Durability of Blended Cements in Contact with Sulphate-Bearing Ground Water
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DURABILITY OF BLENDED CEMENTS IN CONTACT WITH SULPHATE-BEARING GROUND WATER
S L Duerden, A J Majumdar and P L Walton Building Research Establishment, Department of the Environment, Watford, Herts, United Kingdom. VD2 7JR. ABSTRACT
In the concept of radioactive waste disposal developed in the UK, OPC blended with pulverised fuel ash or ground granulated blastfurnace slag is being considered for encapsulation of waste forms, as a material for backfilling and sealing a repository, and for concrete in repository construction. This paper describes a laboratory study of the long term durability of such cements in contact with sulphate-bearing ground water under accelerated exposure conditions. Mineralogical analysis of the cements over the exposure period, carried out with the aid of scanning electron microscope observations and x-ray diffraction studies, provides an indication of the stability of cementitious phases exposed to an aggressive environment. It is shown that for intact cement blocks there is minimal interaction between cement and sulphate-bearing ground water. Sulphate minerals produced by the reaction are accommodated in voids in the cement with no adverse effect on the cement structure. However, crystallisation of C-S-H and sulphate minerals along cracks in hardened cement specimens causes expansion of fracture surfaces resulting in a more accessible route for ground water intrusion and radionucleide migration. The reaction of cement with ground water is greatly accelerated by the use of powdered material. Ettringite formed in the reaction is found to be unstable under these conditions. The mineralogical assemblage after exposure for 1 year is calcite, hydrotalcite, C-S-H and quartz. INTRODUCTION
In the disposal of radioactive waste generated in the nuclear industry inorganic cementitious materials are considered to be eminently suitable for encapsulating some waste types, and for use as the backfilling and sealing materials in the repository. In addition, concrete made from these cements is the most likely material to be used in the construction of the waste repository. After much experimental work, the nuclear industry in the United Kingdom is considering the use of two types of cementitious mixes for the encapsulation of the intermediate level waste and the construction of plants designed for this purpose is well under way [1]. These two types of mixes are mixtures of ordinary Portland cement (OPC) and pulverised fuel ash (pfa) or ground granulated blastfurnace slag (ggbs). It is to be expected that sometime during the design life of the repository the cementitious material will come into contact with the local ground water. It is important, therefore, to study the interactions between the cementitious mixtures being considered and the ground water relevant to a particular repository. At present no specific site has been proposed for construction of a repository in the UK. As clay is likely to be present in significant amounts around a repository, and some clays are rich in sulphates, percolating gro
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