Effect of nanosilica on chloride permeability in cement mortar

An accelerated electro-migration experiment is carried out to examine the chloride penetration of cement mortar admixed with two types of nanosilica and compared with the natural diffusion measurement. The chloride ion concentration in the destination solution is measured by ultra violet–visible spectrophotometer at different time intervals. In the natural condition, the penetration of chloride ions through a 7-mm thick mortar disc takes approximately 24 h, whereas nanosilica incorporated mortar resists the ingress of chloride ions up to around 72 h. Furthermore, in the accelerated condition, the diffusion coefficient is reduced from 6·8 × 10⁻¹¹ m²/s to 4·7 × 10⁻¹¹ m²/s with 3% addition of nanosilica (type I) by weight of cement. The microstructure studies through scanning electron microscopy reveal that the nanosilica refines the pore structure of the mortar and leads to a denser microstructure owing to the formation of additional calcium-silicate-hydrate gel. The pozzolanic reactivity is also determined using a thermogravimetric technique wherein nanosilica and silica fume incorporated cement mortar reduces the calcium hydroxide content by 67 and 30%, respectively. These studies demonstrate that nanosilica efficiently reduces the chloride permeability of cement mortar owing to higher pozzolanic activity.