Accelerated Weathering of Composite Cements Used for Immobilisation
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Accelerated Weathering of Composite Cements Used for Immobilisation Paulo H. R. Borges1, Neil B. Milestone1 and Roger E. Streatfield2 1 Immobilisation Science Laboratory, Engineering Materials, University of Sheffield, Mappin St, Sheffield S13JD, UK 2 Magnox Electric Ltd., BNG, Berkeley Centre, Berkeley, Gloucestershire GL13 9PB ABSTRACT Trying to estimate the long-term durability of cemented wasteforms is a difficult task as the cement matrix is a reactive medium and interactions can occur with the encapsulated waste as well as with the environment. There are few studies of samples that have been stored under controlled conditions for more than 10-15 years. Wasteforms are now being expected to last hundreds of years, much of that likely to be in some form of storage where sample integrity is important. There is also the concern that results from any long-term samples may only be indicative as both formulations and materials change with time. This paper discusses changes in physical properties that occur in composite cements when some of the short-term accelerated procedures employed in construction testing are applied to encapsulating matrices. Changes after increased temperature of curing, wetting/drying and accelerated carbonation are discussed. Many of the encapsulating formulations currently used are composite cements where large replacement levels of OPC with supplementary cementing materials (SCMs) such as PFA or BFS are made, primarily to reduce heat output. Accelerating the exposure conditions, either by increasing temperature or through wetting/drying has the effect of changing the hydration pattern of the composite cement by generating more hydration in the SCMs than would normally occur. The large amount of porosity that occurs because of limited hydration allows intrusion of gases and ready movement of water, so the samples subjected to accelerated testing do not appear as durable as expected if stored at ambient. INTRODUCTION Blended cements containing blastfurnace slag (BFS) and pulverized fly-ash (PFA) are specified for encapsulation of low level waste (LLW) and intermediate level radioactive waste (ILW) in the UK for technical and economic advantages. High levels of replacements of up to 90 wt% of BFS and 75 wt % of PFA are specified in the formulation of grouts used for waste immobilisation. This helps preventing thermal gradients and cracking, which could lead to loss of durability inside steel drums used to confine the ILW and LLW. Previous research [1-3] has shown that the early hydration in these systems containing high levels of OPC replacement is limited. The nuclear industry has studied samples that have been stored under controlled conditions for more than 10-15 years. However, the behaviour of such grouts after longer term is still unknown and the changes in the microstructure can be only assessed if some form of accelerated tests or modelling were carried out. Artificially ageing these materials by submitting them to accelerated tests could help investigations of the changes in microstruc
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