Concrete and cement are used in constructions as well as in conditioning of waste in repositories for radioactive waste. The development of low pH cements for use in geological repositories for radioactive waste stem from concerns over the potential for deleterious effects upon the host rock and other EBS materials (notably bentonite) under the hyperalkaline conditions (pH > 12) of cement pore fluids. Low pH cement (also known as low heat cement) was developed by the cement industry for use where large masses of cement (e.g. dams) could cause problems regarding heat generated during curing. In low pH cements, the amount of cement is reduced by substitution of materials such as fly ash, blast furnace slag, silica fume, and/or non-pozzolanic silica flour.
SKB has (as the Finnish Posiva and the Japanese NUMO) defined a pH limit ≤ 11 for cement grout leachates. Because low pH cement has little, or no free portlandite, the cement consists predominantly of calcium silicate hydrate (CSH) gel with a Ca/Si ratio ≤ 0.8.
