Frequency Characteristics of Acoustic Emission Signals from Cementitious Wasteforms with Encapsulated Al
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0985-NN10-03
Frequency Characteristics of Acoustic Emission Signals from Cementitious Wasteforms with Encapsulated Al Lyubka M. Spasova and Michael I. Ojovan Immobilisation Science Laboratory, Department of Engineering Materials, University of Sheffield, Mappin Street, Sheffield, S1 3JD, United Kingdom
ABSTRACT Acoustic emission (AE) signals were continuously recorded and their intrinsic frequency characteristics examined in order to evaluate the mechanical performance of cementitious wasteform samples with encapsulated Al waste. The primary frequency in the power spectrum and its range of intensity for the detected acoustic waves were potentially related with appearance of different micromechanical events caused by Al corrosion within the encapsulating cement system. In addition the process of cement matrix hardening has been shown as a source of AE signals characterized with essentially higher primary frequency (above 2 MHz) compared with those due to Al corrosion development (below 40 kHz) and cement cracking (above 100 kHz). INTRODUCTION Over many years in the UK cementation of low and intermediate level waste (LILW) has been established as a commercially available technology [1-3]. The LILWs arising from the operation and decommissioning of nuclear facilities and spent nuclear fuel reprocessing consist mainly of sludges, organic ion exchange resins and metallic debris [1]. The physical and chemical characterization of these radioactive wastes is required for development of effective disposal strategies. A particular issue concerned with the cementation of metallic wastes containing ∼980 tonnes of Al [2] is their enhanced corrosion rate in the high pH(>12) for ordinary Portland cement (OPC) environment resulting in internal stresses which may lead to cracking of the encapsulating matrix [4]. Although in composite cement systems, where the OPC is partially replaced by latent hydraulic materials such as blast furnace slag (BFS), Al corrosion rate was found to decrease still the amount of hydrogen gas generated and the accumulation of expansive corrosion products may cause formation of a fracture process zone within the immobilizing cement structure [4]. The degradation of the cementitious wasteforms is associated with generation and release of transient stress waves (AE) which can provide valuable data for assessment of the level of damage induced. Indeed AE technique has been proved to be sensitive to formation of large cracks within the cement-based structures as well as to microcrack nucleation and development [5]. Recently we demonstrated that the AE monitoring of OPC system with encapsulated Al provides information on different stages of Al corrosion and matrix degradation [6, 7]. AE hits rate analysis and parameter-based AE approach [8] have been applied in [7] to customise a procedure for classification of the detected signals and identify the mechanisms and associated processes initiating AE in an OPC sample with encapsulated Al. Moreover the cement hydration at a stage of more than 28 days after mixing
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