Enhancing the Long-Term Performance of Cementitious High-Integrity Containers for Low-Level Radioactive Waste Using Ultrafine Metakaolin and Silica Fume

Cementitious high-integrity container (HIC) concrete used for long-term containment of low-level radioactive waste (LLW) requires sustained durability to maintain its structural and containment functions. This requirement is particularly important under coastal exposure, where chloride ingress may progressively compromise the long-term performance of cementitious barriers. High-integrity container concrete for long-term containment of low-level radioactive waste commonly incorporates silica fume (SF) to improve strength and chloride resistance. This paper explores the potential of ultrafine metakaolin (UMK), either individually or in combination with SF, in improving the performance of HIC concrete, with emphasis on long-term durability under coastal exposure. Three HIC concrete mixtures were investigated: an SF-containing reference mixture (HIC-SF), a mixture incorporating UMK (HIC-UMK), and a mixture incorporating both UMK and SF (HIC-UMKSF). The HIC-SF mixture, based on a previously approved high-performance concrete container formulation in Taiwan, was adopted as the reference mixture. Compressive and splitting tensile strengths, permeable porosity, sorptivity, chloride diffusion, and X-ray diffraction (XRD) of concrete mixtures were evaluated up to 365 days. Thermogravimetric analysis (TGA) was conducted up to 180 days to quantitatively estimate residual portlandite content and complement the qualitative XRD results. XRD showed greater calcium hydroxide consumption and more extensive development of calcium silicate hydrate (C-S-H) and aluminosilicate-bearing hydration products in the UMK-containing mixtures, with HIC-UMKSF exhibiting the most extensive hydration development. At 365 days, HIC-UMKSF achieved the highest compressive strength (114.5 MPa), whereas HIC-UMK exhibited the lowest permeable porosity (2.7%) and highest splitting tensile strength (11.8 MPa). The UMK-containing mixtures also showed lower sorptivity and apparent chloride diffusion coefficients and higher chloride diffusion reduction coefficients, indicating improved long-term chloride resistance. Overall, UMK demonstrated strong potential for enhancing the long-term performance of HIC concrete, particularly through improved pore structure and transport resistance, while its combination with SF provided additional benefits in mechanical performance and chloride resistance. These findings support the use of UMK-modified cementitious HICs for long-term LLW containment under coastal environment.