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According to ASTM Standard C 618-84, fly ashes can be classified into two broad categories depending on their chemical composition. If Si02 + A1203 + Fe203 > 70%, the fly ash is said to be Class F; if 50% < Si02 + A1203 + Fe203 < 70%, it is said to be Class C. The physico-chemical properties of three Class F fly ashes - one French, one Canadian and one American - and of four Class C fly ashes - two American and two French - have been investigated. It has been found that fly ashes from one particular class can behave very differently. Two Class F fly ashes have been found to be pure1 Y Paz pozzolanic, whereas three others, one F and two C, were more or less hydrauli c at an early stage of hydration before behaving like a more or less pozzolanic material. One Class C French fly ash has been found to be hydraulic, then "auto-pozzolanic"; that is, in the presence of water, tis dissolution liberates enough lime to react with its own silica and alumina. Another Class C French fly ash was found to be hydraulic but non pozzolanic, its reactivity with the lime being directly associated to the formation of ettringite.these fly ashes has been explained In each case, the reactivity of by analyzing formation mechanisms of the different hydrates.

This paper reports the findings of an on-going study dealing with the properties of 18 pulverised fuel ashes (pfa) produced in British power stations from bituminous coals. The results reported here deal specifically with the variability of chemical and mineralogical compositions of ashes, both within and between sources (power stations). Physical properties such as particle size distribution, specific surface area and particle shape are also analysed. The importance of the variability of these chemical, mineralogical and physical parameters are discussed in relation to the properties of concrete where pulverised fuel ash is used toreplace 30 percent of ordinary Portland cement. A new method for the measurement of the alkali-soluble glass phase of pulverised fuel ashes is presented and evaluated in terms of the long-term strength properties of the pulverised fuel ash concretes studied. Doubts arise about the current specifications for selecting pulverised fuel ash for use in concrete, since the data accumulated during this study show that a much wider range of pulverised fuel ashes can be successfully used as a cement replacement material for the manufacture of concrete.

This paper presents comprehensive data on the shrinkage and creep behavior of concrete made with PFA coarse aggregates and sand and having 28 day strengths of 30-60 N/mm'. Continuously moist cured concrete showed expansion of about 16-21% of the 500 day shrinkage. At one month some 33% of the 500 day shrinkage and 50% of the one year creep occurred regardless of the concrete strength and exposure condition. The shrinkage took about one year to stabilise, whereas creep mostly stabilised in about six months. When unloaded, the fly ash aggregate concrete was able to recover all of its initial elastic strain on loading, but creep recovery was limited to about 10%. The hyperbolic relation can be used to predict satisfactorily both shrinkage and creep, and these values can be used to evaluate the shrinkage and creep effects on reinforced and prestressed members. The paper points out that the shrinkage and creep of fly ash aggregate concrete compare favourably with those of dense concrete.

FlY ash is usually considered to be a partial replacement of the portland cement in concrete mixtures. This paper presents a new approach to the selection of the mixture proportions of concrete in which fly ash is considered to be the "fourth ingredient*, that is in addition to the portland cement, the aggregate and water. This method enables fly ash to be used more efficiently and generally in greater quantities. High fly ash content concrete (HFCC), as the material has become known, was originally developed as a roller-compacted concrete for dam construction. The uses for the material have now been extended into road construction, both as paver-laid bases and also as pavement-quality concrete in the surface. Pumped (and skipped) structural placements have also been completed including a post-tensioned glued segmental viaduct. In all cases the concrete has performed well both in the fresh and hardened states. The in-situ strength of high fly ash content concrete has been found to be higher than the equivalent strength of conventional portland-cement concretes of equal workability and 28-day compressive strength. The paper traces the development of high flyash content concrete and shows how flyash can contribute as much, if not more, to the strength of a concrete as the same volume of portland cement.

Concrete subjected to elevated temperatures may suffer considerable loss in strength due to the development of microcracks or phase transformations in the matrix. The prevailing mechanism depends on the type of aggregate as well as on the moisture content of concrete. Experiments on different hydrated cement systems showed that under hydrothermal conditions phase transformations in neat cement paste lead to an increase in porosity and reduction in strength. In cement pastes containing fly ash or ground quartz in sufficient amounts, gel-like compounds are formed in pozzolanic reactions during hydrothermal exposure. An increased specific surface area as well as an increase in strength is observed. Concrete exposed to hydrothermal conditions is affected by these phase transformations of the matrix; a loss in strength can be prevented by addition of fly ash or ground quartz. Due to a higher shrinkage of the modified matrix which causes increased microcracking these concretes, however, show a loss in strength when drying during temperature exposure.