The Pozzolanic Reaction of Silica Fume
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The Pozzolanic Reaction of Silica Fume Ole M. Jensen1 1 Technical University of Denmark, Brovej, Bygning 118, 2800 Kgs. Lyngby Denmark. ABSTRACT Silica fume is a very important supplementary cementitious binder in High-Performance and Ultra High-Performance Concretes. Through its pozzolanic reaction the silica fume densifies the concrete micro-structure, in particular it strengthens the paste-aggregate interfacial transition zone. In the present paper different aspects of the pozzolanic reaction of silica fume are investigated. These include chemical shrinkage, isothermal heat development and strength development. Key data for these are given and compared with theoretical calculations, and based on presented measurements the energy of activation of the pozzolanic reaction of silica fume is estimated. The results show that the pozzolanic reaction of silica fume has notable differences from Portland cement hydration. INTRODUCTION Silica fume takes a special position among the concrete mineral admixtures. Compared with other materials it is a physically well-defined and chemically pure and simple component. In its basic form it consists of relatively uniformly sized, sub-micron, spherical particles of almost pure SiO2. Due to its “pure” properties silica fume is a useful model material for examining the actions of pozzolans in concrete. Silica fume has a marked effect on a number of concrete properties. During the early 1980s the introduction of silica fume enabled the development of a new class of so-called high-performance and ultra high-performance concretes. These concretes may for example exhibit strengths up to 10 times that of usual concretes. On the other hand silica fume addition also seems to be closely linked with specific adverse effects of high-performance concrete including autogenous shrinkage and autogenous relative humidity change[1,2]. To understand and to potentially modify the action of silica fume in concrete it is important to have knowledge of its basic properties. Whereas the physical and chemical material properties of silica fume are simple and well-known, basic elements of its pozzolanic action in concrete are less well accounted for. This is the focus of the experimental and theoretical investigation presented in this paper. The contents of the paper origin from a previous project[3]. EXPERIMENTAL DETAILS Materials The materials used consisted of: White Portland cement with a Blaine fineness of 430 m2/kg and the following Bogue calculated phase composition (wt%): C3S: 66.9, C2S: 19.6, C3A: 4.3, C4AF: 1.0, Na2O-eq: 0.15, CaSO4: 3.3, free CaO: 2.4, LOI: 0.8.
Silica fume, a non-compacted, dry powder with a BET-surface of approximately 20 m2/g and the following composition (wt%): SiO2: 92.0, MgO: 1.6, Fe2O3: 1.2, Al2O3: 0.7, Na2O-eq: 1.1, LOI: 2.2. Calcium hydroxide in technically pure quality, 95%. Demineralized water which has been de-aired by boiling and subsequently cooled to room temperature in a closed, collapsible container. Mixtures From these materials three different binders were pro
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