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.