Gamma irradiation resistance of early age Ba(OH)2-Na2SO4-slag cementitious grouts

The gamma irradiation resistance of early age Ba(OH)2-Na2SO4-slag cementitious grouts, formulated for the immobilisation of sulfate bearing nuclear waste, was assessed. The observable crystalline phases were not modified upon heating (50 °C) or upon gamma irradiation up to a total dose of 2.9 MGy over 256 h, but the compressive strengths of the irradiated samples increased significantly. 27Al and 29Si MAS NMR spectroscopy showed that the main binding phase, a calcium aluminosilicate hydrate (C-A-S-H) type gel, had a more ordered and polymerised structure upon heating and irradiation compared to that identified in reference samples. This is associated with a higher degree of reaction of the slag. Samples formulated with the waste simulant Na2SO4, but without Ba(OH)2, became porous and cracked upon heating and irradiation, but still retained their compressive strength. The Ba(OH)2-Na2SO4-slag grouts evaluated in this work withstand gamma irradiation without showing identifiable damage, and are thus a technically feasible solution for immobilisation of sulfate-bearing nuclear wastes.