The appearance quality of concrete is an intuitive manifestation of the quality of concrete. The uniformity of the color and luster of the concrete surface is more and more concerned by construction contractors and owners, and the requirements are getting higher and higher. It has gradually become an important economic factor. Whitening is the most common phenomenon on concrete surfaces. According to incomplete statistics, the blushing phenomenon is as high as 36% or more in concrete engineering, which is a "sickness" that is difficult to deal with in construction projects; one, although it generally does not cause quality accidents in building structures, its presence Both aesthetics and the assessment of the quality grade of construction projects will have a serious impact.
Mechanism of whitening of concrete surface
The surface of the concrete is whitened, commonly known as pan-alkali or blooming.It is generally believed that during the hydration of cement, C3S and C2S are hydrated to form a CSH gel, and a large amount of Ca(OH)2 is formed.Ca(OH)2 is An extremely soluble substance often exists in the free water of concrete in the early stage of hydration. In the early stage of concrete setting and hardening, as the concrete dries, the free water with a high concentration of calcium hydroxide in the structure will gradually migrate outward along the inner capillary pores to compensate for the evaporated water on the surface, and the Ca dissolved in it After the (OH)2 is taken out and reaches the surface of the concrete, Ca(OH)2 will also chemically react with CO2 and moisture in the air to form a white precipitate CaCO3 that is insoluble in water and attaches to the surface of the concrete. This kind of ubiquitin produced during the hardening process is called the first whitening.
In the hardening process of concrete, the hardening speed gradually slows down with the extension of time.Under the erosion of rain and snow, moisture will penetrate into the concrete in some parts, and the entered moisture will dissolve the remaining calcium hydroxide. When the outside temperature rises and evaporates, the calcium hydroxide dissolved in it will be brought to the surface of the structure again, forming a second whitening.
The second whitening is different from the first whitening. In most cases, it does not appear on the entire surface of the concrete, but appears unevenly on the local surface. This blushing is related to cement variety, dosage, density, water absorption and porosity. The parts with rough surface and easy to accumulate water, and the internal loose water absorption rate are most likely to produce whitening for many times.
Causes and preventive measures of whitening on concrete surface
At present, there is no admixture that can effectively prevent the precipitation of calcium hydroxide water in the concrete to the surface. Only preventive measures can be taken for the cause of the whitening.
Mixed concrete appears bleeding
The more water seeping out of the surface of the structure during initial setting, the greater the severity of blushing. Mainly manifested in the following aspects:
(1) The water-cement ratio is too large. The larger the water-cement ratio, the more free water in the mixture, the greater the possibility of concrete bleeding, and the more concrete blushing. Therefore, under the premise of satisfying the construction operation, the water consumption for mixing and the water-cement ratio should be reduced as much as possible.
(2) Excessive amount of admixture will also cause bleeding of the concrete mixture. In this case, the amount of admixture should be reduced.
(3) Related to cement. Affected by the source of raw materials, production control and process conditions, the mineral content of cement produced by different manufacturers is also different. In general, for cement with high C3A content, due to its fast hydration, the adsorption capacity of admixture is large, under the same conditions, the over-admixture of the admixture is avoided, and the secretion caused by the excessive amount of the admixture is not easy to occur. water. For cement with low C3A content, the admixture should be appropriately reduced.
(4) It is related to the mix ratio of concrete and aggregate gradation. Mixtures with too low sand rate or poor gradation are prone to bleeding.
(5) Under the condition that other conditions remain unchanged, the mixing station generally hopes to adjust the admixture to adapt to the changes in raw materials and meet the pumping requirements of concrete with different labels. Adding proper amount of water-retaining, plastic-retaining and air-entraining components to the admixture is beneficial to improve the workability of concrete and reduce bleeding and sedimentation.
The content of soluble salts and alkali in the building materials is high
(1) Cement is the main component of concrete. The higher the alkali metal oxide content of cement itself, the more alkali can be precipitated under certain conditions, and the greater the possibility of whitening of the concrete surface. At the same time, the alkali content of cement has a great influence on the effect of the admixture. Therefore, when choosing cement, low alkali cement should be used as much as possible.
(2) The level of soluble salt in the aggregate is also an important factor affecting the whitening of the concrete surface. If conditions permit, there should be a certain selectivity for the aggregate, and its soluble salt content should be strictly controlled.
(3) Naphthalene-based concrete admixtures are currently the most widely used, most of which are low-concentration. The content of Na2SO4 is relatively high, which increases the soluble matter in the concrete, thus increasing the possibility of whitening of the concrete surface. Minimizing the alkali content of the admixture is not only beneficial to prevent the alkali-aggregate reaction, but also improves the use effect of the admixture.
