The effect of amorphous nanosilica particles on the mechanical properties and durability of two high-strength concrete (HSC) mixes was investigated. Nanosilica in powder form was used as a partial replacement of cement at dosages of 1 wt% and 2 wt%, and significant improvements in performance were observed for 2 wt% replacement of cement by nanosilica. Micromechanical studies were performed on the nano-modified HSCs to determine the impact of nanosilica on pozzolanic reactivity. Durability assessments such as the rapid chloride penetration test, water sorptivity test and water absorption test revealed significant resistance to chloride penetration, sorptivity and water absorption. These improvements can be mainly attributed to the larger specific surface area of nanosilica, which effectively stimulates both pozzolanic reactivity and the filler effect over the cementitious matrix. Notation a exposed area of specimen (mm 2 ) d density of water (g/mm 3 ) I absorption I 0 , I 30 , I 60 … I 330 , I 360 current (A) at 0, 30, 60 … 330, 360 min m t change in specimen mass (g) at time t Q charge passed (C)

Fly ash (FA)-based geopolymer mortar is known to exhibit less strength development at ambient temperature due to slow progress in the polymerisation reaction. In this paper, studies are reported on geopolymer mortar made of FA, ground-granulated blast-furnace slag (GGBS) and nano silica at ambient temperature. The investigations on mechanical and durability properties of FA/GGBS-based geopolymer mortar were carried out by adding nano silica at 1% and 2% weight of FA into the mixture. For all the mixtures, the concentration of sodium hydroxide (NaOH) was maintained as 3M and the sodium silicate (Na2SiO3)/sodium hydroxide weight ratio was 2·06. The alkaline liquid/binder ratio of 0·3 and ambient curing temperature (28°C) was maintained for all the mixtures. Geopolymer mortar with an addition of 2% nano silica showed significant improvement in compressive strength, split tensile strength, flexural strength and fracture properties. The water absorption, rapid chloride penetration test and sorptivity showed better resistance in mixtures with a 2% addition of nano silica. The improved properties could have been due to an enhancement of geopolymeric reaction by the addition of amorphous nano silica, which formed additional sodium aluminosilicate hydrate (Al2H2Na2O7Si) or geopolymer gel while ultrafine particles of nano silica reduced the size of empty space in the geopolymer matrix.

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