Effect of Aggregate, Cement, and Mineral Admixtures on the Microstructure of the Transition Zone
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EFFECT OF AGGREGATE, CEMENT, AND MINERAL ADMIXTURES ON THE MICROSTRUCTURE OF THE TRANSITION ZONE P.K. MEHTA AND P.J.M. MONTEIRO
Department of Civil Engineering, University of California, Berkeley, CA 94720
ABSTRACT This paper contains a review of the results from the studies at the University of California at Berkeley on various factors influencing the microstructure of the transition zone in concrete. Two types of aggregate, two different cement, and three mineral admixtures were investigated. Using cement paste-polished aggregate composite specimens cured up to three years, X-ray diffraction, scanning electron microscopy, and microhardness testing techniques were used for characterization of the transition zone. Compared to the transition zone between a quartz aggregate and an ASTM Type I portland cement, transition zones with smaller and less preferentially oriented crystals of calcium hydroxide were obtained when using a Type K expansive cement, or limestone aggregate, or mineral additives, such as condensed silica fume, granulated blast-furnace slag, and fly ash.
INTRODUCTION Beginning with Farran's work [1], there has been a growing interest in the study of transition zone between coarse aggregate and cement paste [2-14]. This is because many important properties of concrete, such as permeability, durability, and strength are controlled by the the transition zone characteristics. Due to differences in materials and characterization techniques although contradictory results on the transition zone microstructure are sometimes reported in the published literature, the studies described in this paper seem to confirm the following: 1. In concrete, the microstructure of hardened cement paste in the transition zone is different from the microstructure of the cement paste in the bulk matrix. 2. Compared to the bulk cement paste, the cement paste in the transition zone is characterized by higher porosity, and larger size crystals of hydration products. 3. Due to the inherent weaknesses in the microstructure as described above (#2) when a concrete is subjected to any internal or external stresses the microcracks form first in the transition zone. 4. The improvements in the properties of concrete from the use of mineral admixtures, modified cements, and certain aggregate types can be explained from the resulting pore refinement and grain refinement processes, which occur in the microstructure of the transition zone. This paper contains a review of the results from the transition zone characteristics studies at the University of California at Berkeley. Two aggregate types (a quartz and a limestone), three mineral admixtures (a condensed silica fume, a fly ash and a granulated blast-furnace slag), and two cements (an ASTM Type portland cement and a Type K expansive cement) were used. Material characteristics, details of experimental procedure, and preliminary results were reported by the authors in earlier publications [14-18], and a summary was presented at the 8th International Congress on the Chemistry of Cements [1
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