The Use of Activated Slags as Immobilisation Matrices for ILW
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The Use of Activated Slags as Immobilisation Matrices for ILW
NB Milestone1 , Y Bai1 , C.H. Yang 2 , Y.J. Shi 2 , X.C. Li 3 1 . Immobilisation Science Laboratory, University of Sheffield, U.K. 2 . Department of Building Materials & Engineering, Chongqing University, P.R. China 3 . Yunnan Construction Concrete Co. Ltd, P.R. China ABSTRACT Composite cements where large amounts of blast furnace slag (BFS) replace Portland cement are currently used for immobilisation of ILW. Hydration of BFS is activated by the small amount of OPC present but the amount of reaction that occurs is limited at ambient temperatures. Increasing the temperature increases the hydration of the BFS but large amounts still remain unreacted, leaving a porous matrix where the capillary pores remain filled with a highly alkaline solution. This solution causes corrosion of reactive metals giving rise to expansive reactions and hydrogen release, and it can destroy the structure of zeolites releasing the adsorbed species. Apart from OPC, BFS hydration can be activated by other compounds such as hydroxides, sulphates, silicates, and calcium aluminate cements. The use of these alternatives gives rise to binders such as ettringite and strätlingite which have a different chemistry where the pore solution has a lower pH. Corrosion of metals does not readily occur in these binders. This may be due to the reduced pH but could also arise from the lack of pore water, as these binders bind more water in their structure so that it is not available for transport of ionic species. This extra water binding also has potential for immobilisation of sludges where high w/s ratios are necessitated by the need to transport the sludge. This paper will review some of the alternative activators for slag hydration and present experimental results on several systems where slag has been activated with compounds other than OPC. INTRODUCTION The conventional method for encapsulating low and intermediate level nuclear waste (LLW and ILW) in the UK is to use composite cement blends based on high levels of replacement of ordinary Portland cement (OPC) with supplementary cementing materials (SCM’s) such as pulverised fuel ash (PFA) and blast furnace slag (BFS). Typically levels up 9:1 BFS:OPC or 4:1 PFA:OPC are used. The use of cement for encapsulation/immobilisation of nuclear waste offers a number of advantages. These include low cost, ready availability and a known consistent performance, good workability and ease of remote working, high alkalinity which helps immobilise many radionucleides as hydroxides or carbonates, and formation of a strong, dense, low permeability matrix with high adsorption properties and known durability. The composite cement formulations have been chosen, not for their strength development, but rather for their ability to reduce the exothermic temperature rise associated with the large volumes of cement used in containers which can be as large as 3m3, although for ILW only 500L drums are routinely used. Despite the use of coarse ground cement and BFS, tem
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