Composition and selected properties of low pH mortars and concretes for radioactive waste repositories

Conventional cementitious materials as tunnel supporting materials are utilised in the construction of the final repository for spent nuclear fuel. However, the use of cementitious material releases alkaline ions from pH12 to pH13 plumed into groundwater. Such a high pH is detrimental to the performance of the bentonite functioning, which may possibly enhance the dissolution and alteration of the fracture buffer and filling materials. Instead, low-pH cementitious materials are being developed for use in geological repositories. This study is aimed at evaluating the usability of low-pH cementitious materials containing 40% silica fume or composites blended with 20% silica fume and 40% fly ash. Engineering properties were analysed and verified through experimental research using the flow, compressive strength, pH measurement and hydraulic conductivity. Test results show that the replacement level with 40% of silica fume or 20% of silica fume and 40% of fly ash was suitable for the mixture of low-pH cementitious. Compared to the compressive strength and water permeability of ordinary cementitious, those of low-pH cementitious enhanced better engineered performances at the age of 91 days. The information is contributed us to establish the long-term durability and environmental requirements of disposal repositories in Taiwan.