Processing Characteristics and Strength of Magnesium Phosphate Cement Formulations Compatible with UK Nuclear Waste Trea
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Processing Characteristics and Strength of Magnesium Phosphate Cement Formulations Compatible with UK Nuclear Waste Treatment Plants W. Montague1, L. Vandeperre1 and M. Hayes2. 1
Centre for Advanced Structural Ceramics, Department of Materials, Imperial College London, South Kensington Campus, London SW7 2AZ, U.K. 2 National Nuclear Laboratory, Chadwick House, Warrington Road, Birchwood Park, Warrington, WA3 6AE, U.K. ABSTRACT Cementation is the dominant encapsulation technology for UK intermediate level waste (ILW) from nuclear operations. However, Portland cement (PC) based encapsulation systems encourage corrosion of reactive metals such as uranium, aluminium and magnesium. Thus, the development of alternative systems is required. A candidate alternative system is magnesium potassium phosphate cement (MKPC) and in this paper the results of a wide testing programme to establish a formulation envelope for MKPC is presented. This envelope determines a region where bleed production, fluidity and workability are within a range suitable for plant processing and encapsulation. Additionally, the evolution of the compressive strength with curing age for multiple formulations and curing temperatures was determined. Fly ash (PFA) inclusion was found to be highly beneficial to slurry workability and its addition also improved strength; high PFA content formulations reached compressive strength values of 34 ±2 MPa and 44 ± 2 MPa at 28 days and 180 days respectively. A decreased in achieved compressive strengths was observed on increasing the curing temperature from 47 °C to 72 °C. INTRODUCTION The currently favoured UK packaging method for ILW begins with encapsulation in a composite Portland cement (PC) matrix within stainless steel containers which are then sealed, producing a stable wasteform. However, a number of problems occur when a PC-based product is used to encapsulate metallic uranium waste: water contained in porosity within PC-composites drives corrosion of uranium which results in the formation of expansive products (primarily H2 and UO2), a phenomenon which is worsened by the high solubility of UO2 at the system pH (§12) preventing the formation of a protective oxide layer. Expansive product formation could compromise wasteform integrity and ultimately limit wasteform life-times. As a result investigations are underway to identify potential alternative cementitious systems with improved reactive metal compatibility. This investigation is concerned with the composite system of magnesium potassium phosphate cement (MKPC) with pulverised fly ash (PFA). MKPC is an acid-base cement which sets by a neutralization reaction between the basic MgO and acidic phosphate solution to form the crystalline magnesium potassium phosphate hexahydrate, or K-struvite, binder phase: MgO + KH2PO4 + 5H22ĺ0J.324.6H2O
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Boric acid is used as a set retarder to improve ultimate strength and provide acceptable setting times and PFA inclusion has been found to reduce heating of the cement due to the setting reaction exotherm as
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