How do air-entraining admixtures work? Surfactants in the production of concrete

Published: 30-05-2018

Concrete, as one of the most popular building materials, is constantly modified and improved through the use of modern admixtures and additives modifying its properties. Already in antiquity, various substances were used to give the concrete the desirable characteristics of use. For example, the introduction of powdered eggs or bovine blood into the concrete mix accelerated the cement binding and increased the strength of the finished concrete. In ancient Rome, volcanic ashes were also a popular additives to concrete.

Nowadays, cement concrete (called “artificial stone”) is created by mixing cement, suitable aggregate and water. Contact of cement with water causes hydration, as a result of which heat is released. This results in the binding of a concrete mix in which densely packed hydration products are present, such as hydrated calcium silicates and hydrated calcium aluminates.

What affects the strength of concrete?

The parameters of the finished concrete and elements made of concrete depend on many factors. As for its strength, the type of cement used, type and quality of the aggregate, as well as the W/C ratio (water / cement) are very important. It is also extremely important to choose the right additives and admixtures that affect the workability, water resistance and frost resistance of the finished concrete. The changes of temperatures during the autumn and winter season cause that the structure of elements made of concrete can be weakened as a result of repeated freezing and thawing of water present in its pores. The change in the volume of water accompanying these phenomena causes cracks in the concrete elements, which can lead to serious damage. The most effective ways to improve the resistance of concrete to repeated cycles of freezing and thawing is proper care, as well as ensuring proper air-entraining. Here the quantity, shape and size of pores in its structure are particularly important.

Air-entraining admixtures for concrete

The last of the mentioned methods – proper aeration of concrete – is based primarily on the use of modern chemical admixtures. These are products added in small amounts to concrete mixes. The main group of ingredients used in their production are surfactants. The introduction of suitable surfactants into the concrete mix results in the formation of fine and stable air bubbles. When the fresh mixture begins to cure, the bubbles within it become mineralized and become an integral part of it. In this way, an additional space is created inside the concrete structure inside which the freezing water expands. This prevents an increase in internal pressure, which is responsible for the formation of cracks during low temperatures. Even distribution of pores with air in the entire volume of concrete improves its frost resistance. The presence of air bubbles in the concrete mix also improves its workability. Bubbles act as bearings that increase plasticity without having to change the W/C ratio (water / cement). Air bubbles also reduce the friction that occurs during the concrete pumping stage.

In the production of air-entraining admixtures, anionic and amphoteric surfactants as well as non-ionic surfactants can be used. Among the anionic additives, the most important are lauryl sulfates and lauryl ether sulphate salts. The use of alkyl benzene sulphonate salts, e.g. sodium salt in the form of ABSNa, will also provide high performance in the formation of fine and stable air bubbles. In turn, the introduction of ethoxylated alcohols with additional wetting properties to the concrete mix will enable stabilization of the bubbles formed. Amphoteric additives, among them amidobetaines, are so-called co-surfactants that support the air-entraining of concrete and also stabilize the durability of air bubbles.

The invention of concrete in antiquity created many new possibilities for architecture, thanks to which buildings such as the Pantheon and the Coliseum were created and have survived to this day. Today, knowledge about this construction material is much greater, and the constant optimization of concrete production technology and the development of new types of admixtures make it possible to create, with its help, increasingly durable and more complicated forms and elements. What’s more, the development of the construction industry thanks to designing the fair-faced concrete allows not only the creation of durable structural elements, but also provides new buildings with an original appearance and interesting design.

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