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