Conversion of magnesium bearing radioactive wastes into cementitious binders Samuel Alexander Walling A thesis submitted in partial fulfilment of the requirement for the degree of Doctor of Philosophy Department of Materials Science and Engineering The University of Sheffield June 2016 i Abstract The UK radioactive waste inventory contains sizeable portions of magnesium hydroxide rich Magnox sludges due to corrosion of historic wastes. These require disposal in suitable wasteforms, with one potential being solidification in a composite Portland cement matrix. This thesis investigates magnesium hydroxide as a key component in the production of a cementitious binder, attempting to maximise waste loading and improve wasteform integrity through integral usage of these wastes. Hydrated magnesium silicate cements were produced through reaction with amorphous silica, creating stable products comprising a poorly crystalline M-S-H gel. The formulations for this product were optimised, water contents reduced through the use of a polyphosphate dispersant, and the nature of the M-S-H binder investigated further. This was determined to be a lizardite-like structure, which is a stable mineral. This system was modified through the addition of sodium aluminate, resulting in formulations with varying ratios of silicon to aluminium, each of which produced various zeolitic phases along with a magnesium aluminium hydrotalcite phase. This addition improved the setting characteristics of the binders, but did not produce any additional magnesium silicates binders. Additionally to this, sodium carbonate activated slag cement binders blended with magnesium hydroxide were assessed. These were slower setting, low heat cements which formed stable mineral phases, largely unaffected by the addition of magnesium hydroxide. The chemistry of these binders was assessed over an 18 month period, remaining stable throughout. Ultimately it was proved that magnesium hydroxide can be utilised to form cementitious binders, but only in the absence of competing calcium based binding systems. i Acknowledgements This thesis owes a great deal to the support and encouragement of countless people, most notably my academic supervisors Prof. John Provis and Dr. Hajime Kinoshita, without whom none of this would have been possible. I would also like to especially thank Dr. Susan Bernal López for getting me back on track when I’ve been lost, and for proof reading a great deal of this thesis in various stages. Thanks of course to my family and Laura Gardner for their (nearly) unending patience and help. Also thanks everyone within the Immobilisation Science Laboratory, especially Dr. Claire Corkhill, Dr. Oday Hussein, Dr. Martin Stennett and Prof. Neil Hyatt. I would like to express my gratitude to my funding bodies, the Engineering and Physical Sciences Research Council for funding, via the Nuclear FiRST program, and to the Nuclear Decommissioning Authority for funding also. I am indebted to Dr. Nick Collier and Dr. Steve Palethorpe for industrial input, to Dr. Dinu Iuga for assistance undertaking high-field NMR experiments, and to Andrew Mould, Ben Palmer and Dr. Nik Reeves for ongoing technical assistance. ii Publications Journal publications 1. Walling, S.A., Provis, J.L. (2016) “Magnesium based cements – a journey of 150 years, and cements for the future?” – Submitted to Chemical Reviews 2. Walling, S.A., Provis, J.L. (2016) “A discussion of the papers “Impact of hydrated magnesium carbonate additives on the carbonation of reactive MgO cements” and “Enhancing the carbonation of MgO cement porous blocks through improved curing conditions”, by C. Unluer & A. Al-Tabbaa”, Cement and Concrete Research, 79, 424-426. 3. Walling, S.A., Kinoshita, H., Bernal, S.A., Collier, N.C., Provis, J.L. (2015) “Structure and properties of binder gels formed in the system Mg(OH)2-SiO2- H2O for immobilisation of Magnox sludge”, Dalton Transactions, 44, 8126- 8137. 4. Gardner, L.J., Bernal, S.A., Walling, S.A., Corkhill, C.L., Provis, J.L., Hyatt, N.C. (2015) “Characterisation of magnesium potassium phosphate cements blended with fly ash and ground granulated blast furnace slag”, Cement and Concrete Research, 74, 78-87. 5. Walling, S.A., Bernal, S.A., Kinoshita, H., Collier, N.C., Provis, J.L. (2014) “Cementitious binders in the system Mg(OH)2-NaAlO2-SiO2-H2O”, Advances in Applied Ceramics, 113, 496-501. iii Conference publications 1. Walling, S.A., Bernal, S.A., Kinoshita, H., Provis, J.L. “Magnesium aluminium silicate hydrate (M-A-S-H) cements for Magnox sludge immobilisation”, Workshop on Waste Cementation, Sheffield, 17th September 2014. (Extended abstract and poster presentation) 2. Walling, S.A., Bernal, S.A., Collier, N.C., Kinoshita, H., Provis, J.L. “Sodium carbonate activated slag binders for the immobilisation of Magnox nuclear waste”, 34th Cement and Concrete Science Conference, Sheffield, UK, September 14-16th 2014. (Extended abstract and poster presentation) 3. Gardner L.J., Bernal S.A., Walling S.A., Corkhill C.L., Provis J.L., Hyatt N.C. “High-resolution multi-nuclear NMR characterisation of a novel GBFS/MKPC binder for nuclear waste encapsulation”, 34th Cement and Concrete Science Conference, Sheffield, 14-16th September 2014. 