The main factors for the yellowing of the surface of the water-resistant putty After material research, a large number of experiments and engineering practice, the author believes that the main factors for the yellowing of the surface of the water-resistant putty are as follows:

Reason 1. Calcium hydroxide (ash calcium powder) back to alkali causes yellowing Calcium hydroxide, molecular formula Ca (OH) 2, relative molecular weight 74, melting point 5220, pH value ≥ 12, strong alkaline, white fine powder, slightly Soluble in water, soluble in acid, glycerin, sugar, ammonium chloride, soluble in acid to release a lot of heat, relative density is 2.24, its clear aqueous solution is a colorless, odorless alkaline transparent liquid, gradually Absorbed, calcium oxide becomes calcium carbonate. Calcium hydroxide is moderately strong alkaline, its alkalinity and corrosiveness are weaker than sodium hydroxide, calcium hydroxide and its aqueous solution are corrosive to human skin, clothing, etc., but non-toxic, and should not be in direct contact with skin for a long time.

Calcium hydroxide is an active filler in water-resistant putty to form a hard film with heavy calcium carbonate and high-gloss rubber powder. Because of its strong alkalinity and high alkali content, part of the water in the putty will be absorbed by the wall base during construction. The same strongly alkaline cement mortar bottom, or sand-lime bottom (lime, sand, a small amount of cement) is absorbed, as the putty layer gradually dries and the water volatilizes, the alkaline substances in the grassroots mortar and putty and some of them are unstable after hydrolysis Substances in the putty (such as ferrous iron, ferric iron, etc.) will come out through the tiny pores of the putty, and a chemical reaction will occur after encountering air, causing the surface of the putty to turn yellow.

Reason 2. Volatile organic chemical gases. Such as carbon monoxide (CO), sulfur dioxide (SO2), benzene, toluene, xylene, formaldehyde, pyrotechnics, etc. In some engineering cases, there have been situations where the putty surface has turned yellow due to the use of paint and a fire to keep warm in the room where the water-resistant putty has just been scraped, or even burning incense in the room, and many people smoking at the same time.

Reason 3. The impact of climate and environmental factors. In the northern region, during the season exchange period, the surface of the putty usually turns yellow from November to May of the next year, but this is only an isolated phenomenon.

Reason 4. The ventilation and drying condition are not good. The wall is wet. After scraping the water-resistant putty, if the putty layer is not completely dry, closing the doors and windows for a long time will easily cause the surface of the putty to turn yellow.

Reason 5. Grassroots issues. The bottom of the old wall is generally a sand-gray wall (lime, sand, a small amount of cement, and some mixed with gypsum). Lord, but there are still many areas where the walls are plastered with lime and plaster. Most of the wall materials are alkaline. After the putty touches the wall, some water will be absorbed by the wall. After hydrolysis and oxidation, some substances, such as alkali and iron, will come out through the tiny pores of the wall. A chemical reaction occurs, causing the surface of the putty to turn yellow.

Reason 6. Other factors. In addition to the above possible factors, there will be other factors, which need to be further explored.

Solution to prevent water-resistant putty from returning to yellow：

Method 1. Use a back-sealing agent for back-sealing.

Method 2. For old wall decoration, the low-grade ordinary putty that is not water-resistant and easy to pulverize has been scraped before. Before using high-grade water-resistant putty, technical treatment should be done first. The method is: first spray water to wet the wall surface, and use a spatula to wipe it Remove all the old putty and paint (until the hard bottom) and clean it up. After the wall is completely dry, clean it up again and apply the backing agent to cover the backing treatment, then scrape the water-resistant putty. yellow.

Method 3. Avoid volatile chemical gases and fireworks. During the construction process, especially when the putty is not completely dry after construction, do not smoke or light a fire indoors for heating, and do not use volatile chemicals such as paint and its thinners indoors within three months.

Method 4. Keep the site ventilated and dry. Before the water-resistant putty is completely dry, do not close the doors and windows tightly, but open the windows for ventilation, so that the putty layer can dry as soon as possible.

Method 5. An appropriate amount of 462 modified ultramarine can be added to the water-resistant putty. Specific method: According to the ratio of 462 modified ultramarine: putty powder = 0.1: 1000, first add ultramarine into a certain amount of water, stir to dissolve and filter, add the ultramarine aqueous solution and water into the container, and then press the total water: putty powder = 0.5 ?: 1 weight ratio, put the putty powder into the container, stir it evenly with a mixer to form a creamy milk, and then use it. The test shows that adding a certain amount of ultramarine blue can prevent the surface of the putty from turning yellow to a certain extent.

Method 6. For putty that has turned yellow, technical treatment is required. The general treatment method is: first apply a primer on the surface of the putty, and then scrape and apply high-grade water-resistant putty or brush interior wall latex paint.

Summarize the above points：

The surface yellowing of water-resistant putty and imitation porcelain paint involves many aspects such as raw materials, environmental conditions, climate conditions, wall base, construction technology, etc. It is a relatively complicated problem, and further research and discussion are needed.