ABSTRACT
Admixtures are chemicals that are added to the concrete during mixing and significantly change its fresh, early age or hardened state to economic or physical advantage. This chapter considers the five of the distinct types which together make up more than 80" of the total quantities used in concrete – plasticisers, superplasticisers, accelerators, retarders and air-entraining agents (AEAs) – and briefly mentions others. Plasticisers, also called workability aids, increase the fluidity or workability of a cement paste or concrete. As the name implies, superplasticisers are more powerful than plasticisers and are used to achieve increases in fluidity and workability of a much greater magnitude than those obtainable with plasticisers. Retarders delay the setting time of a mix. The mode of action of retarders involves modification of the formation of the early hydration products, including the calcite crystals. The major reason for entraining air is to provide freeze–thaw resistance to the concrete.
