Effect of Elevated Curing Temperature on Early Hydration and Microstructure of Composite Cements
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II10.3.1
Effect of Elevated Curing Temperature on Early Hydration and Microstructure of Composite Cements J Hill1, B R Whittle1, J H Sharp1, M Hayes2 1
Immobilisation Science Laboratory, University of Sheffield, Department of Engineering Materials, Mappin Street, Sheffield, S1 3JD, UK 2 BNFL plc, Sellafield, Seascale, Cumbria, CA20 1PG, UK
Abstract The heat of hydration of a number of composite cement systems has been studied using isothermal conduction calorimetry (ICC) from ambient temperature to 90°C. The resulting hardened cement paste, from the high temperature regime, was then examined by scanning electron microscopy. Results showed that increasing the hydration temperature increased the rate of heat output for all systems and, at early ages, decreased the porosity of the sample. Introduction The use of composite cements based on the partial replacement of Portland cement by waste materials has become commonplace because they offer cost reduction, energy saving and, arguably, superior longer term products. The replacement materials fall into two principal groups, pozzolanic materials, such as pulverised fuel ash (PFA), rice husk ash, volcanic ash and silica fume, and latent hydraulic materials, notably ground, granulated blastfurnace slag (BFS), which was the cement used in this study. In both groups, the replacement materials participate in the hydraulic reactions, contributing significantly to the composition and microstructure of the hydrated product. Latent hydraulic materials have a chemical composition intermediate between that of a pozzolanic material and Portland cement. They act as hydraulic cements when mixed with water in the presence of a suitable activator. BFS typically has a CaO content of about 40 weight % and can be activated by alkalis, such as portlandite formed from the hydration of alite. Under optimum conditions, which include effective blending of the components, these composite cements can have excellent properties, including high ultimate strength, low heat of hydration, low permeability and good durability in a wide range of media. As such, cements containing up to about 70% by weight of BFS are now well established in the construction industry and fully characterised in the scientific literature [1-2]. Composite cements with higher levels of replacement are less well documented [3], but may have advantages in specific applications, for example when very low heat evolution is required as in the encapsulation of radioactive waste. It is in this latter capacity that the cement systems presented here have been investigated. Isothermal conduction calorimetry (ICC) provides valuable information on the heat evolution and early hydration of cements. The rate of heat evolution plots obtained are not only affected by the composition of the cement but also by the presence and composition of replacement materials and are strongly dependent on the temperature of hydration. Previous studies have only examined the effect of temperature up to 60°C [4-9]. However, in this study, curing temperature
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