7.2 Surface microstructure
The surface microstructure of samples at 60 times in three experiments was shown in Fig. 12, 13, 14. Quartz, feldspar and other grains of fresh sandstone were closely arranged. The grains were filled with white cement with clear boundary contour and smooth surface. As the experiment went on, all samples experienced granular disintegration. On the other hand, the surface color of samples turned dark and black spots were visible. Besides, a small number of pores and micro-cracks reflected that the structure got loose. Some yellow spots appeared on the samples under the effects of acid rain cycle. The deterioration of samples in acid rain solution (especially at higher H+ concentrations) was more serious and rapid than that of samples affected by dry-wet and freeze-thaw cycles.