when the cement contact the stone, some material easy to absorb wet enter into the stone, it make the stone surface wet mark which is not easy to be naturally dry. It is called water spot.
1,The cause of water spot
due to no protection for stone
due to protection material not averagely brushed on the stone surface.
Back and side surface someplace no protection material
2,The distinguish of the water spot
Firstly use clean water to make the water spot wet, and then use PH test paper to test its PH value, if test paper show to be blue color (PH value > 7), it is alkaline. Furthermore, whatever the weather change, the water spot never be dried. Even baked with fire, it is difficult to make bake it dried.
3,Variety of water spot
From how it come into being, mainly the following kinds:
(1) Hydration gel type
Cement after hydration, it become silicic acid gel etc. the stone pore make the gel material move to the surface level and solidify after the surface water vaporize.
Cement gel is of level (net) structure, its surface area reach to 200-600m2/g, gel hole the size is 1.5-3nm, with very strong water absorbability, and simple heat drying method could not make the gel to become decompose. The gel container water look half-transparent, it change the light reflectivity, so it look like to be wet by water.
Some stone with loose density, if use wet sticking, usually it occur such kind of water spot.
(2) Web absorbability saline-alkali type
It is the high saline-alkali exist in cement and the solubility saline-alkali with the add-in material for cement, these kind of saline-alkali is easy to absorb the water, form the hydrate which cause the water spot. More seriously, sometimes it cuase the obvious saline-alkali spot on the stone surface.
The saline-alkali exist in cement: the oxid exist in cement include Na2O、K2O, ect. The alkali content is 0.6～1.5%, mix with water and become NaOH、KOH etc solubility alkali. Part of them react with the active Sio2 and become alkali-silicon acid gel, part of them react with the calcite and become sodium carbonate and Calcium hydroxide, the leftover part will go out through stone pore.
Use cement add-in will have some solubility saline-alkali material: like NaCl（anti-freeze）、CaCl2、Triethanolamine etc.
In actual situation, the water spot, usually not only one kind. Quite often they are hydrate gel type and water absorb saline-alkali type, usually the saline-alkali type have lower steam pressure, they absorb the water in the air so to reach the balance of outer world steam pressure. While the stone pore and the dydrate gel, the extra pressure caused by them seems to be helpful for the absorbing more water. That is why whatever how the weather is, the water spot on the surface is always difficult to be dried, and also it is not easy to clean this kind of water spot.
To construction stone, water is the evil, after the water spot appear on the stone, other kind of disease will come out also, for example, water spot+saline-alkali , water spot-rusty spot. Etc.
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