"Getting Close to Meihui" Vol. 7 | Collection of Issues on Exterior Wall Efflorescence in the New Issue
Release time:
2021-11-02
Source:
01. The exterior wall has efflorescence and is bulging in large areas. When knocked down, there is a layer of white substance inside. If we clean off the white substance and apply anti-salt nitrate, will it be effective? Will there be more efflorescence later?
Analysis:
First, we should understand that the white substance is alkali.
Secondly, we need to understand why alkali is produced. The reason for alkali production is related to water. Simply put, areas with efflorescence have water seepage: either water seeps from the inner wall to the outer wall, or water seeps in through cracks in the outer wall.
Finally, after we solve the water seepage problem, we can start reworking the wall. After cleaning off the white substance, use diluted oxalic acid for cleaning (5% dilution). If the efflorescence is severe, clean with diluted oxalic acid twice; if the situation is not too serious, one cleaning is sufficient. After cleaning with diluted oxalic acid, rinse the wall thoroughly with clean water 3 to 4 times to dilute any remaining oxalic acid on the wall. Then apply anti-salt nitrate, and after it dries, you can proceed with putty scraping and finishing layer construction.
After this treatment, efflorescence will not occur again.
02. Many years ago, the exterior wall with stone-like paint has efflorescence in large white patches. How should this be handled? The exterior wall has a layer of mesh during the base coating, and it has cracked? What should we do?
Analysis:
If it is a large area of whiteness, then it is similar to our first question. It is basically either cracking or water seepage from the inner wall, which causes this phenomenon.
Treatment method: After solving the water seepage problem, start reworking the wall. After cleaning off the white substance, use diluted oxalic acid for cleaning (5% dilution). If the efflorescence is severe, clean with diluted oxalic acid twice; if the situation is not too serious, one cleaning is sufficient. After cleaning with diluted oxalic acid, rinse the wall thoroughly with clean water 3 to 4 times to dilute any remaining oxalic acid on the wall. Then apply anti-salt nitrate, and after it dries, you can proceed with putty scraping and finishing layer construction.
The exterior wall already had a layer of mesh during the base coating, but it still cracked, which involves a cracking issue. Cracking issues in construction can be divided into many categories: structural cracking, thermal expansion cracking, settlement cracking, etc.
First, we need to understand how the cracking occurs. If it is large-scale vertical cracking (in the vertical direction), it is a crack between entire floors. In most cases, we cannot do much about this; this phenomenon is mostly a structural issue.
There is also a type of crazing problem. Crazing mainly occurs because the cement mortar is not fully cured before the coating work begins. The cement mortar continues to shrink during the curing process, leading to cracks in the coating layer. The shrinkage of cement mortar is significant, and even if the coating layer has added anti-cracking fiber mesh, it cannot prevent cracking.
Therefore, to prevent cracking in the exterior wall, attention should be paid to the junctions between the wall and columns, or at the junctions of newly constructed walls, where steel mesh must be laid before plastering. After completing the work, the cement mortar layer should have a curing period of 28 days before further construction. This will help avoid many issues.
If such a problem has already occurred, and it is structural cracking, this issue cannot be resolved at present. The cracking phenomenon can only be filled to ensure aesthetics, but it will crack again later.
If it is overall crazing of the wall, as long as the cracks are not too severe, it will not significantly affect the quality, but it will affect aesthetics. If reworking is necessary, the base layer must be removed and then re-plastered and scraped.
03. Why does the wall show water stain-like whiteness six months after the stone-like paint application? How should this be handled?
Analysis:
The main reason for this problem is efflorescence at the bottom, which is the alkali from the cement mortar.
If the cement mortar is coated with stone-like paint shortly after application, the alkalinity of the cement mortar is very high (pH value greater than 10), and this alkali will slowly seep out. Initially, there may not be any visible problems, but after drying, the alkalinity will gradually seep out, leading to corrosion of the paint film—corroding the emulsion and pigments in the coating layer, resulting in the wall turning white. Later, delamination and powdering may occur. Currently, there is no mature method in the industry to address this issue, as it is a problem caused by insufficient preparation work in the early stages.
Meihui Stone Coating, Efflorescence, Approaching Meihui
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Address: Quanhui Petrochemical Park, Fujian Province
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