Effect of pH on primary and secondary crack propagation in sandstone under constant stress (creep) loading

Time-dependent, chemically assisted crack propagation behavior in rocks is fundamental to understanding the long-term stability of civil engineering structures. In this study, we investigated the propagation of primary and secondary cracks in macrofractured sandstone in distilled water (pH = 5.6) and in an aqueous solution of hydrochloric acid (HClaq, pH = 1.82) using digital image correlation (DIC), comparing the results to those from experiments performed in air. Results show that for the macrofracture angles gamma = 0 and 90 degrees, the nucleation position r of the primary crack is not affected by pH, and always occurs at the fracture tip (r = 0) and fracture center (r = 1), respectively. For gamma = 30, 45, and 60 degrees, r increases quasi-linearly with the increase of gamma in semi-log plots, and an increase in pH moves the primary crack away from the fracture tip. For a given gamma, an increase in pH produces an increase in the kink angle k1 of the secondary crack. The influence of pH on secondary crack propagation is most pronounced at high gamma values. As the pH increases, the location of the secondary cracks gradually turns counterclockwise with a decreasing swing range, from 37 degrees at pH 1.82-35 degrees at pH 5.6. The secondary crack is inclined at a maximum of 30 degrees and 32 degrees to the maximum principal stress direction at pH 1.82 and pH 5.6, respectively. Right-lateral shear secondary cracks with well-developed tensile tail fractures are more prevalent in higher pH environments. The comparison with dry experiments indicates that distilled water is more aggressive than HClaq. The significant loss of calcium ions in distilled water and the reduction of large pore volume fraction in HClaq are considered the main reasons for the deterioration of the pH effect.