The appearance of concrete is a visual representation of its quality. The evenness and luster of the concrete surface is of increasing concern to construction contractors and property owners, and their requirements are becoming more and more stringent. This has gradually become an important economic factor.

Bleaching is the most common phenomenon on concrete surfaces. According to incomplete statistics, the phenomenon of bleaching is found in more than 36% of concrete projects, and is one of the difficult “diseases” in construction projects. Generally, it does not cause quality accidents in the building structure, but its existence will have a serious impact on the aesthetics of the building and the assessment of the quality of the project.

Mechanism of frost heaving on concrete surfaces

Frost heaving on concrete surfaces, commonly known as efflorescence or frost heaving, is generally believed to occur during the hydration of cement. While C3S and C2S hydrate to form C-S-H gel, a large amount of Ca(OH)2 is also formed. Ca(OH)2 is a highly soluble substance that often exists in the free water of concrete during the early stages of hydration.

In the early stages of concrete setting and hardening, as the concrete dries, the free water within the structure with a high concentration of calcium hydroxide will gradually migrate outward along the internal capillaries to compensate for the water evaporating from the surface. After bringing the Ca(OH)2 dissolved in it to the surface of the concrete, the Ca(OH)2 will react with CO₂ and moisture in the air to form a white, water-insoluble precipitate CaCO₃, which will adhere to the surface of the concrete. This efflorescence that occurs during the hardening process is called initial efflorescence.

As the concrete hardens, the hardening process gradually slows down over time. Under the erosion of rain and snow, water will seep into the interior of the concrete in some places and dissolve the remaining calcium hydroxide. When the water evaporates with the rise in outside temperature, it will bring the calcium hydroxide dissolved in it to the surface of the structure again, forming secondary efflorescence.

The second whitening is different from the first whitening, and in most cases it does not appear on the entire surface of the concrete, but is unevenly distributed on the surface. This whitening is related to the type of cement, the amount used, the density, the water absorption rate and the porosity. Parts with a rough surface that is prone to water accumulation, internal porosity and a high water absorption rate are most likely to experience multiple whitening.

Causes of whitening on the surface of concrete and preventive measures

01

Water bleeding in the mixed concrete

The more water that seeps out of the surface of the structure during initial setting, the more severe the whitening. This is mainly manifested in the following aspects:

(1) An excessive water-to-cement ratio. The greater the water-to-cement ratio, the more free water there is in the mixture, and the greater the likelihood of bleeding and more concrete efflorescence. Therefore, as long as construction operations are not affected, the amount of mixing water should be reduced as much as possible and the water-to-cement ratio should be lowered.

(2) An excessive amount of admixture can also cause bleeding in 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 composition content of cement produced by different manufacturers is also different. In general, for cement with high C3A content, because of its fast hydration and high adsorption of admixtures, under the same conditions, over-dosage of admixtures is avoided, and bleeding caused by excessive admixture is not likely to occur. For cement with low C3A content, the admixture dosage should be appropriately reduced.

(4) It is related to the concrete mix ratio and aggregate gradation. A mixture with a low sand rate or poor gradation is prone to bleeding.

(5) With other conditions remaining unchanged, the mixing plant generally hopes to adapt to changes in raw materials and meet the pumping requirements of concrete with different grades by adjusting the admixture. Adding an appropriate amount of water retention, plastic retention and air entrainment components to the admixture is beneficial to improving the workability of the concrete and reducing bleeding and settlement.

02

The content of soluble salts and alkalis in the building materials is high

(1) Cement is the main component of concrete. The higher the alkali metal oxide content of the cement itself, the more alkali can be precipitated under certain conditions, and the greater the possibility that the surface of the mixed concrete will turn white. The alkali content of cement also has a significant impact on the action of admixtures. Therefore, when selecting cement, try to use low alkali cement.

