Geopolymers as Candidates for the Immobilisation of Low- and Intermediate-Level Waste
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Geopolymers as Candidates for the Immobilisation of Low- and Intermediate-Level Waste Dan Perera, Eric Vance, Satoshi Kiyama, Zaynab Aly, and Patrick Yee Institute of Materials and Engineering Science, Australian Nuclear Science and Technology Organisation, New Illawarra Road, Menai, NSW 2234, Australia ABSTRACT Geopolymers should be serious waste form candidates for intermediate level waste (ILW), insofar as they are more durable than Portland cement and can pass the PCT-B test for high-level waste. Thus an alkaline ILW could be considered to be satisfactorily immobilised in a geopolymer formulation. However a simulated Hanford tank waste was found to fail the PCT-B criterion even for a waste loading as low as 5 wt%, very probably due to the formation of a soluble sodium phosphate compound(s). This suggests that it could be worth developing a ìmixedî GP waste form in which the amorphous material can immobilise cations and a zeolitic component to immobilise anions. The PCTñB test is demonstrably subject to significant saturation effects, especially for relatively soluble waste forms. INTRODUCTION Geopolymers (GPs) are potential substitutes for Portland cement and they are argued to produce only ~20% of the CO2 per unit mass that is produced by cement. They also have better fire and acid resistance. Two common methods of making GPs are to add reactive aluminosilicate precursors - fly ash or metakaolin (MK; kaolinite clay heated to ~7500C for several hours and thereby rendered X-ray amorphous) - to strongly alkaline silicate solutions for dissolution, polymerisation and solidification. Curing at 25-900C in a humid atmosphere for hours or days at the lower temperatures completes the curing process. Aside from their potential as building materials, for which a key feature is to minimise costs through the use of cheap additives, GPs have possible candidacy for immobilisation of radioactive waste. While there are a variety of data on the reaction of GPs with aqueous media [1-4], including the context of radioactive waste immobilisation [5-9], only a few workers have used regulatory tests [5,7-9]. The volumes of radioactive wastes which are candidates for immobilisation in GPs can be relatively small and the costs of shielding to facilitate handling are such that the costs of the additive materials are relatively negligible in the whole immobilisation process. Thus in our efforts to investigate the candidacy of GPs to immobilise radioactive waste we have concentrated on relatively reproducible alkaline silicate solutions and metakaolin as the essential constituent materials. GPs made from metakaolin and having Na/Al ~ 1 and Si/Al ~ 2 and ~ 40% of total water at the outset of formation can pass [9] the US-based Product Consistency Test [10] which addresses their resistance to water leaching in the first instance. The equivalent leach rates derived from these tests are ~ 1 g/m2/day. Thus at first sight, typical aqueous durabilities are akin to those of borosilicate glasses, rather than cement. However it has al
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