Effects of fly ash and silica fume on interfacial porosity of concrete

This paper presents experimental results on porosity and pore size distribution of fly ash and silica fume modified cement pastes and mortars using mercury intrusion porosimetry. It is found that replacement of cement by fly ash increases the porosity but decreases the average pore size of the pastes at the ages of 28 and 56 days. The additional replacement of cement by up 5% silica fume did not significantly change the pore size distribution and porosity of either the plain cement pastes or the fly ash cement pastes. Interfacial porosity, however, was significantly influenced by incorporating fly ash and silica fume in the mortars. The interfacial porosity was much reduced when fly ash and silica fume were incorporated. Compared with the porosity and average pore size of the pastes and mortars, the interfacial porosity better explains the results of a chloride diffusion test of the fly ash and silica fume concrete in most cases. However, for the concrete with 55% fly ash replacement at 28 days, the drop of ionic concentration of the pore solution seems to be more responsible for the low result of coulomb passed in the test.