- Author
-
Clifton, J. R.
|
Brown, P. W.
|
Frohnsdorff, G. J. C.
- Title
- Reactivity of Fly Ashes With Cement.
- Coporate
- National Bureau of Standards, Washington, DC
- Book or Conf
- Cement Research Progress. Chapter 15. Proceedings. 1977,
321-341 p.,
1977
- Keywords
-
cements
|
fly ash
- Abstract
- The reactivity of fly ashes incorporated in blended cements or added to portland cement concrete, and methods used to estimate the magnitude of this reactivity, are covered in this chapter. For the purpose of this review, the reactivity of a fly ash is defined loosely as the rate at which strength-giving phases are produced by the reactions between the fly ash and a portland cement in the presence of water. However, it will be evident from the text that the term reactivity has been given several different meanings by different investigators. It should be recognized that, because of the compositional heterogeneity of fly ashes and cements, and the heterogeneous nature of their pozzolanic reactions, reactivity is not a unique property of a fly ash. Even with a single cement, standard curing conditions and mix proportions, reactivity will probably vary with the progress of the reaction. Further, as discussed in this review, reactivity does depend on the curing temperature, the cement and the mix proportions. Because fly ash reactivity is difficult to measure, few direct measurements of it have been made. Therefore, this review includes references to the rates of chemical changes involving the hydration of fly ash-cement mixtures and to the rate of strength development, which is often used as an indirect method of estimating reactivity. It should be noted that relationships between reactivity and strength development are still not well-defined, as will be apparent from this review. Fly ash is composed of small particles produced by the combustion of coal, as in steam generation, which are separated from the combustion gasses by dust collectors. The mineralogical composition of fly ash varies, depending on the source of the coal. Reactive fly ashes can be classified as artificial pozzolans; i.e., they contain constituents which at normal curing temperatures will react with lime in the presence of water to form insoluble cementitious materials. Certain fly ashes from lignite and subbituminous coals have sufficiently high free lime contents to have hydraulic properties. The reactivity of fly ash is of particular impor tance because of the increasing use of fly ash in cement and concrete systems.