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As part of research to develop a highly durable concrete container for radioactive waste disposal in chloride and sulfate bearing granite groundwaters, a variety of cement pastes were studied. A sulfate resisting portland cement was used with various replacement levels of Class F fly ash and pelletized blast furnace slag at a water to solids ratio (W/S) = 0.36. Blends with fly ash, slag, and silica fume were also combined with a super water reducer at W/S = 0.25. Results are presented for strength development, permeability to water, and pore size distribution after 7, 28, 91, and 182 days moist curing. As a direct measure of durability, after 91 days moist curing, paste prisms were immersed in both de-ionized water and a synthetic chloride and sulfate bearing groundwater at 70°C.
While all three supplementary cementing materials (mineral admixtures) reduced ultimate permeabilities, silica fume was more effective in reducing permeability at early ages. Silica fume was also the most effective in reducing calcium hydroxide contents of the pastes while slag was the least effective; only reducing calcium hydroxide levels by dilution of the portland cement. From preliminary analysis, there does not appear to be a way of accurately predicting permeability from porosity or pore size parameters alone.
cement paste, slag, fly ash, silica fume, permeability, mercury porosimetry
Concrete materials engineer, Civil Research Department, Ontario Hydro Toronto, Ont.