Optimization of Particle Sizes in High Volume Fly Ash Blended Cements

摘要

One major objective of the sustainable construction movement is to decrease the energy and carbon dioxide footprints of concrete. Since the production of portland cement is a major contributor in both of these areas, concretes with increasing quantities of supplementary cementitious materials replacing cement are drawing renewed interest. While cement is typically the majority component of the binder powder in a blended cement concrete, the physical and chemical properties of all of the powder components will strongly impact concrete performance. Following on a recent patent application filing by Roman Cement LLC on 'High Early Strength Pozzolan Cement Blends' (patent granted in September 2010), the current study focuses on evaluating the properties of cement-fly ash blended cements, examining the influence of volumetric proportions of fly ash/cement and the individual particle size distributions (PSDs) of the cement and fly ash on a variety of performance properties including admixture requirements, setting times, heat release, strength, and autogenous deformation. Each of the three independent variables is examined at four different levels; a fractional factorial experimental design is employed to reduce the requisite number of mixtures from 64 to 16. To provide for the fairest comparison, all mortars are prepared with constant volume fractions of components: water, sand, and binder powders (cement + fly ash). The basic objective of the study is to evaluate if these blended cements can match the performance of a 100 % portland cement system, as exemplified by providing equivalent or superior strengths at both 1 d and 28 d. The results indicate that by controlling the PSDs of the cement and fly ash, 20 % fly ash replacements for cement on a volume basis can readily achieve this performance goal, while 35 % replacements can approach this goal and even achieve it when the fly ash source (class) is carefully selected.