The Immobilisation of Clinoptilolite Within Cementitious Systems
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The Immobilisation of Clinoptilolite Within Cementitious Systems L. E. Gordon(a)*, N. B. Milestone(a), M. J. Angus(b) (a)* Immobilisation Science Laboratory, Department of Engineering Materials, University of Sheffield. Mappin Street, Sheffield, S1 3JD , UK. Phone: + 44 (0) 1142225973, Fax: + 44 (0) 114 222 5943, E-Mail: [email protected] (b) Nexia Solutions, Sellafield, Cumbria, UK. ABSTRACT The zeolitic ion exchanger clinoptilolite was encapsulated within various cementitious systems in order to assess their suitability for the retention of the radioelements, Cs and Sr. The pozzolanic reaction of clinoptilolite is reduced in composites containing BFS and PFA and appears not to continue after 7 days of hydration. Ca(OH)2 persists up to 360 days of hydration in a 9:1BFS:OPC system with 10% clinoptilolite added, despite the presence of unreacted pozzolan. This may be due to low pH of the pore solution, if Na and K act as counter cations in the aluminous C-S-H, a product of pozzolanic hydration or are exchanged onto the clinoptilolite. Saturation of the pore solution with Ca may prevent further dissolution of Ca(OH)2. Cs leaching occurs in all samples during accelerated tests due to breakdown of the clinoptilolite structure. The alternative cement system calcium sulfoaluminate cement (CSA) has a different hydration chemistry and properties to OPC and OPC composites with a lower pore solution pH. Clinoptilolite appears to react in a hydrating CSA system with significant reaction continuing between 28 and 90 days of hydration. Leaching of Cs from CSA is higher than from an OPC system, in which almost all of the clinoptilolite crystallinity is lost. The major product of CSA hydration is ettringite. Cs may be adsorbed within cation sites of the C-S-H in an OPC system but not by ettringite which does not retain Cs so Cs has high mobility and leachability through the CSA matrix. INTRODUCTION 137
Cs and 90Sr can be successfully exchanged on a naturally occurring zeolite, clinoptilolite in a clean up process at Sellafield, UK. No decision has yet been made on the encapsulation route for this ILW, although cementation is the accepted route for ILWs in the UK. Encapsulation of zeolites within a traditional OPC system results in a pozzolanic reaction(1,2) where the crystalline aluminosilicate framework reacts with Ca(OH)2(3). This work considers the potential encapsulation of clinoptilolite in two types of composite cement formulations based on fly ash and blast furnace slag where high levels of replacement of OPC with fine pozzolans may reduce the pozzolanic reaction of the encapsulated zeolite, and encapsulation within an alternative cementitious system, calcium sulfoaluminate cement, where the hydration reactions are different from those of the OPC based composites. The major products of CSA hydration are crystalline ettringite and Al(OH)3. (4) The factors that may enhance the stabilisation of clinoptilolite in CSA cements relative to OPC based systems are: Firstly, the pH of the pore solution in CSA cements is report
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