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What are the Reasons for Concrete Carbonization?

Thomas Huang posted this article in June 30, 2020

In recent years, the environment has been deteriorating, and the carbon dioxide content in the air has been increasing. The concrete exposed to the air for a long time reacts with the carbon dioxide in the air to form carbonates and other substances, which reduces the alkalinity of the concrete, resulting in accelerated corrosion of the steel bars in the concrete. The cracking and spalling of concrete have seriously affected the durability of concrete and reduced the service life of concrete. Therefore, it is an important subject to study the reasons that affect concrete carbonation, find out the important influencing factors, and propose measures to control the influencing factors of concrete carbonation.
1. Concrete carbonation mechanism
The concrete is not dense, and there are defects such as large and small pores, capillaries, and bubbles inside. These pores, capillaries, and bubbles communicate with each other to form small channels. When the carbon dioxide in the air penetrates into the pores, capillaries and bubbles inside the concrete, it will react with calcium hydroxide, tricalcium silicate and dicalcium silicate in the cement hydration products to produce carbonates and other substances , Which in turn reduces the alkali content in concrete. The decrease of alkali content in concrete and the increase of carbonate content cause the internal compactness of concrete to deteriorate, which in turn leads to the phenomenon of steel corrosion, increased concrete cracks, and poor durability.
2. Influencing factors of concrete carbonation
Since a large amount of concrete is exposed to the outdoor environment, long-term exposure to moisture, carbon dioxide, and other substances in the air that are not conducive to the durability of concrete, the carbonation mechanism of concrete shows that the main causes of concrete carbonation are as follows:
2.1 Raw materials
2.1.1 Water-cement ratio

On the one hand, the size of the water-cement ratio determines the density of the concrete to a certain extent. The larger the water-cement ratio, the more pores the concrete hardens and the greater the contact surface with carbon dioxide in the air; on the other hand, as the water-cement ratio The greater the increase, the stronger the ability of carbon dioxide to diffuse in concrete. Some studies have shown that there is a mathematical function relationship between the water-cement ratio and the carbonation depth:
K=12.1w/c
w/c——water-cement ratio
2.1.2 Cement dosage
The amount of cement affects the compactness of concrete. The more cement, the better the compactness of concrete. The less pores inside the concrete, the smaller the area of contact with carbon dioxide, the less the degree of concrete carbonation. Importantly, the greater the amount of cement, the greater the alkali content in the cement, and the higher the PH value in the pore solution, which reduces the concentration of calcium carbonate precipitates formed after partial carbonization in the concrete, improving the compactness of the concrete.
2.1.3 Admixture
Fly ash, mineral powder, silica fume and other admixtures are added to the concrete. A large amount of active substances in the admixture replace part of the cement to participate in the hydration process and react with carbon dioxide in the hydration product of the cement. This process not only reduces The amount of cement has greatly reduced the alkalinity of the concrete. The added admixture is also mixed with the hydration product generated by the secondary hydration reaction of cement to fill the internal pores of the concrete again, and the integrity of the concrete is improved again.
2.1.4 Additives
There are many types of additives, and the active substances in each additive are also different. When mixed into different cements, the anti-carbonization effect is also different. The combination of high-quality admixtures and suitable cement can reduce the water consumption of concrete to a certain extent and increase the overall compactness of concrete. It is the most superior property of concrete and reduces the carbonization reaction.
2.2 Environmental factors
2.2.1 Humidity
The relative humidity of air has a decisive influence on the diffusion rate of carbon dioxide in concrete. If the humidity of carbon dioxide in concrete is high, and the moisture in concrete is large, the moisture in the pores of the concrete hinders the diffusion of carbon dioxide in the air in the pores of the concrete, making the contact area of carbon dioxide and concrete smaller, and slowing down the carbonization rate of concrete .
2.2.2 Temperature
The temperature also affects the carbonation of concrete. From the analysis of physical knowledge, the higher the ambient temperature, the faster the speed of ions movement, and the speed of carbon dioxide movement in concrete will also accelerate. Generally speaking, if the temperature rises by 10℃, the rate of chemical reaction will increase twice. There are also theories that the increase in temperature accelerates the carbonization of concrete because the dissolution rate of carbon dioxide decreases. There are various opinions on the mechanism of carbon dioxide's effect on concrete carbonation, and there is no generally accepted conclusion.
2.3 Construction factors
Construction factors such as mixing, vibration, and maintenance conditions determine the density of the concrete and also affect the carbonation performance of the concrete. Measures such as full and even mixing, vibration and compaction, heat preservation and moisturizing, and constant temperature curing can reduce the occurrence of cracks, holes, and looseness in the concrete, and reduce the possibility of concrete carbonization. The curing conditions also have an important effect on the formation of concrete hydration products and the size of the pores in the concrete. In general, the steam curing of ordinary concrete is 1.5 times higher than the carbonization rate of ordinary natural curing.
3. Conclusion
Concrete is currently the largest building material in the world, and its durability has received the attention and research of experts and scholars from various countries. The currently recognized influencing factors for concrete carbonation are: raw materials, environment and construction factors. The development of concrete carbonization will cause concrete diseases, which will affect the safety of concrete structure and bring hidden dangers to the project. Therefore, the in-depth study of concrete carbonation and the formulation of specific measures to prevent carbonization are still the main directions of research.

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