Nanoscience and nanoengineering of cement-based materials - Nanotechnology in Eco-efficient Construction - page 23

Carbon nanotubes

Carbon nanotubes (CNT) are the strongest and stiffest material known to
date 4 and hold great promise for reinforcing cement-based composites
(Makar and Beaudoin, 2004; Makar et al. , 2005; Nasibulin et al. , 2009; Chai-
panich et al. , 2010; Konsta-Gdoutos et al. , 2010b; Manzur and Yazdani, 2010;
Metaxa et al. , 2010; Wille and Loh, 2010; Kumar et al. , 2012). They are allo-
tropes of carbon with a cylindrical-tubular geometry of diameter that ranges
from 1 to 100 nm and lengths up to a few millimeters. Aspect ratios (length-
to-diameter ratio) beyond 100,000,000:1 have been reported in the litera-
ture which allows crack-bridging capabilities at fi ner scales. Owing to their
geometry and covalent sp 2 bonds formed between the individual carbon
atoms, they exhibit superb mechanical, electrical, and thermal properties
(among others) coupled with impressive deformation characteristics which
makes them excellent candidates for additives to various structural materi-
als, including concrete (Salvetat et al. , 1999). It is in fact impressive that with
only a tiny portion of reinforcement the composite system can potentially
infl uence the mechanical response. CNTs can occur as a single layer of
graphite rolled in a tubular shape (SWCNT) or multiple wall concentric
tubes (MWCNT, also known as CNF, whereas F stands for fi ber). SWCNT
have reported values of 1 TPa for elasticity and 60 GPa for strength whereas
the respective values for CNF are in the order of 400 GPa and 7 GPa
respectively. The mechanical reduction of MWCNTs is due to the weak
shear interactions between adjacent tubes. This limitation has recently been
addressed by applying high-energy electron irradiation, which crosslinks
inner shells and tubes, and can effectively increase the strength of these
materials to 60 GPa.

In order to harness the real benefi ts of the fi ller, any fi ber-reinforced
composite should ensure the effi cient transfer of the stresses between the
hosting matrix and the reinforcing material. To ensure this condition, two
requirements should be met: (a) the fi bers should be uniformly dispersed
in the matrix thus avoiding possible agglomeration of particles, through van
der Waals forces, in the system, and (b) the fi ber-matrix interface should
be strong enough such as to avoid pull-out phenomena during service. The
selection of chemicals that will ensure proper dispersion and bonding is
complicated by the requirement to comply with the chemical system, i.e.,
not to interfere in a negative way with the hydration process. As a result,
many of the well-known surfactants that have been used effectively to
disperse CNTs in other systems have been discarded for use in concrete.
While the actual protocol that will allow proper dispersion of CNT in the
matrix and good bonding qualities is still the subject of intense research,

∼

￿ ￿ ￿ ￿ ￿ ￿

4

The recent invention of graphene, an allotrope of carbon, might relegate CNT to second
in rank.