Fly ash and slag: standards and specifications—help or hindrance?

Abstract

From technical, economic and ecological points of view, siliceous by-products are now recognized and accepted as desirable and vital constituents of concrete. Current specifications for such materials tend to be restrictive to some extent, however, and in many countries there are separate standards for each admixture. These in turn inhibit the wider use of these mineral additions to concrete. This paper presents a review of the more important aspects of current standards for siliceous admixtures in this context. It is shown that there is not only a lack of uniformity in the requirements of these standards, but also wide variations in the limits set for a given requirement. It is suggested that these variations reflect not only the industrial or agricultural background of the by-products but also the inevitable wide range of compositions arising from it. The paper discusses selected items of specifications such as alkali contents, the role of physical properties and mineralogical compositions in the development of strength, the relationship of composition to hydraulicity and the relevance of the pozzolanic and slag activity index tests. Guidance is given on the development of a reliable slag activity index test related to its performance in concrete. An accelerated test is also recommended as an acceptance criterion. The integrity and durability of concrete structures is closely linked with the characteristics of the materials used, but there is no clear understanding of this interrelationship with design and construction. Under these circumstances, material specifications are likely to be prescriptive and sometimes restrictive. The need, however, to correlate requirements for quality control and quality assurance with the real performance of the concrete in the structure cannot be overemphasized. The paper proposes a more positive and engineering-oriented approach to the use of these materials in concrete, so that a wider range of the applications of concrete in practice can incorporate these mineral admixtures as partial cement replacements.