Pore structure and chloride ion permeability of mortars containing silica fume

Abstract

The addition of silica fume in concrete causes a remarkable increase in strength and a drastic reduction in chloride ion permeability. These effects may be due primarily to microstructural changes both in the cement paste phase and in the interfacial zone around aggregates. The standard method of test for rapid determination of the chloride permeability of concrete, AASHTO T 277–831, has increasingly been used to evaluate the permeability of concrete. However, for the concrete containing silica fume, the results of the AASHTO T 277–831 test, which is expressed in terms of electrical charge passed, do not necessarily reflect the real diffusion index of chloride ion through the concrete. There seems to be factors other than the pore structure which govern the results of the AASHTO T 277–831 test in the concrete containing silica fume. In this study, the effects of silica fume to reduce the chloride ion permeability of the mortar were investigated based on the results of pore size distribution measurements, X-ray diffraction analysis, SEM observations and pore solution extraction. The application of the AASHTO T 277–831 test to the evaluation of the chloride ion permeability of the concrete containing silica fume was discussed.