According to Bache [1981], the principal hydration effect of silica fume in concrete is
that of a filler which because of its fineness, can fix into spaces between cement grains
in the same way that sand fills the spaces between particles of coarse aggregates and
cement grains fill the spaces between sand grains. According to Delage and Aitcin
[1983] and Mehta et al., [1982], incorporation of silica fume in concrete essentially
eliminates pores between 500 and 0.5 microns in size and reduces the size of pores in
the 50 to 500 micron range.