Nanoscience and nanoengineering of cement-based materials - Nanotechnology in Eco-efficient Construction - page 24

some preliminary results have already been reported in the literature (Gay
and Sanchez, 2010; Yazdanbakhsh et al. , 2010; Collins et al. , 2012). Approaches
to disperse the fi bers include their surface modifi cation through function-
alization usually in conjunction with sonication (Cwirzen et al. , 2008).
Surface treatment with ozone gas (Chung, 2005), sulphuric and nitric acids
(Li et al. , 2005), use of gum Arabic (Sáez de Ibarra et al. , 2006), polyacrylic
acid and polyallylamine hydrochloride (Grunlan et al. , 2008), growing CNTs
on cement particles (Nasibulin et al. , 2009; Nasibulina et al. , 2010) and
polymer grafting on CNT (Li et al. , 2005) have also been suggested.
Admixtures that are commonly used in cementitious materials have also
been tested with the polycarboxylate and lignosulfate to show good results
(Collins et al. , 2012). Cwirzen et al. (2008) demonstrated that stable and
homogeneous water dispersions of MWCNTs can be obtained by using
functionalized CNTs with COOH and additional treatment with polyacrylic
acid polymers. The mixing method included stirring combined with sonica-
tion at 50 Hz. The cement paste specimens produced revealed an increase
in compressive strength values in comparison with the reference pure
cement paste specimens. The highest increase in the compressive strength
was nearly 50% in cement paste incorporating only 0.045% of the poly-
acrylic acid polymer-treated MWCNTs. These results indicate the existence
of chemical bonds between the OH groups and probably the C-S-H phase
of the cement matrix which enabled the stress transfer.
The quality of the interface characteristics is an even more complicated
fi eld and is still in its infancy. CNTs have been treated using a H 2 SO 4 /HNO 3
mixture solution which potentially leads to the formation of carboxyl acid
groups on their surfaces (Li et al. , 2005). The presence of carboxylic acid
groups enhances the interface effi ciency through a series of chemical reac-
tions. In general, when properly dispersed and well bonded, CNTs enhance
compressive, tensile, and fl exural performance by creating bridging mecha-
nisms between the fi bers and matrix, thus capturing microcracks and causing
more material to go into plastic deformation prior to failure. Furthermore,
there appears to be a densifi cation of the C-S-H matrix. Through nanoin-
dentation experiments, it has been suggested that CNT promote the forma-
tion of the HD C-S-H (Shah et al. , 2009). The CNT can potentially be
employed in any cement-based material ranging from fl y-ash mix (Chaipan-
ich et al. , 2010) to ultra high performance composites (Sáez de Ibarra et al. ,
2006). Finally the exploitation of CNTs in the matrix for monitoring and
sensing properties is currently under investigation (Wansom et al. , 2006; Li
et al. , 2007; Han et al. , 2011).

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Titanium dioxide

Titanium dioxide nanoparticles in the form of anatase have received con-
siderable attention in the construction industry in recent years due to their