Ba(OH)2-Na2SO4-BFS cement composites for the encapsulation of sulphate bearing nuclear waste

Soluble sulphate ions in nuclear wastes can have detrimental effects on cementitious wasteforms and disposal facilities based on Portland cement. For safe immobilisation of sulphate-bearing nuclear wastes, Ba(OH)2-Na2SO4-Blast Furnace Slag composites were synthesised via a newly developed single-step method and studied. These composites promote precipitation of insoluble BaSO4 and the consequent activation of the BFS. Six month aged Ba(OH)2-Na2SO4-BFS composite samples, successfully produced via the single-step method, revealed that phases formed are low solubility salts of BaSO4 and BaCO3, calcium aluminosilicate hydrate (C-A-S-H) with some barium substitution as the main binder phase, with barium also present in Ba-substituted AFt, and a hydrotalcite-type layered double hydroxide. Long term stability investigation, up to 18 months, showed that the BaCO3 initially formed reacts with the sulphate contained in different phases to form less soluble BaSO4 without disturbing the physical stability of the bulk materials.
Gamma irradiation resistance of the proposed composite cement system is the focus of the second half of this thesis. An early age BFS-PC system is studied as a reference to provide information regarding the expected behaviour of slag-rich cements under irradiation, and then early age Ba(OH)2-Na2SO4-BFS composites are studied to evaluate their performance under exposure to gamma rays by comparison. Irradiated samples were compared with control and also heat-treated samples to identify potential effects of the temperature rise during irradiation exposure. The gamma irradiation resulted in the formation of cracks in the BFS-PC system as a result of the radiolysis of the free water, whereas Ba(OH)2-Na2SO4-BFS composites could withstand gamma irradiation without any obvious reduction in the integrity of the products.