In general magazines or media, one often reads or hears that concrete consists of sand, gravel, cement and water. If sand and gravel are regarded as one unit, we speak of a 3-component system. In the early days of industrial concrete production, the majority of concretes were classified in such a 3-component system. This also worked very well for years up to certain limits. It is only since researchers and concrete technologists have been pushing the limits of what is possible that it has been found that, with the help of concrete chemistry (admixtures) and finely dispersed powdery substances (additives), the performance of concrete could still be significantly increased. The 5-component system was born.
In the relevant technical literature and technical regulations, the differences between concrete admixtures and additives are essentially described as follows. First of all, it should be said that both are already added to the concrete during the mixing process:
Admixtures are liquid or powdery substances (also granulates) that are added to the fresh concrete in small quantities (usually < 5 % by mass of the cement content). Through chemical and/or physical action, they influence the properties of the fresh or hardened concrete, such as workability, setting and hardening behaviour or frost resistance.
Typical admixtures are concrete plasticisers, superplasticisers, air entraining agents, retarders, setting accelerators, stabilisers and shrinkage reducers.
The effect of admixtures such as high-performance superplasticisers sometimes lasts only a few minutes, but can also last up to several hours depending on the product used (use of retarders). Other admixtures, such as air entraining agents, change the concrete in such a way that it remains permanently under the influence of the admixture even after hardening.
Concrete additives are finely dispersed, mineral or organic powdery substances that influence certain properties of the fresh and hardened concrete. The amount of additions added is significantly higher than that of admixtures and can be well over 100 kg/m3. The amount of concrete additives added must be taken into account in the calculation of the absolute volume.
Additives are primarily used as a substitute for cement in order to save costs on the one hand and to improve the ecological footprint of concrete on the other. Some additions are by-products that are generated as waste in industrial processes. Additions should ideally improve the properties of the fresh and or hardened concrete. There are two different types of additives. Virtually inactive (inert) substances such as limestone or quartz flour, which serve as fillers, and puzzolanic or latent hydraulic substances such as fly ash, ground granulated blast furnace slag or silica fume, which actively participate in strength development and which usually also positively influence the durability and long-term strength of the concrete.
Many of today's buildings and constructions would not be feasible without admixtures and additives. No concrete consisting of a 3-component system could be pumped to a height of several hundred metres and still arrive at the site as a homogeneous mass. Self-compacting concrete would be inconceivable without these admixtures and additives. Without adding cement substitutes, massive concrete foundations, some several metres thick, could not be built without cracking under the effects of the heat of hydration. Extremely densely reinforced components require highly flowable concrete compositions to ensure complete covering of the steel with concrete. Furthermore, concretes with special durability requirements usually contain a high proportion of additives that can be used to prevent, for example, reinforcement corrosion or an alkali-silica reaction, as required.
The basic prerequisite for the successful application of concrete admixtures and additions is the consideration of the recognised concrete technology rules. Thus, an improvement in one concrete property can result in a deterioration of another. If one knows the advantages, but also the possible disadvantages of these substances, then one can produce extremely efficient concretes and thus structures and push the limits of what is feasible in terms of construction technology to a certain extent.