Concrete additives are fine materials which are generally added to concrete in significant proportions (around 5–20 %). They are used to improve or obtain specific fresh and/or hardened concrete properties. European Standards lists 2 types of inorganic concrete additive:
Virtually inactive materials such as lime fillers, quartz dust and colour pigments.
Pigmented metal oxides (mainly iron oxides) are used to colour concrete. They are added at levels of 0.5–5 % of the cement weight; they must remain colour-fast and stable in the alkaline cement environment. With some types of pigment the water requirement of the mix can increase.
2- Rock flours (quartz dust, powdered limestone)
Low fines mixes can be improved by adding rock flours. These inert materials are used to improve the grading curve. The water requirement is higher, particularly with powdered limestone
Pozzolanic or latent hydraulic materials such as natural pozzolans (trass), fly ash and silica dust. Fly ash is a fine ash from coal-fired power stations which is used as an additive for both cement and concrete. Its composition depends mainly on the type of coal and its origin and the burning conditions. Silica dust (Silicafume) consists of mainly spherical particles of amorphous silicon dioxide from the production of silicon and silicon alloys. It has a specific surface of 18–25 m² per gram and is a highly reactive pozzolan. Standard dosages of silica dust are 5 % to 10 % max. of the cement weight.