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.

Fly ash in China contains low CaO (5%) and is obtained by burning bituminous coal. Thus such a fly ash concrete exhibits low early strength at an optimal dosage of 10-20%. Two methods were adopted to increase the dosage of fly ash (1) introducing calcium directly during burning or (2) as an additive. The effect of dosage of such fly ash on strength, shrinkage, frost resistance, carbonation and steel corrosion of concrete was investigated. Method 1 is effective in improving the activity of fly ash and permits the optimal dosage of fly ash to increase by 30-4O%. Early strength of concrete is also increased. Method 2 is applicable to steam-cured fly ash concrete at a high dosage (5O-60%). X-ray diffraction analysis was used to study the process of reaction between fly ash and cement.

SP91 This contains 78 symposium papers, bringing together the expertise of representatives from industry, government and universities. These volumes present the latest advances in the use of fly ash, silica fume, slag and natural pozzolans in concrete. New technologies are explored to provide ways in which these valuable mineral by-products can best be used to conserve both resources and energy. Case studies include: the effect of fly ash on physical properties of concrete; evaluation of kiln dust in concrete; effect of condensed silica fume on the strength development of concrete, and the influence of slag cement on the water sorptivity of concrete.

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.

The adsorption of a naphthalene sulfonate formaldehyde (NSF) superplasticizer on the surface of a silica fume (SF) has been studied by adsorption isotherms and zeta potential measure-ments. A model is proposed for the fixation of NSF on a SF surface in the presence of cement, which can explain the high efficiency of this type of superplasticizer on the Portland cement - SF - Water system. The rheological efficiencies of several SF have been eva-luated by the determination of the water content of one fresh SF mortar composition at a constant workability. An hypothesis is suggested for explaining the higher water demands by the five silicon or ferro-silicon SF tested in this study, compared to a zirconia SF.