Low-pH concrete developed for tunnel and plugs used in nuclear waste containment

Innovative research has been done to develop and validate the performance of low-pH concrete used in high level radioactive nuclear waste containment structures. The unique type of concrete is used for deposition tunnel end plugs acting as hydraulic and mechanical barriers in Finland's deep underground repository. Due to long-term material safety requirements, the concrete pH leachate must be lower than normal, thus the concrete mixtures have been made with very low amounts of cement while utilizing 40% or more of alternative binders such as silica fume and fly ash. The mixtures must also have high workability, thus the research has focused on both rheology and high durability performance to ensure the long service life in the challenging environmental conditions. It was possible to achieve two different mixtures, having compressive strengths over 50 MPa, adiabatic temperature rise under 10°C, minimal shrinkage, extremely low permeability and a pH under 11 in groundwater leachate. These excellent performance results provide two new potential concrete alternatives, either as binary or ternary blends of binder, to be used in nuclear repository conditions very long-service life structures. The results are being utilized in the design specifications and construction of the Finnish repository facility of Posiva Oy.