ABSTRACT
Concrete is a composite material consisting of aggregates (stones and sand) bound together by hardened cement paste. Although the aggregates can comprise more than 80% of the concrete, they are natural materials and not generally conducive to engineering. us, they must simply be selected from available sources as the most appropriate for the application. For example, certain silica-or carbonate-containing aggregates can react adversely with alkalies from the cement or from the environment and, therefore, should be avoided if there is likely to be exposure to alkalies. Moreover, natural aggregates can be modi-ed for certain purposes, for example, for low-density concrete, expanded shales, clays, or slate are produced as lightweight aggregate by ring the natural material. However, this topic is beyond the scope of this chapter.
