## 4 Discussion
Sulphuric and nitric acids, which are supplied by acid precipitation because of air pollution, take over from carbonic acid in weathering reactions of albite, which induced a decrease in the atmospheric/soil CO2 consumption by weathering [32]. The high air pollution in the urban environment causes dissolving CO2 in water and carbonic acid formation afterward. This process promotes the mineral weathering of albite feldspar in kurkar sandstone and results in albite kaolinization [32; 33; 34]: \[\text{Albite}+5.5\text{H}_{2}\text{O}+\text{CO}_{2}=\text{HCO}_{3}{}^{-}+ \text{Na}{}^{+}+2\text{H}_{4}\text{SiO}_{4}+0.5\text{Kaolinite}\]  
Generally, kaolinization is a long-term process. However, this process is strongly enhanced by the hot, humid climate and acidic pH, and its time is reduced dramatically [35]. Biotite is also a mineral of a highly sensitive nature, and it has a strong tendency to kaolinization when exposed to elevated temperatures and a non-neutral acidic pH [36]. Furthermore, it is might also be partially degraded upon low and high alkali pH values [32; 33; 34] (b) Kurkar sandstones in the Panorama Sea Walls have always been exposed to the non-stop dry-wet cycles in acidic and alkaline environments with high levels of air pollution (acidic pH ca. 4.0 (estimated) and seawater breeze (alkaline pH ca. 8.5-9.0), respectively. The dissolution reactions of albite and calcite minerals that form cement in sandstone are too spontaneous in an acidic environment. In an alkaline solution, the dissolution reaction of quartz in sandstone is spontaneous [25; 28]. The dissolution of the large amounts of sandstone cement explains kurkar crumbling (see Figure 5) and a sharp decrease in the mechanical properties of naturally weathered kurkar (see Table 2). A sharp rise in capillary absorption could be partially attributed to quartz (aggregate) dissolution leading to the high sandstone porosity, see Figure 7. A dissolution of silicon and a release of iron (II) ions from biotite subjected to the solutions with the low and near neutral (alkali) pH is found to occur within the first ten months of exposure [32; 34]. The oxidation of iron (II) to iron (III) in Panorama Sea Walls' sandstone might be a rapid process given the seawater chlorides absorbed by kurkar. Furthermore, the rusty coloration of kurkar, see Figure 9, has resulted from these redox reactions of iron phases released by biotite because of its degradation. 3. The walls were cast in-situ against sandstone veneer siding. Thus, one can conclude with confidence that kurkar stone has been, for at least 28 days, in direct contact with fresh and hardening cement-based matrix, which means that kurkar absorbed high pH alkali solutions from concrete. It is common knowledge that the pH of fresh and hardening concrete is ca.12 to 13, and alkalis in it are Ca2\({}^{+}\) (major alkali) and Na\({}^{+}\)& K\({}^{+}\) (minor alkalis).). Furthermore, salt-containing seawater breeze and acid air pollution in Acre Bay have promoted the formation of significant efflorescence at kurkar face surface, see Figures 7 and 10 (The chemical composition of such efflorescence might be variable and consists of the following salts: halite, NaCl, thenadite, Na2 SO4, mirabilite, Na2 SO4, 10H2O, thermonatric, Na2 CO3:H2O, gypsum anhydrite, CaSO4 and gypsum, CaSO4 2H2O, depending on (a) seawater moisture content in kurkar sandstone, (b) concentration of alkalis absorbed by sandstone from fresh and hardening mortar/concrete and (c) levels of the local air pollutions [28; 29; 40; 41; 42]. Those mentioned above physical and chemical processes caused a loss of kurkar components, see Figure 7; breaking out the stone matter, see Figure 8, and the rusty coloration of kurkar, see Figure 9.  
The study of the mineralogical composition by petrographic analysis and XRD test has shown that the secondary minerals in kurkar sandstone have been responsible for some weathering processes in the Panorama Sea Wall.