a basis for linking crystallographic behavior to macroscopic swelling performance of bentonite barriers.