Civil Engineering Reference
In-Depth Information
between the C-S-H gel. This result also agrees well with the experimental
results of Li et al. (2005).
20.6 Calcium-leaching with nanosilica particles
addition
Much of the research to date on nanomodification of concrete with nanoparticles
has been with nano-silica (nano-Sio
2
) (Bjornstrom et al. 2004; Ji 2005, Jo et
al. 2007; H. Li et al. 2004a, 2006; D.F. Lin et al. 2008; K.L. Lin et al. 2008;
nazari and Riahi 2010, 2011a,b; Qing et al. 2007, 2008; Sobolev et al. 2009)
and nano-titanium oxide (nano-TiO
2
) (Chen et al. 2012; H. Li et al. 2006,
2007). A few studies exist on including nano-iron oxide (nano-Fe
2
o
3
) (H.
et al. Li 2004a), zinc oxide (Nazari and Riahi 2011c), nano-alumina (nano-
al
2
o
3
) (Z. Li et al. 2006; nazari and Riahi 2011d, 2011e), and nanoclay
particles (Chang et al. 2007; Kuo et al. 2006). SEM images of four different
types of nano and microsilica are depicted in Fig. 20.10 (Quercia et al.
2012). nanosilica was shown to accelerate the hydration reactions of C
3
S
and to improve concrete workability and strength (Givi et al. 2011; Jalal et
al. 2012; H. Li et al. 2004b; Sobolev and Ferrada-Gutiérrez 2005; Sobolev
et al. 2009; Zhang and Islam 2012). also it was found to increase resistance
to water penetration (Ji 2005) and to help control the leaching of calcium
(Gaitero et al. 2008).
Calcium leaching is a degradation process concerning the progressive
dissolution of the cement paste as a consequence of the migration of calcium
ions to the aggressive solution. the kinetics of the process depend of on
several parameters with the porosity (the natural path for chemical attack),
the composition of the paste (each phase degrades at a different rate), and
the nature of the aggressive solution (generally soft water) being the most
important ones. although the phenomenon of calcium leaching is nowadays
reasonably well understood, a way of stopping or, at least, reducing it has not
been found yet. Silica, generally in the form of silica fume, has long been
used to improve the performance of the cement paste in terms of strength
and refinement/reduction of the porosity. Furthermore, this takes place by
mean of a pozzolanic reaction that results in the reduction of the amount of
portlandite, the hydrous phase most severely affected by calcium leaching.
Silica fume is composed of silica particles with diameters over 100 nm,
i.e. microsilica, whereas silica nanoparticles have typical diameters well
below 100 nm. the use of silica nanoparticles has several advantages in
comparison to other types of silica (Qing et al. 2007). As a consequence of
their reduced size the nanoparticles act as nucleation site for the growth of
hydration products, accelerating the hydration rate. Furthermore, their large
surface area and purity (up to 99 %), in addition to their amorphous nature,
provide them with great reactivity.
