This study explores the permeability of the partial replacement of cement using ultra-fine reactive fly ash (URFA) and silica fume. URFA is an industrial by-product from thermal power plants and it has a smaller spherical particle size (0.1 µm to 10 µm) than the general fly ash. The fixed water-to-binder ratio was 0.45 and the replacement of cement was used as 5% and 10%. The combinations of URFA and silica fume in composites were used as 5% URFA plus 5% silica fume and 8% URFA plus 2% silica fume. The results show that the inclusion of URFA was enhanced to increase the workability, and help to produce pozzolanic reaction and C-S-H colloids. The colloids were filling the pore structures to increase the compactness of specimen, which is consistency with the increased the compressive strength and reduction of the dry shrinkage. It also was reduced the chloride ion penetration and non-steady state migration coefficient, which is helpful for improving the mechanical properties and permeability. The composites with 10% URFA had a better performance. For blending specimen, it was also a tendency to improve mechanical properties and permeability; and the combination of 5% URFA and 5% silica fume in composites enhanced better performances.
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