Carbonization of concrete, or neutralisation, is a chemical reaction between carbon dioxide in the air and calcium hydroxide and hydrated calcium silicate in the concrete.
The global environment has been deteriorating in recent years, and the carbon dioxide content in the air has been increasing. Concrete exposed to air for a long time reacts with carbon dioxide in the air to produce carbonates and other substances, which reduces the alkalinity of concrete and leads to accelerated rusting of reinforcement in concrete and cracking and spalling of concrete. It seriously affects the durability of concrete and reduces the service life of the concrete.
Therefore, it is important to study the causes of the carbonization of concrete, find out the essential influencing factors, and propose measures to control the influencing factors of the carbonization of concrete.
The Mechanism for the Carbonization of Concrete
Concrete is not dense. There are large and small pores, capillaries, bubbles, and other defects inside it. These pores, capillaries, and bubbles are interconnected and constitute tiny channels. When the carbon dioxide in the air penetrates the pores, capillaries, and bubbles inside the concrete, it reacts physicochemically with the calcium hydroxide, tricalcium silicate, and dicalcium silicate in the cement hydration products to generate carbonates and other substances, which reduces the alkali content in the concrete. The decrease of alkali content in concrete and the increase of carbonate content generated cause the internal compactness of concrete to deteriorate, which produces the generation of reinforcement corrosion, concrete cracks increase, and durability deterioration.
Factors Affecting Carbonization of Concrete
A large amount of concrete is exposed to the outdoor environment and long-term exposure to airborne moisture, carbon dioxide, and other substances that are detrimental to the durability of concrete. The mechanism for the carbonization of concrete shows that the main reasons affecting the carbonization of concrete are as follows.
The water-cement ratio determines the compactness of concrete to a certain extent. The larger the water-cement ratio is, the more pores the concrete has after hardening, and the larger the contact surface with carbon dioxide in the air. As the water-cement ratio increases, the greater the ability of carbon dioxide diffusion in the concrete. It has been shown that there is a mathematical function between the water-cement ratio and the depth of carbonization.
w/c – water-cement ratio
Amount of cement
The amount of cement affects the compactness of concrete. The more the amount of cement, the better the compactness of the concrete, the fewer the pores inside the concrete, the smaller the area in contact with carbon dioxide, and the less the degree of carbonization of the concrete. 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, so that the concentration of calcium carbonate precipitation generated after partial carbonization in the concrete will be reduced, improving the compactness of the concrete.
Fly ash, mineral powder, silica fume, and other admixtures are added to the concrete. The admixtures have a large number of active substances to replace part of the cement in the hydration process and react with the carbon dioxide in the hydration products of cement. This process not only reduces the amount of cement but also greatly reduces the alkalinity of the concrete. The added admixture is also mixed with the secondary hydration reaction of cement, and the generated hydration products fill the internal pores of concrete again. The integrity of the concrete is improved.
There are many different kinds of additives. The active substances in each additive are also different. Admixtures mixed into different types of cement produce different anti-carbonization effects. High-quality additives used in combination with compatible cement can reduce the water consumption of concrete to a certain extent, increase the overall compactness of concrete so that the concrete can play the most superior properties, and reduce the carbonization reaction.
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, there is more water in the concrete. The water in the pores of the concrete hinders the diffusion of carbon dioxide from the air in the pores of the concrete. It makes the contact area of carbon dioxide with concrete smaller and slows down the rate of carbonization of concrete.
The temperature level also affects the carbonization of concrete. Analysis from physical knowledge shows that the higher the ambient temperature, the faster the movement of ions and the movement of carbon dioxide in concrete. Generally speaking, a 10°C increase in temperature doubles the rate of chemical reactions. It is also theorized that the increase in temperature accelerates the carbonization of concrete because of the decrease in the dissolution rate of carbon dioxide. The mechanism of the effect of carbon dioxide on concrete carbonization varies widely, and there is no generally accepted conclusion.
Construction factors such as mixing, vibrating, and curing conditions determine the degree of compactness of concrete and also affect the carbonization properties of concrete. Measures such as adequate and uniform mixing, dense vibrating and pounding, heat preservation and moisturizing, and constant temperature curing can reduce the occurrence of cracks, holes, and looseness in concrete, which reduces the possibility of carbonization of concrete. Curing conditions also have an important influence on the generation of concrete hydration products and the size of the internal pores of the concrete. In general, steam curing of normal concrete has a carbonization rate of 1.5 times higher than that of normal natural curing.
As the most used construction material in the world, the durability performance of concrete has received attention and research from experts and scholars in various countries. The currently recognized factors affecting concrete carbonization are raw materials, environmental, and construction factors. The development of carbonization of concrete can cause concrete disease, which in turn affects the safety of the concrete structure and brings hidden dangers to the project. Therefore, the in-depth study of concrete carbonization and the development of specific measures to prevent carbonization are still the main direction of research.
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