The SEM images for control (P), 45% fly ash (F3) and combination of 37.5% fly ash plus 7.5%
nanosilica (FS3) contained specimens are shown in Figures 3 a, b and c respectively. In Figure 3 b, large amount
of different sized, spherical-shaped fly ash can be seen filling the pores in the system [3], while less binder
material of calcium silicate hydrate can be detected to hold together this large amount of fly ash. For the FS3
specimen the contribution of nanosilica in strengthening the boundaries between the pastes with fly ash particles
can be detected, and due to the reaction of nanosilica with portlandite, an adequate amount of calcium silicate
hydrate was formed to hold the fly ash in the system. The promoted effect of nanosilica in increasing the
pozzolanic activity in the presence of fly ash as well as its filler effect resulted in strengthening the
microstructure for the mortars. This phenomenon is the cause of strength enhancement in mortars containing a
combination of fly ash and nanosilica.