Influence of pH on moisture-absorbing swelling cracks of red layer in central Sichuan and its micro-mechanism

In this study, the influence of pH on the moisture-absorbing swelling cracks of red mudstone in southwest China was studied and its microscopic mechanism was revealed. A self-designed water absorption device combined with digital speckle correlation (DSCM), scanning electron microscope, XRD diffraction experiment, mercury injection experiment and other means were used. The results indicated that compared with an alkaline environment, an acidic environment resulted in the self-absorption and cracking of mudstone. The maximum water absorption rate of mudstone and crack rate in a pH of 3 were determined as 15.77 and 18.91%, respectively; whereas for pH = 11, they were 11.71 and 7.11%, respectively. In the acidic environment, the maximum combined displacement value and maximum principal strain value of mudstone self-priming and swelling evolution were both greater than in an alkaline environment. Simultaneously, in the former, the moisture absorption failure characteristics of mudstone exhibited increased swelling strain zones and swelling cores. The three possible mechanisms by which pH affects the expansion of mudstone hygroscopic cracks were proposed: hydration-expansion softening, adsorption-wedge failure, and humidity stress field. However, it was found that the solutions with different pH values may affect these three microscopic mechanisms through the following four physical or chemical processes: mineral dissolution, ion absorption and exchange, particle association, and pore change. The results of this study are expected to be of significance and aid in roadbed design, tunnel support, slope treatment, and other projects in the southwest region.