Introduction to Concrete Admixtures

Concrete admixtures are defined as materials other than hydraulic cement, water, or aggregates that are added immediately before or during mixing. Additives or additions such as grinding aids are added to cement during manufacture. An addition is a material that is interground or blended during the manufacture of cement. Most concrete used in North America contains at least one admixture. The admixtures are added to improve the quality of concrete in the fresh and hardened state. Publications and patents on admixtures are voluminous. Interest in the development of admixtures is evident from the number of patents taken every year. The total number of patents reported in the years 1985, 1986, 1987,1988,1989,1990,1991, and 1992 were respectively 44, 64, 99, 133, 242, 57, 53, and 149. 

Categorization of admixtures into distinct groups is not easy as one material may belong to more than one category. Broadly, however, they could be divided into the following categories. 

Chemical admixtures They are generally water-soluble, added mainly to control setting and early hardening of fiesh concrete, and to reduce water requirements. Chemical admixtures include accelerators, normal water reducers, superplasticizers, and retarders. The accelerators may contain calcium chloride, alkali hydroxide, calcium formate, calcium nitrate, etc. Examples of retarders are Na, Ca, or NH4 salts of lignosulfonic acids, hydroxy carboxylicacids,orderivativesofcarbohydrates.Normal water reducers may contain salts of refined lignosulfonic acids, hydroxycarboxylic acids, hydroxylated polymers, etc. Superplasticizers may contain sulfonated melamine formaldehyde, sulfonated naphthalene formaldehyde, car-boxylated synthetic polymers, etc. 

Air-entraining agents These are used to improve the durability of concrete. They comprise resins such as neutralized vinsol resin, synthetic detergents, salts of petroleum acids, etc. 

Mineral admixtures These finely divided materials improve workability, durability and other properties of concrete examples are fly ash, slag, silica fixme, rice husk ash, etc. 

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In this chapter an introduction to the role of different t5q)es of admixtures on cement and concrete properties is given. This should serve as a useful guide for following the application of thermal techniques to the evaluation of the effect of admixtures on cement and concrete. 

Incorporation of chemical andmineral admixtures in concrete results in many beneficial effects such as enhanced physical and mechanical properties and durability. Many t)q)es of admixtures are currently in the market and their effect on concrete is determined by complex factors. Hence, Ch. 4 has been included to describe t)q)es of admixtures and their roles in concrete technology. This chapter should serve as an introduction to the subsequent chapters devoted to the application of dieimal analysis techniques for the investigation ofthe role of admixtures in concrete. 

This report is a fully revised version of TRCS 1, Admixtures for concrete, which was first published in 1968. It is intended as a non-technical introduction and guide to admixtures for site engineers, consulting engineers, architects and ready-mix and precast personnel who are not necessarily materials specialists. 

Starting from the OPC, several modifications may be obtained with different kinds of aggregate, by the introduction of special pore structure of appropriate parameters, by variation of quality and quantity of cement and also by application of a large variety of available admixtures and reinforcements. The application of various technologies also contributes to enlarging the field of materials based on ordinary concrete. 

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