8.1 Mineral and chemical composition analysis
The mineral and chemical composition of samples before and after the experiment is displayed in Figs. 19, 20, 21, Tables 4, 5, 6. The mineral composition of fresh sandstone comprises quartz, K-feldspar, plagioclase, clay minerals and calcite; and clay minerals comprise smectite, illite and chlorite. After the experiment, the results indicate that plagioclase, K-feldspar, calcite and clay minerals contents decreased. The drop in the mineral content of the samples is the lowest inside the sample; the highest on the surface; and in the middle beneath the sample surface. Since the sandstone surface is more prone to carry out WRI, and water acts on sandstone surface for a long time, while water infiltrates into the inner for a certain time. These caused the surface hardness to drop faster than the P-wave velocity. Calcite content decreases the most due to its instability, especially the surface calcite of pH=2.6 in the main disappears, demonstrating the weathering rate of calcite between grains is much higher than that of grains, and to slow down the sandstone weathering rate, the key is thus to control the dissolution of inter-granular material. Although dry smectite is stable in natural state, it often drives water swelling and drying shrinkage in response to the change in humidity, hence causing granular disintegration (Li & Chikaosa, 2005). The chemical properties of quartz are stable with strong weathering resistance, resulting in the increase of quartz after the experiment.
There are no new minerals generated in the dry-wet and freeze-thaw cycles. We deduce that the dry-wet cycle and freeze-thaw cycle on the deterioration of sandstone are physical deterioration. In fact, mineral grains and cement of sandstone are continuously lubricated and softened with the increasing dry-wet cycles (Sumner & Loubser, 2010). Furthermore, the water stored in the pores of sandstone freezes and expands under freeze-thaw cycles, causing tensile stress and micro-cracks in the sandstone. When the pore water or crack water melts, water enters into the micro-pores and micro-cracks inside the sandstone if the temperature alternates positively and negatively (Tan et al., 2011). Nevertheless, the chemical reaction between calcite and acid rain solution reduces the content of CaCO3. Other minerals that are non-reactive to corrosion are enriched on the surface of the samples which meanscould change the color of the samples. This might explain the occurrence of yellow spots on the samples (Sun, Li, Zhang, Wang, & Wang, 2010). Kaolinite, a new mineral, is produced on the sandstone after acid rain cycle. The main reason is that plagioclase and K-feldspar are more unstable. The following chemical reactions are likely to occur (Lee et al., 2011):
2Na[AlSi3O8]+2H++H2O=2Na+4SiO2+Al2[Si2O5][OH]4↓(kaolinite);
2K[AlSi3O8]+2H++H2O=2K+4SiO2+Al2[Si2O5][OH]4↓(kaolinite);
Ca[AlSi3O8]+2H++H2O=Ca2++Al2[Si2O5][OH]4↓(kaolinite);
CaCO3+2H+=Ca2++H2O+CO2↑.
Besides, chemical substances dissolved in acid rain solution would be taken away removed from the sandstone surface, and acid rain cycle is affected by the effects of dry-wet cycle which would accelerate chemical corrosion inside the sandstone.