Reactivity of Fly Ash At High pH

Abstract

AbstractThe reactivity of fly ash in cementituous systems has been an object of many studies, and is only understood in broad terms. It is generally agreed that the particle size distribution of fly ash correlates with strength development. It has also been demonstrated that temperature and pH development of the blended cement paste have an effect on fly ash reactivity. It is however not clear whether ash chemistry and/or ash glass structure have an influence on its reactivity, as is the case with some blast furnace slag cements. In order to discriminate between the many variables controlling fly ash reactivity, the dissolution kinetics of class-F fly ash have been studied. The fly ash was first separated into equal size and density fractions. These were reacted with NaOH, with a pH ranging from 13.0 to 13.7 and temperatures between 20 and 40°C. If the results of the dissolution experiments are corrected for differences in glass content and particle size, the effects of temperature and pH seem to be of more importance than differences in ash chemistry. The dissolution proceeded virtually congruent. TEM micrographs of some of the ash fractions indicate that a wide variety of glasses may be encountered in fly ashes; most glasses show evidence of phase separation. The results are discussed with respect to glass structure and glass dissolution-theory.