4. Walling, S.A., Bernal, S.A., Kinoshita, H., Collier, N.C., Provis, J.L. rd “Alternative binders in the systems Mg(OH)2-NaAlO2-SiO2-H2O”, 33 Cement and Concrete Science Conference, Portsmouth, 2-3rd September 2013 (Extended abstract and presentation) 5. Walling, S.A., Bryan, N.D., Kinoshita, H. “Magnesium silicate hydrate cement for Magnox sludge encapsulation” 32nd Cement and Concrete Science Conference, Belfast, 17-18th September 2012 (Extended abstract & poster presentation) iv Other abstracts / posters 1. Walling, S.A., Bernal, S.A., Kinoshita, H., Provis, J.L. “Conversion of magnesium bearing radioactive wastes into cementitious binders”, Nuclear Decommissioning Authority PhD Research Seminar for NDA-Sponsored PhD Projects, 14th January 2015, Manchester. (Abstract and poster presentation) 2. Walling, S.A., Collier, N., Kinoshita, H. “Magnox sludge as a cementitious binder” – Nuclear Decommissioning Authority PhD Research Seminar for NDA-Sponsored PhD Projects, 29th January 2013, Manchester. (Abstract) v Cement Chemical Nomenclature A = Al2O3 C = CaO F = Fe2O3 H = H2O M = MgO S = SiO2 Ĉ = CO2 C3A 3CaO·Al2O3 Ca3Al2O6 C4AF 4CaO·Al2O3·Fe2O3 2Ca2AlFeO5 C2S 2CaO·SiO2 Ca2SiO4 C3S 3CaO·SiO2 Ca3SiO5 CH CaO·H2O Ca(OH)2 MH MgO·H2O Mg(OH)2 vi Contents Abstract ........................................................................................................................ i Acknowledgements ..................................................................................................... ii Publications ................................................................................................................ iii Journal publications ................................................................................................ iii Conference publications .......................................................................................... iv Other abstracts / posters ........................................................................................... v 1 Introduction ......................................................................................................... 1 2 Literature Review ............................................................................................... 3 2.1 Nuclear power ............................................................................................... 3 2.2 Nuclear power in the UK ............................................................................... 7 2.2.1 History .................................................................................................... 7 2.2.2 Today.................................................................................................... 10 2.3 Nuclear waste .............................................................................................. 15 2.3.1 Nuclear waste in the UK ...................................................................... 16 2.4 Geological disposal ..................................................................................... 20 2.4.1 Geological disposal in the UK ............................................................. 20 2.5 Waste treatment ........................................................................................... 23 2.5.1 Vitrification .......................................................................................... 23 2.5.2 Cementation ......................................................................................... 24 2.5.3 Alternative ILW immobilisation technologies ..................................... 29 2.6 Magnox sludges ........................................................................................... 31 2.6.1 Other sources of Magnox sludge.......................................................... 34 2.6.2 Magnox sludge encapsulation .............................................................. 35 2.6.3 Alternative processing techniques........................................................ 36 2.6.4 Cement encapsulation of Magnox sludges ........................................... 37 2.7 Summary ..................................................................................................... 38 3 Review of magnesium cements ........................................................................ 39 3.1 Introduction ................................................................................................