Architecture：Introduction to Concrete Admixtures.pdfVIP

143 4 Introduction to Concrete Admixtures 1.0 INTRODUCTION 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. 143 144 Chapter 4 - Introduction to Concrete Admixtures 1. Chemical admixtures: They are generally water-soluble, added mainly to control setting and early hardening of fresh concrete, and to reduce water requirements. Chemi- cal admixtures include accelerators, normal water reduc- ers, 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 carboxylic acids, or derivatives of carbohydrates. Normal water reducers may contain salts of refined