Long-term changes in the chemical, microstructural, and transport properties of a low-pH cement shotcrete during operation of the Horonobe Underground Research Laboratory, Japan

The properties of low-pH cement used in the geological disposal of radioactive waste may change through atmospheric carbonation and degradation caused by groundwater during the long-term operation of a repository. In this study, we investigated the effects of atmospheric carbonation and groundwater contact on the chemical, microstructural, and transport properties of shotcrete made from low-pH, high-fly-ash silica-fume cement (HFSC) over a period of 16 years in an underground research laboratory. In both carbonated and degraded zones of the HFSC shotcrete, capillary porosity increased for pores of < 300 nm in diameter, and the total porosity was higher than in undegraded zones. These changes in porosity may be associated with the decalcification of calcium–silicate–hydrate and decomposition of ettringite. Such changes were minor in altered zones of OPC shotcrete, indicating that HFSC shotcrete is less resistant to atmospheric carbonation and groundwater leaching under the studied conditions. However, the hydraulic conductivity in HFSC was low enough to fulfill the specific functional requirements of low-pH cements for geological disposal. • Carbonation and degradation altered the properties of low-pH cement shotcrete. • Elemental distributions changed and porosity increased in altered shotcrete zones. • Hydraulic conductivity was below the low-pH cement functional requirements.