Use of nanomaterials in the construction sector is gaining widespread attention as significant improvements are expected to be achieved in the desired properties of construction materials. The most commonly used nanomaterials in the cement are nano-silica, nano-titania, nano-alumina, carbon nano-tubes (CNTs) etc. (Sanchez and Sobolev
2010). Among all, nano-silica has been proven an effective additive to cement matrix for accelerating cement hydration due to its high reactivity, ability to refine the microstructure and thus, leading to a reduced porosity (Toutanji et al.
2004). Various types of nano-silica (powder or in suspension) are available commercially, having specific density, specific surface area, pore structure and reactivity (Quercia et al.
2014). Several researchers (as reviewed by Singh et al. (
2013)) reported that the mechanical properties and durability can be improved by adding nano-silica (powder or colloidal) in cement-based materials. The enhancement in compressive strength of cement mortar with 0.25 % powder nano-silica was achieved 63.9 and 95.9 MPa at the age of 1 and 28 days, respectively (Flores
2010). The characteristics of cement mortar with powder nano-silica particles showed that nano-silica behaves not only as a filler to improve the microstructure but also as an activator to promote the pozzolanic reaction (Jo et al.
2007). The performance enhancing properties of nano-silica are achieved through two mechanisms: firstly, the ultrafine particles are able to fill the voids between the cement particles improving ‘‘packing’’ and creating a less permeable structure. Secondly, the nano-silica also reacts with the calcium hydroxide (CH) produced with the cement hydration to form additional C–S–H (Gaitero et al.
2008,
2009). The porosity and capillary pores decreased while the gel pores increased as a result of the inclusion of silica fume and fly ash in the cement-based composites (Lin et al.
2009). Several researchers (as reviewed by Shi et al. (
2012)) have studied the role of mineral admixtures in concrete durability, methods of measuring chloride ingress into concrete, challenges in assessing concrete durability from its chloride diffusivity, and the service life modeling of reinforced concrete in chloride-laden environments. The ingress of gases, water or ions in aqueous solutions into concrete takes place through pore spaces in the cement paste matrix and paste-aggregate interfaces or microcracks. For the durability of concrete, permeability is believed to be the most important characteristic (Baykal
2000), related to its microstructural properties, such as the size, distribution, and interconnection of pores and microcracks (Savas
2000). The water permeability test shows that the nano-silica concrete has lower water permeability as compared to normal concrete (Ji
2005). The incorporation of nanoparticles (Fe
2O
3, Al
2O
3, TiO
2 and SiO
2) and nanoclays (montmorillonite) reduces the diffusion coefficient of the mortar as well as electrochemical impedance spectroscopy. The tests indicate that such effects are especially significant using nano-SiO
2 and nanoclays (He and Shi
2008).
In contrast, colloidal nano-silica (CNS) denotes small particles (1–100 nm) consisting of an amorphous silica core with a hydroxylated surface, which are insoluble in water (Coenen and Kruif
1988). The accelerating effects of colloidal silica on C
3S phase dissolution; C–S–H gel formation and silica polymerisation in cement paste hydration were studied (Bjornström et al.
2004). The surface treatment of colloidal nano-silica was found effective in decreasing water absorption of cement mortar at 50 °C, but a negligible effect at 20 °C and filled coarser pores (>50 nm) (Hou et al.
2014). The addition of 6 % CNS improves the compressive strength of mortar from 18.3 to 46.3 MPa, at 7 days (Jo et al.
2007). The improvements are attributed to three reasons: the acceleration effect of CNS on cement hydration, pozzolanic reaction of CNS and the improved particle packing of matrix. Cement with 2–4 % addition of CNS do not lead to an immediate mechanical strength gain due to the formation of agglomerates, later on hydration evolution takes place due to consumption of calcium hydroxide (Kontoleontos et al.
2012).
Nano-silica is extensively used in cement matrix, though their mixing is a challenge which needs to be addressed. When nanoparticles are added into the cement with water, they form agglomerates and may not reflect its original reactivity (Kong et al.
2013). In order to address this issue (i.e. homogeneous mixing of nanomaterials) dispersible silica nanoparticles were prepared and introduced into cement mortar. Further, the experiment comprises the comparison of powdered and colloidal NS with respect to their effect on gel/space ratio, compressive strength, pozzolanic reactivity of both nano-silica and silica fume and quantification of the C–S–H using thermogravimetric analysis were conducted. Moreover, chloride penetration of plain and nanomodified cement mortar was investigated.