Fly ash for use in concrete, part II - a critical review of the effects of fly ash on the properties of concrete

Abstract

Fly ash may be employed as a pozzolan to replace a portion of the portland cement used for the manufacture of concrete. Such replacement is often a means to achieve energy and cost savings and to impart specific engineering properties to the finished product. To use fly ash effectively and economically it is important to develop a fundamental understanding of the differences between fly ash concrete and portland cement concrete. The differences in the rate of strength development between the two types of concrete and the ways in which this may be influenced by methods of mix proportioning are of particular importance. Within limits, the application of appropriate proportioning methods and the use of a high quality fly ash permit the economic manufacture of concrete of adequate early strength. When properly proportioned and placed, fly ash concrete generally shows improved workability, pumpability, cohesiveness, finish, ultimate strength and durability. It has been found that fly ash is of particular value in high-strength concrete. Its use has often been shown to improve the performance of concretes exposed to sulphate attack or to deterioration caused by alkali-aggregate interactions. In common with many areas of technology, the use of fly ash in concrete has largely preceeded a fundamental understanding of its properties. As a result, much of the reported research has been concerned with studies wherein fly ash concretes have been compared directly with concretes made with portland cement. Most of these studies have ignored rational proportioning methods and, as a result, the unique properties of fly ash concrete have not been adequately examined. It is concluded that research should be carried out to aid in improving the knowledge of fly ash as a concrete material and to assist in developing specifications and test methods that recognize its usefulness. In particular, research is required to elucidate the physical and chemical mechanisms whereby fly ash interacts with portland cement and water. Such factors as degree of hydration, gel-space ratio, strength, porosity, permeability and their inter-relationships should be examined. More extensive studies on the durability of rationally proportioned fly ash concretes, are also required.