How can we improve the workability of concrete
Overview: The most important concrete admixtures and substances
At this point we cannot introduce all the concrete admixtures that are available. We would like to mention the air-entraining agents, if only because their effect, compared to the additives mentioned so far, is a little “out of line”. Normally you want to minimize air pores in concrete as much as possible. However, air-entraining agents have exactly the opposite effect. But that is also required from time to time. The substances ensure that many small pores with a diameter of less than 0.3 mm are created in the concrete. Although this reduces the strength of the material, it increases its resistance to frost. The reason: When the water in the larger capillary pores of the concrete freezes during frost, the small mini-pores absorb some of the pressure. This reduces the likelihood that the concrete will crack.
So-called additives can also influence the properties of the concrete. We have already encountered some of these substances in the other specialist knowledge articles on the subject of concrete. Steel or glass fibers, for example, are used to increase the tensile strength of fiber-reinforced concrete. And silica fume plays a crucial role in the production of high-strength concretes. Compared to the additives, the additives are added in relatively large quantities, so they influence the volume of the concrete. Therefore, they must also be taken into account in the "material space calculation", which is used to calculate how large the concrete volume will be if the raw materials (cement, water, aggregates, additives) are mixed in the previously specified quantities.
Additives often act as pure “fillers” in the concrete matrix, which do not enter into any chemical reaction with cement and water. This applies, for example, to the fibrous materials mentioned above, to pigments for coloring the concrete, but also to powdered rock, which only compress the concrete structure in a mechanical way. There are also additives that react with the cement-water mixture (cement paste) - that is, they contribute to the setting of the concrete. Examples are the silica dust mentioned above, as well as trass and coal fly ash.
The example of fly ash in particular shows how various additives can influence the properties of concrete. Hard coal fly ash is actually an industrial waste product. They are obtained by filtering the flue gases that arise when coal is burned. If part of the cement in the concrete mix is replaced by fly ash, it acts like a liquefier on the fresh concrete. On the other hand, the compressive strength can also be increased because the fine fly ash particles increase the density of the material structure and at the same time react hydraulically with the cement paste. Incidentally, this is no wonder: Because coal fly ash has a very similar chemical composition to natural pozzolans (volcanic ash), which the ancient Romans discovered as a binding agent for their “opus caementitium”.
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