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Microstructural studies on recycled aggregate concrete 441

glass, gypsum, ceramics and metals (Silva et al., 2014; Favaretto et al., 2017;
Akhtar and Sarmah, 2018; Juan-Valde ´s et al., 2018).
Regardless of the category, RA share common microstructural features, with
considerably more complex structure than NA (Akhtar and Sarmah, 2018). RAs
mainly consist of adhered old mortar (or materials such as brick, gypsum or wood,
also with porous nature) and primary aggregates. On the one hand, the composition
of RA is in itself variable, depending on cement and cement paste characteristics,
and on the primary fine and coarse aggregates composition, fraction and relative
fraction. On the other hand, old mortar adhered to primary aggregates results in the
inherent presence of an ITZ within the secondary aggregate (Akhtar and Sarmah,
2018)(Fig. 14.12D, arrow). This variability also influences the density value of
resulting aggregates, which is usually lower than that of NA because of the higher
porosity fraction introduced by the presence of adhered old mortar (Etxeberria
et al., 2006; Silva et al., 2014). There is also a clear relationship between the size
of RA and their density, since the amount of mortar adhered to the surface is deter-
mined by the number of comminution stages during beneficiation (Silva et al.,
2014). The use of sequential mechanical processes decreases the amount of cement
paste adhered to coarse aggregates and as RA are progressively broken up, cement
paste accumulates in the fine fraction; also for this reason, the density of fine RA is
lower than that of coarse RA from the same origin (Silva et al., 2014). RA are
rougher, more irregular and more angular than standard sands used in the produc-
tion of concrete (Silva et al., 2014). However the size, shape and roughness of RA
depends on the type and number of fragmentation stages, with a higher number of
stages leading to more spherical and less sharp particles (Silva et al., 2014).
Previous studies (Nagataki et al., 2000; Shima et al., 2005; Ulsen et al., 2013;
Pedro et al., 2015) have shown that the number and type of comminution processes
adopted, as well as the type of treatment carried by the RA have a major influence
on the morphological, physical and chemical properties of RA. If the RA have the
same number and type of comminution processes as the NA, the properties ofre-
cycled gggregate concrete will resemble those for natural aggregate concrete
Several treatment techniques have been tested to improve the quality of the RA.
Those can be categorised in two main approaches: one corresponds to the improve-
ment of the RA’s quality and of their surface properties by removing the detrimental
materials (adhered mortar, light contaminants and organic matter, among others);
the other corresponds to the improvement of the quality of the RA by enhancing
their properties, either at surface or through the bulk aggregate volume. The former
treatments include the removal of mortar adhered to the RA, either by mechanical
grinding (Ulsen et al., 2013; Pepe et al., 2014; Bonifazi et al., 2015; Pedro et al.,
2015) or by using ultrasound agitation to detach loose particles (Katz, 2004), with
improved results achieved in all cases. Heat treatment methods have also been
tested (Fathifazl et al., 2009; Al-Bayati et al., 2016a), leading to improvement on
the physical properties of the RA for temperatures up to 350oC; as for temperatures
%
higher than those, the RA will suffer adverse thermal expansion and cracking. Some
other studies include the use of acid soaking procedures on the RA in order to
remove adhered mortar (Tam et al., 2007; Ismail and Ramli, 2013, 2014;