The variation of swelling pressure with layer charge ( X ) has been primarily investigated using natural smectites with varying X values and inherent geochemical differences, making it infeasible to isolate the effect of X . This paper presents an integrated experimental and theoretical investigation of the effects of X on the swelling pressure of a sole-source Na-montmorillonite (NaM) by artificially tuning its X . A series of charge-reduced montmorillonites (CRMs) were prepared via Li-fixation, and one-dimensional constant-volume swelling experiments were conducted to measure their swelling pressure on samples of varying dry densities. Experimental results show that reducing X decreases swelling pressure by a factor of 3–5, due to the transformation of some expandable layers into pyrophyllite-like ones. In addition, Li-montmorillonite (LiM) exhibits 1.2–1.4 times higher swelling pressure than NaM, due to the Li + cation’s higher ionic potential. A theoretical model was developed based on the concept of intergranular stress to predict the swelling pressure of the CRMs through introducing two innovative concepts: (1) the immobility of relatively large, negatively charged clay particles constitutes a virtual, Donnan-type, semi-permeable membrane that is permeable to ions and water only, but not clay particles; and (2) the Donnan osmotic pressure in the mineral–ion–water system is equivalent to the macroscopic swelling pressure of the CRMs. The measured and predicted swelling pressures agree remarkably, corroborating the practical significance and viable capability of intergranular stress, instead of traditional effective stress, in predicting the swelling pressure of expansive clays without the need to explicitly address complex electromagnetic interactions.
Ma et al. (Sun,) studied this question.