6.2 Experimental parameters and analytical method
All samples were first dried at 105 ℃ for at least 48h, and cooled down until a constant mass was reached. After that, the mass, surface hardness and P-wave velocity were measured and the surface microstructure of samples was photographed at fixed points using a microscope. For each sample, all these parameters were re-determined after every period. At the end of the experiments, the XRD and SEM of samples were measured in order to analyze the sandstone weathering mechanism. Fig.8 shows that the sampling locations of XRD and SEM are the surface (red), middle (blue) and inner (green) parts. Three experiments were conducted in 50 periods that lasted for 350 days.