Page 32 - New Trends in Eco efficient and Recycled Concrete

Construction and demolition waste 9

that lignosulphate-based water reducing admixtures lead to a loss in water reduction
potential with increasing RA content (Pereira et al., 2012b). The reason for this is
that these admixtures act mainly by electrostatic repulsion and partially by steric
hindrance, adsorbing onto the surface of the particles. Therefore, because RA pres-
ent greater surface area when compared to NA, a higher admixture content is
required to obtain the same level of consistence. Polycarboxylic acid water reducing
admixtures, however, appear to have negligible loss of effectiveness with increasing
RA content (Pereira et al., 2012a,b).
A considerable number of studies have been carried out concerning the use of
mineral additions—e.g., fly ash (Kou and Poon, 2009a,b; Kou et al., 2007; Berndt,
2009), silica fume (Pedro et al., 2017a), ground granulated blast furnace slag
(Berndt, 2009; Kou et al., 2011), metakaolin (Kou et al., 2011)—in the production
of RAC to further reduce its environmental footprint and to improve specific prop-
erties. The literature findings generally indicate that RA have an inert role when in
the presence of these additions and, thus, no products of hydration are expected
between them. On the other hand, it has been reported that there may be some poz-
zolanic activity between the fine RCA’s adhered mortar and mineral additions due
to the former’s greater specific surface area when compared to coarse RCA
(Ravindrarajah and Tam, 1987; Kou and Poon, 2009b; a).
Concerning the high-water absorption capacity of RA, depending on the level of
water compensation, this aspect also has a considerable influence on the strength
development of concrete. From a workability perspective, the compensation of the
RA’s water absorption, by pre-saturating them or by adding an extra amount during
the mixing process, is likely to have similar outcomes. However, it was reported
that by simply compensating the water, which is absorbed during the mixing pro-
cess up until the slump test, will result in specimens with improved compressive
and splitting tensile strength and modulus of elasticity when compared with those
containing pre-saturated RA (Ferreira et al., 2011). The use of this water compensa-
tion method, apart from resulting in stable consistence levels, as typically resulted
in smaller losses in mechanical performance (Ferreira et al., 2011; Pereira et al.,
2012a; Soares et al., 2014a,b; Bravo et al., 2015, 2017b).
The increasing incorporation of RA in concrete, under given conditions, may
result in improved mechanical performance (Khalaf, 2006; Ridzuan et al., 2005; Ho
et al., 2013; Domingo-Cabo et al., 2010; Domingo-Cabo et al., 2009; Salesa et al.,
2017; Zhou and Chen, 2017; Thomas et al., 2016; Carneiro et al., 2014). This
enhanced strength is likely a result of effective w/c ratio reduction as a result of the
use of partly dry RA in concrete mixes with the same initial free water content as
that of control mixes. With elapsing time, the free mixing water is progressively
absorbed by the uncompensated RA, resulting in a cementitious matrix with lower
effective w/c ratio. This leads to greater heat of hydration (Koenders et al., 2014), a
finer microstructure (Garcı ´a-Gonza ´lez et al., 2015; Gonzalez-Corominas et al.,
2016) and improved mechanical behaviour (Carneiro et al., 2014). However, the
use of completely dry RA may lead to unworkable mixes, hinder their compaction
and also the cement’s hydration (Amer et al., 2016; Gupta et al., 2016), causing a
significant compressive strength decline (Pepe et al., 2016; Koenders et al., 2014).