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Microstructural studies on recycled aggregate concrete 443

Bravo et al., 2016). Therefore, their moisture condition during the mixing process
will affect the ITZ density and strength. Etxeberria et al. (2006) analysed several
samples of recycled aggregate concrete using SEM and concluded that RA with
partial water content produced an effective ITZ between the new cement matrix
and the old adhered mortar, with very low local w/c ratios. These finding were also
observed by other researchers (Poon et al., 2004; Zegardło et al., 2016; Boudali
et al., 2017), who have also stated that due to the lower water content of the RA
migration of the cement paste to the inner RA structure takes place, increasing the
mechanical bond at the ITZ. These results have also been established by other
authors (Guedes et al., 2013; Zhao et al., 2015) for fine RA concrete. In opposition,
the presence of excessive water in the aggregates may lead to water bleeding, that
will increase the size and quantity of pores at the ITZ (Bonifazi et al., 2015). This
was also empirically stated by Ferreira et al. (2011). Several other authors have
tested the quality of the ITZ using different mixing techniques (Tam et al., 2005;
Li et al., 2012). Tam et al. (2005) developed the ‘two-stage mixing approach’
method that intends to improve the quality of recycled aggregate concrete by
adjusting the order and mixing intervals of each of the concrete’s constituents. This
induces a thin cement slurry layer to form on the surface, permeating to the inner
zones of the RA at a later period of the mixing procedure. As a consequence, this
slurry will fill the existing voids and seal cracks in the RA structure, as shown in
SEM micrographs presented by the authors. More recently, Li et al. (2012) used the
same mixing technique to test the quality of the ITZ in recycled aggregate concrete
by means of SEM and nanoindentation. The authors concluded that the mixing pro-
cedure can effectively reduce the porosity of the new ITZ by reducing the local w/c
ratio, reducing the formation of CH crystals and increasing formation of C S H.
Applying enhancement treatments to RA affects the ITZ as stated before.
Treatments with nano-silica seem to counter this effect by forming C S H gel around
the old mortar, leading to a denser ITZ (Hosseinietal.,2009). This has also been con-
firmed by other authors (Li et al., 2009) that used surface coating, fly ash, silica fume
and bottom furnace slag, in combination with an optimised mixing technique. The use
of heat treatment has also proven itself as an effective method to reduce the ITZ poros-
ity, as stated by Al-Bayati et al. (2016b). For heating temperature up to 250 Cthe

Ca/Si ratio will decrease significantly, corresponding to an enhancement of the ITZ.
Increasing the temperature beyond that value will produce detrimental effects, as the
number and length cracks in the RA will increase. Carbonation treatment proved itself
to be very effective, as the ITZ shrinks and stiffens, compared to RA that are not yet
carbonated (Zhan et al., 2014; Yue et al., 2018).

14.6 Conclusion

Microstructural characterisation of concrete and of RA concrete is a powerful tool
to establish the contributions behind the development and features of most of the
mechanical, rheological and durability related properties of this complex and