Silica fume (Elkem Materials, Pittsburgh, PA, microsilica
EMS 965) was used for the sake of comparison in three
proportions by weight relative to cement, namely, 0:100,
50:50, and 100:0. The silica fume had particle size ranging
from 0.03 to 0.5 Am, with an average size of 0.2 Am. Its
composition is shown in Table 2. Comparison of Tables 1
and 2 shows that silica fume contains much more SiO2 and
much less Fe2O3 than fly ash.