(2) The level of soluble salts in the aggregate is also an important factor affecting the whitening of the concrete surface. If conditions permit, the aggregate should be selected carefully and its soluble salt content strictly controlled.

(3) Naphthalene-based concrete admixtures are currently the most widely used, with low concentrations being the most common. The relatively high content of Na₂SO₄ increases the amount of soluble substances in the concrete, thereby increasing the likelihood of the concrete surface whitening. Minimizing the alkali content of admixtures is not only beneficial for preventing alkali-aggregate reaction, but also improves the effectiveness of admixtures.

(4) When preparing concrete, add an appropriate amount of reactive silica admixture. During the hydration of cement, SiO₂ can react with Ca(OH)₂ produced by the hydration of cement mineral components C3S and C2S to form C-S-H gel, which is more likely to react with the strong alkalis NaOH and KOH in the cement, thereby reducing the free alkali content in the concrete. This means that after the concrete has hardened and dried, even if water penetrates into the substrate, there will be very little available to leach out as salt or alkali. A high-quality mineral admixture has a good effect on preventing the concrete surface from whitening.

03

Concrete itself is permeable

Due to construction requirements, the water-to-cement ratio of concrete mixtures is always greater than the theoretical value required for cement hydration. Excessive water often occurs during the concreting and vibrating process, forming hydrophobic channels inside the concrete or remaining inside the concrete and accumulating under the coarse aggregate. It is evaporated during the later hardening stage to form internal voids. In addition, after cement hydration, the volume reduction of hydrates can also cause internal pores or gaps, resulting in a certain degree of permeability in the concrete.

(1) The incorporation of admixtures can reduce the water-to-cement ratio, reduce excess water, and greatly improve the compactness of the concrete. The incorporation of a certain air-entraining component in the admixture improves workability, prevents bleeding and settlement, reduces large capillary pores on the interface between the aggregate and cement paste, produces a large number of tiny air bubbles distributed in the voids of the concrete, and accumulates in the passageways of the capillaries, cutting off the capillaries and greatly improving the impermeability of the concrete.

(2) Strengthen construction management. The concrete should be vibrated and compacted to prevent external water from entering the interior of the component. Strengthen the plastering. It is best to do another layer of plastering before initial setting to close the microcracks on the surface caused by water evaporation and make the cement slurry float layer compact, thereby improving the surface impermeability and preventing whitening.

(3) For projects with high requirements for the appearance of the concrete, a layer of transparent waterproof paint can also be sprayed on the surface of the concrete to fill the pores on the surface of the concrete tightly and prevent penetration.

04

If the maintenance is not carried out in time, the method is not appropriate, or the maintenance time is insufficient

Concrete should be protected promptly after it has set, so that the rate of drying and evaporation is slowed down. When water penetrates from the surface of the concrete to the interior, carbon dioxide in the air enters the interior of the concrete and reacts chemically with the calcium hydroxide dissolved in the water, producing calcium carbonate precipitation. If the concrete is not protected promptly, when it hardens and dries, the water migrates from the interior to the exterior, carrying soluble substances to the surface of the concrete. As the surface water evaporates, the white soluble substances remain on the surface, causing a whitening effect. The curing and covering should not be stopped prematurely before the strength of the poured structure has fully reached the point of drying. Generally, the curing and covering should be carried out for no less than 14 days after construction to maintain heat preservation and moisture. It has been found in practice that the concrete test blocks placed outdoors are shiny the day after hardening, and begin to turn white on the third day. However, those that were cured in time did not turn white. The reason for the serious whitening of the test blocks on the construction site is mostly that they were placed in the open air without the necessary curing.

In short, there are many factors that cause the concrete surface to turn white, such as weather, temperature and humidity, and the type of admixture. These factors are interrelated and influence each other. Only by taking practical preventive measures based on local raw material conditions and actual work, can the phenomenon of the concrete surface turning white be effectively reduced, and the high requirements for the appearance of concrete be met.