Effect of gamma radiation on early age strength and pore structure development of metakaolin-based geopolymer used for conditioning cesium and strontium radioactive waste

This paper presents the effect of gamma radiation on early age strength and pore structure development in metakaolin (MK)-based geopolymers containing realistic cesium and strontium loading determined based on Boom Clay, a hypothetical host formation under consideration in the Belgian geological disposal concept. The effect of gamma-induced heat was decoupled from the ionizing nature of gamma radiation and assessed separately to elucidate its impact on strength and microstructure development. Changes in pore structure were evaluated using nitrogen physisorption and mercury intrusion porosimetry. The results indicate that both gamma radiation and temperature, analogous to irradiation-induced heat, negatively impacted the strength and pore structure development. Gamma radiation exposure of fresh geopolymer samples resulted in a coarser microstructure, leading to lower strength. No dose rate effect was observed, but the type of gamma radiation source had a significant impact, particularly on pore structure. Geopolymer samples exposed to Cs-137 from spent nuclear fuel exhibited larger pore structure alteration than those exposed to Co-60 at the same cumulative dose and similar dose rates. This suggests that a higher degree of pore structure alteration than previously reported in the literature could be anticipated in real-world Cs and Sr immobilization. The pore structure alteration is attributed to both gamma-induced heat and gamma-assisted water radiolysis and subsequent H₂ evolution and escape, with water radiolysis being the dominant mechanism of microstructural damage. Nevertheless, the MK-based geopolymer exposed to gamma radiation during hardening maintained satisfactory compressive strength, demonstrating strong radiation resistance. This indicates that MK-based geopolymer is promising for the immobilization of Cs and Sr-containing wastes. This study not only provides insights on formulating waste forms with realistic waste content in line with the foreseen geological disposal concept, but also advances our knowledge on mechanical and pore structure development under gamma irradiation, which have positive implications on radioactive waste management.