Experimental investigation on microstructure and surface morphology deterioration of limestone exposed on acidic environment

Understanding evolution of microstructure and surface morphology is essential for studying the strength of building material in acidic enviorment. In this study, the microstructure of limestone samples immersed in acidic solutions of varying pH values was evaluated by nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM), while the joint surface morphology evolution process was determined by three-dimensional (3D) scanning experiments. Further, direct shear tests on the immersed jointed samples were conducted for shear properties analysis. It is found that the pore size distribution gradually changes from nanopore to micropore with the decrease in pH value, while the single pore area and perimeter increased and the fractal dimension decreased. Therefore, the increased pore distribution, enlarging in area, and change in the shape of the pores altered the microstructure and caused the increase in porosity. Also, with the decrease in pH value, the decrease rate of 3D relative fluctuation height, angle, and 3D area ratio increase. The 3D average fluctuation angle decreases the most, whereas the 3D area ratio decreases the least. In an acidic environment, the 3D surface morphology is significantly influenced by the 3D average fluctuation angle, which eventually changes joint roughness. Under the condition of acidic underground water, the microstructure of the joint wall and surface morphology of the joint degraded, thereby affecting the joint shear behavior. Therefore, the degradation of joint roughness coefficient and joint wall compressive strength should be considered for evaluating the joint mechanical properties in an acidic environment.