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Silica fume has been used commercially for five years in Canada, particularly in the province of Quebec where most of the material is produced. Field problems such as plastic shrinkage that were experienced in the early stages and the distinct nature of this pozzolan caused some concern among prospective users who, while recognizing the unique properties and potential of silica fume, were hesitant to specify it. Added to their apprehension was the unavailability of a recognized standard. These considerations prompted the Concrete Materials and Construction Committee of the Canadian Standards Association (CSA) to develop a specification for silica fume. Findings from research by the University of Sherbrooke, Quebec, and the Canadian Center for Mineral and Energy Technology, Ottawa, Canada, provided input for the new specification which is now part of CSA Standard A23.5, Supplementary Cementing Materials and Guidelines for their Use in Concrete. During the preparation of the specification, close contact was maintained with Committee 226 and ASTM Committee C618 which are in the process of preparing a state-of-the-art report and a specification, respectively. The Canadian specification, believed to be the first of its kind in the world, will eventually be updated particularly as regards to the ASTM C 311 methods of tests which were originally developed for pozzolans that are not as finely divided as silica fume.

This paper examines recent trends in European and British cement and concrete standards, concerning the production and use of blended cements and concrete mixer combinations of cements and additions, such as slag and pulverized fuel ash. The implications of the new categorization of six types of cement, changes in the definition of "cement," and acceptance of equivalence of performance in terms of concrete strength grade, are discussed. The principles being adopted in the UK for the approval of the use of additions in concrete include the adoption of the standard for blended cement as the basis, recognition of the cementitious properties of standardized additions and certification procedures for confirming compliance of concrete mixer blends with the standards of blended cements. Provided these procedures are followed, the answer to the question--blended cements or concrete mixer blends?--lies in a clearer definition of responsibility for their fitness for purpose, the relative costs of using blended cements or concrete mixer blends, and the local market demands.

Concern is growing that prescriptive and separate standards are an obstacle to widely using such by-product mineral admixtures as fly ash, granulated slag, and condensed silica fume. Since natural as well as by-product pozzolanic and cementitious admixtures offer similar technical benefits when used in concrete, it is desirable to develop a single performance-oriented standard. With this objective, the principal chemical and physical requirements of a few selected standards are reviewed, with a focus on fly ash and its relevance to current materials. A rational approach is suggested and specific recommendations made toward developing a performance standard to cover all mineral admixtures.

Many of the current standards for fly ash to be used in portland cement suffer from prescriptive requirements not necessarily related to the material's performance in concrete. The development of performance specifications is being encouraged, and classifying fly ash by reference to type of coal is being questioned. To use fly ash as a mineral admixture, the potential strength contribution to a reference mortar seems to be the most important consideration. To overcome the inconsistency with cement in the ASTM C 311 pozzolanic activity index, many countries have revised the test by incorporating fixed fly ash/cement ratio by weight, fixed water/cement + ash ratio, and accelerated curing at various temperatures for up to 7, rather than 28, days. Various studies to improve pozzolanic test methods are summarized, and recommendations made to simplify present fly ash standards, and to change their orientation from prescriptive to performance.

In drafting the durability section of the Australian concrete code, the committee faced both an urgent problem and a lack of precise data. This paper outlines the concepts in the draft code with a particular emphasis on the protection of reinforcement and the influence of fly ash on durability. Based on a qualitative understanding of the fundamental parameters, some engineering decisions and classifications are made. Where ample curing or self-curing is available, fly ash is a desirable component of durable reinforced concrete. For building exteriors where minimal curing is provided, achieving quality concrete cover is difficult. In this respect, concrete containing fly ash may be more affected than plain concrete.