Composites with Nanoparticles
Nanoparticles have a high-surface area to volume ratio providing high-chemical reactivity. They act as nucleation centers, contributing to the development of the hydration of Portland cement. Most investigations use nanosilica while some already used nano-Fe2O3. The production of nanoparticles can be obtained either through a high-milling energy (Sobolev and Ferrada-Gutierrez 2005) or by chemical synthesis (Lee and Kriven 2005). Porro et al. (2005) mentioned that the use of nanosilica particles increases the compression strength of cement pastes. The same authors state that the phenomenon is not due to the pozzolanic reaction, because calcium hydroxide consumption was very low but, instead, due to the increase use of silica compounds that contributes to a denser microstructure. According to Lin et al. (2008), the use of nanosilica on sludge/fly ash mortars, compensates the negative effects associated to the sludge incorporation in terms of setting time and initial strength. Sobolev et al. (2008) reported that nanosilica addition led to an increase of strength by 15% to 20%. Other authors (Gaitero 2008; Gaitero et al. 2009) believe that nanosilica leads to an increase of C-S-H chain dimension and also to an increase of C-S-H stiffness. Chen and Lin (2009) used nanosilica particles to improve the performance of sludge/clay mixtures for tile production. The results show that nanoparticles improved the reduction of water absorption and led to an increase of abrasion and impact strength. Others (Vera-Agullo et al. 2009) also confirm that the use of nanoparticles (nanotubes, nanofibers, nanosilica or nanoclay) is responsible for a higher hydration degree of cementitious compounds, as long as a higher nanoparticle dispersion can be achieved. Nasibulin et al. (2009) reported an increase in strength by 2 to 40 times for electric conductivity, which means a high potential for sensing ability. Several authors confirm the suitability of mortars with Fe2O3 nanoparticles to act as sensing materials (Li et al. 2004; Qing et al. 2008; Lin et al. 2008). Chaipanich et al. (2010) mentioned that 1% of carbon nanofibres (by binder mass) can compensate the strength reduction associated with the replacement of 20% fly ash. Gdoutos-Konsta et al. (2010) also studied the effect of carbon nanofibres on cement pastes (0.08% by binder mass) observing an increase in the mechanical strength. Those authors used ultra-sounds to achieve a high-nanofibre dispersion stating that this is a crucial step in order to obtain a high performance of nanotubes in the cement matrixes. Nevertheless, the fact that carbon nanotubes are not cost-efficient prevents the increase of its use in commercial applications in a near future.