XRD: To detect the changes in the compounds from replacing cement with fly ash and incorporating nanosilica
into the system an XRD test was performed on controlled cement paste, pastes containing 45% fly ash and
pastes containing 7.5% nanosilica + 37.5% fly ash. Cement paste compounds having a crystalline nature, such
as calcium hydroxide, calcium silicate and silica, can easily be detected, while amorphous compounds such as
calcium silicate hydrate can rarely be distinguished by this technique [3, 28].
The XRD diagrams for the specimens are shown in Figure 4. Replacing cement by fly ash resulted in a slight
decrease in the calcium hydroxide peaks due to the pozzolanic reaction, while the incorporation of nanosilica
decreased the amount of calcium hydroxide significantly [17, 29]. This phenomenon supports the idea of
increasing calcium silicate hydrate (the binder material) due to conversion of calcium hydroxide to calcium
silicate hydrate by nanosilica, which by turn explains the significant increase in strength for specimens
containing nanosilica. Furthermore, a decrease in the silica peak in specimens containing nanosilica and fly ash
indicates the effect of nanosilica in accelerating the reaction of coarser particles of silica with calcium hydroxide
to produce larger amounts of high-density calcium silicate hydrate.