The Microstructure of ggbfs/OPC Habdened Cement Pastes and Some Effects of Leaching
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THE MICROSTRX
)RE OF ggbfs/OPC HABDENED CEMENT PASTES AND SOME EFFECTS OF LEACHING
I.G. RICHARDSON, SALLY A. R *ER" AND G.W. GRUVES Department of Metallurgy and Science of Materials, University of Oxford, Parks Road, Oxford OXI 3PH, U.K. ABSTRACT Ground granulated blast-furnace slag (ggbfs) /Ordinary Portland cement (OPC) blends are possible materials for use in intermediate and low-level radioactive waste repositories. The microstructural development in neat OPC is described. The effect of increasing the loading of ggbfs on the composition and microstructure of the hardened paste has been examined by a number of techniques, including transmission electron microscopy. The implications for performance are discussed. A ggbfs/OPC 9:1 blend which had been exposed, after normal hydration to aqueous leaching was also examined. Marked changes in the microstructure and composition were
observed.
INTRODUCTION Rapidly cooled blast-furnace slag is a glassy material known as 'granulated blast-furnace slag'. It is itself hydraulic, but the reaction with water is very slow at 20'C and to achieve hardening at a practically useful rate the reaction must be either thermally or chemically activated. The usual way of doing this is to combine the slag with Ordinary Portland cement. It is generally assumed that calcium hydroxide, produced by the hydration of the OPC, is the main activator for the hydration of the slag, with gypsum and alkali sulphates, present in the OPC, also having an effect. A wide range of compositions may be used for the ggbfs/OPC blends, with proportions as high as 9:1. Besides its economic advantages, the use of ggbfs with OPC may produce property benefits as compared to pinplended OPC, for example, greater mechanical strength at longer times Ili and possibly greater durability in terms of resistance to sulphate attack [2], resistance to alkali-aggregate reaction [3] and lower chloride diffusion rates [4]. A lower heat evolution upon hydration as compared to neat OPC may be of benefit in mass concrete applications. In addition to their use in civil engineering, ggbfs/OPC blends may be of value for encapsulating low and intermediate level radioactive waste. For these purposes a good understanding of the microstructure of these materials, and its development, is desirable. In this paper the principal features of the microstructural development in hydrating OPC and ggbfs/OPC blends of typical compositions is outlined. Over a long term the microstructure of a hardened paste may be subject to various changes depending on the environment. One which may be of concern is the effect of leaching. The final part of the paper describes some observations of a leached 9:1 ggbfs/OPC hardened paste. EXPERIMENTAL Thermal analysis was carried out on a Mettler DSC 20 differential scanning calorimeter at a heating rate of 100C min-i in an air atmosphere. A Wexham Developments Limited isothermal conduction calorimeter was used to obtain heat evolution profiles for hydrating pastes. Details of sample preparation techniques for el
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