(4) When preparing concrete, appropriate amount of active siliceous admixture is added. During the cement hydration process, SiO2 can react with the Ca(OH)2 produced by the hydration of the cement mineral C3S and C2S to form a CSH gel, which is more likely to react with the strong alkalis NaOH and KOH in the cement and reduce the freeness in the concrete. Alkali content. In this way, after the concrete is hardened and dried, even if moisture penetrates into the substrate, the amount of salt or alkali available for precipitation is very small. High-quality mineral admixture has a good effect on preventing whitening of concrete surface.
The concrete itself is permeable
Due to the requirements of construction, the water-cement ratio of concrete mixes is always greater than the theoretical value required for cement hydration.Excess water often bleeds during the vibrating process of concrete pouring, which causes the formation of hydrophobic channels inside the concrete, or stays in Inside the concrete, it gathers under the coarse aggregate and evaporates in the later period of hardening to form internal voids. In addition, after the cement is hydrated, the volume reduction of hydrate will also cause internal pores or cracks, resulting in a certain permeability of concrete.
(1) The admixture can reduce the water-cement ratio, reduce excess water, and improve the compactness of commercial concrete. Adding a certain air-entraining component to the admixture improves the workability, prevents bleeding and settling, reduces the large capillary holes on the interface between the aggregate and the cement stone, and generates a large number of tiny bubbles distributed in the voids of commercial concrete It is collected in the capillary channel, and the capillary is cut off, and the impermeability of commercial concrete is greatly improved.
(2) Strengthen construction management, commercial concrete should be vibrated and compacted, so that external moisture is not easy to enter the interior of the component. To strengthen the surface, it is better to re-apply the surface before the initial setting, so that the micro-cracks caused by the evaporation of water on the surface are closed, and the cement slurry floating layer is dense, thereby improving the surface impermeability and preventing whitening.
(3) For projects that require high quality of concrete appearance, a layer of transparent waterproof material can also be sprayed on the surface of the concrete to fill the pores of the concrete surface densely to achieve the purpose of preventing penetration.
Insufficient maintenance, improper methods or insufficient maintenance time
After the concrete is finally set, it should be cured in time to slow down the drying and evaporation rate. When the water penetrates from the concrete surface to the inside, the carbon dioxide in the air enters the concrete and chemically reacts with the calcium hydroxide dissolved in the water to generate calcium carbonate precipitation. If it is not cured in time, when the concrete is hardened and dried, the water migrates from the inside to the outside, and the soluble substances are carried to the surface of the concrete. As the surface moisture evaporates, the white soluble substances remain on the surface, causing whitening.
Before the strength of the pouring structure is completely dry, the curing and covering should not be stopped prematurely. Generally, the heat preservation and moisturizing after construction should be no less than 14 days. Practice has found that the concrete test blocks placed outdoors are still oily on the second day after hardening, and begin to blush on the third day, but no blushing occurs in time. Most of the reasons for the serious blushing of the test blocks on the construction site are due to the fact that they are placed in the open air without necessary maintenance.
Treatment of whitened surfaces
After whitening occurs after a long time of use after treatment, it needs to be removed if it needs to be re-decorated on its surface. The general and easy way is to use pickling method. The author has treated several times at the foot of the wall where the pan-alkali corrosion is severe. The method is to use 1:9 ~ 10 (hydrochloric acid: water) of dilute hydrochloric acid to clean. Rinse the area with clean water on the surface to be cleaned, and then wash it with diluted hydrochloric acid for about 30 minutes to work. At this time, rinse the surface with plenty of clean water to remove the whitening.
In addition, the pressure sand blasting method can also be used to remove the white. Under the impact of the dry pressure fine sand particles, the whitening layer will be removed to expose the original shape. For example, the colored surface will make the color more colorful, but it must not damage the surface to form another disadvantage.
The phenomenon of blushing will naturally disappear with the prolongation of the use time even without manual removal. When the carbon dioxide and various moisture in the air act on the structure for a long time, the main component of the whitening, calcium carbonate, becomes calcium bicarbonate. Because calcium bicarbonate is a trait that is easily soluble in water, it is subject to rain and snow. Long-term wetting shock will flow away and restore the surface to its natural state.
There are many factors that cause the whitening of the concrete surface, such as weather, temperature and humidity, and the variety of additives. Various factors are related to each other and affect each other. Only by combining the conditions of local raw materials and working conditions and taking practical and preventive measures can we effectively reduce the appearance of whitening on the concrete surface and meet people's increasing requirements for the appearance quality of concrete.
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