Efflorescence in concrete occurs when the calcium hydroxide and salts in concrete dissolve and come to the surface of the concrete. This creates a thin white layer of crystals on the surface of the concrete. Efflorescence not only affects the durability of concrete but its aesthetic appeal as well. Discover how to stop efflorescence in concrete. Explore the causes, factors, and preventive measures in this comprehensive article. Preserve the quality of your concrete surfaces today.
What Factors Cause Efflorescence in Concrete?
A humid environment plays a important role in the occurrence of efflorescence. Rainwater, snow melt, surface waters, ground water or the water in industrial waste all penetrate concrete through the pores in the surface of the concrete and through cracks and grout joints. Efflorescence occurs more frequently in the rainy winter season and is rarely seen in the summertime. However, efflorescence reappears in the cold and rainy months that follow the hot dry season.
Efflorescence occurs rapidly in the first months after concrete is poured, but subsides over time. It generally stops almost completely after 3-4 years.
Effects of water from different sources on dissolution of calcium hydroxide:
- Rainwater and snowmelt are both very soft, so they are very effective in dissolving calcium hydroxide.
- The cooler the water temperature is, the faster calcium hydroxide dissolves. This is why snow melt has a greater effect on the dissolution of calcium hydroxide than rainwater does.
Acidic waters also facilitate the dissolution of calcium hydroxide. Ground water and sea water contain various ions such as sulfate chloride, sodium and potassium.
As a result of the reactions of the main components of calcium silicate in cement with water, calcium-silica-hydrate gels and calcium hydroxide crystals Ca(OH)2, which provide the binding property of the cement, are formed.
Both the water that penetrates concrete and the aggregate used in the production of concrete contain various salts. These salts and calcium hydroxide are dissolved by the water that penetrates concrete and move towards the surface of the concrete in a capillary fashion.
When the water that reaches the surface evaporates, the calcium hydroxide and salts form a layer of precipitate on the surface of the concrete that is generally 3 mm to 15 mm thick. Calcium hydroxide comes into contact with the carbon dioxide in the air and is transformed into CaCO3 (calcium carbonate). This is why the thin layer that forms is white.
Ca(OH)2 + CO2 →CaCO3 + H2O
A white stain on the surface of the concrete spoils its aesthetic appearance. The dissolution of a small amount of the calcium hydroxide and salts in the concrete do not greatly affect the strength of the concrete.
However, as the efflorescence continues and excess calcium hydroxide dissolves from the concrete, the concrete becomes even more porous. The fact that the concrete becomes more porous reduces the durability of the concrete. Therefore, this has a negative effect on concrete durability. It becomes easier for harmful water to enter this kind of concrete and cause further damage.
The amount of water that penetrates concrete must be reduced. The following precautions should be taken to achieve this:
- Concrete should have a minimum amount of empty space.
- Aggregate gradation should be chosen appropriately,
- The water/cement ratio should be kept as low as possible,
- The concrete mixing materials and ratios should be chosen for the purpose of reducing concrete permeability,
- Bleeding of fresh concrete should be prevented,
- The permeability of the concrete should be reduced by making the placement and compaction of the concrete appropriate,
- The concrete should be cured for an adequate amount of time.
- Precautions should be taken to prevent water penetration into the concrete.
- Adequate amount of joint should be used in order to prevent the accidental formation of cracks. Joint gaps should be closed with waterproof material,
- The surface of concrete in contact with the soil should be properly insulted from ground water.
- The upper surfaces of the structures should have smoothness and slope that will not cause water accumulation.
- The amount of calcium hydroxide and salt in concrete should be reduced as much as possible.
The pozzolanic substances found in mineral admixtures used in concrete (fly ash, blast furnace slag, etc.) and blended cements (trass or blast furnace slag cement) react chemically with calcium hydroxide. For this reason, there is less calcium hydroxide in concretes that contain blended cements or those that use mineral admixtures.
There should be no salt or foreign matter in the aggregate, concrete mixing water or the water used during curing. Aggregate should be washed.
In this article, we addressed the problem of efflorescence in concrete. To learn more about cement and the world of cement, feel free to glance through our other articles.