Hydration Reactions in Cement Pastes Incorporating Fly Ash and Other Pozzolanic Materials
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HYDRATION REACTIONS IN CEMENT PASTES INCORPORATING FLY ASH AND OTHER POZZOLANIC MATERIALS
F.P. GLASSER*, S. DIAMOND**, AND D.M. ROY*** *Department of Chemistry, University of Aberdeen, Old Aberdeen AB9 2UE, UK **School of Civil Engineering, Purdue University, Lafayette, IN 47907 ***Materials Research Laboratory and Department of Materials Science and Engineering, The Pennsylvania-State University, University Park, PA 16802
ABSTRACT A model for reactions that occur in hydrating portland cement is now generally well developed. Incorporation of various by-products to form blended cements modifies both the hydrat~ion reactions and the physical properties of the resulting pastes. A review of recent progress in understanding the effects of blending agents on these reactions is presented. The blending agents considered are low-calcium (Class F) fly ash, high calcium (Class C) fly ash, blast furnace slag, silica fume, biosilica and natural pozzolans. Effects of the blending agents on physical properties such as rheology are also considered. Particular attention is given to the essential role of alkalies in pore solutions and the beneficial reactions that occur with high silica content blending agents.
INTRODUCTION The use of coal combustion by-products presents not only economic opportunities, but also a number of technical challenges. A principal use of fly ash (FA) is in the formulation of blended cements, either "formal" blended cements produced and marketed by a cement manufacturer, or more commonly, "informal" blended cements batched as part of the concrete making operation by concrete producers. The technical challenges involved in optimizing such use may be considerable. The behavior of cements blended with FA is in many respects similar to the behavior of cements blended with other blending components, such as ground blast furnace slag (BFS), natural or artificial pozzolans, and silica fume (SF). In this paper we have attempted to consider relevant aspects of cement hydration reactions as influenced by some of these alternative components, notably BFS and SF, as well as by FA. These blending components are ordinarily available as a consequence of some other industrial process and their characteristics and compositions tend to be governed by constraints imposed by that process, rather than by potential use in concrete. For example, metallurgical requirements control the bulk composition of BFS, and power generating requirements control many of the characteristics of FA. When selecting materials for use as blending components in concrete, it is thus important to have adequate procedures in place to ensure their suitability. The incorporation of blending agents is sometimes regarded as a cheap way of extending cement. However, economic and performance considerations go together, because blending agents can also markedly improve the performance of cement-based systems. This potential is not, however, automatically obtained but arises from careful formulation and optimization of mix properties, adequate curing and go
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