Page 253 - New Trends in Eco efficient and Recycled Concrete

Life cycle assessment applied to recycled aggregate concrete 221

them during storage (Butera et al., 2015). Furthermore, Souto-Martinez et al. (2017)
proposed a model to predict such environmental credit as a function of total con-
crete volume, total exposed concrete surface area, cement type, cement content per
unit mass of concrete, secondary cementitious materials type, per cent-replacement
of cement with secondary cementitious materials, CO 2 exposure classification, and
time. Finally, from the perspective of the carbonation level of the recycled aggre-
gates, a diminishing effect on the leachability of cationic metals can be expected
(Van Gerven et al., 2004), which can lower possible toxic impacts. Nonetheless,
more information is needed in this regard since Butera et al. (2015) reported that
the 15% carbonation required to compensate the CO 2-eq of CDW resulted in
increased carcinogenic human toxicity and ecotoxicity due to the increased oxya-
nion leaching.
Besides those environmental benefits, it cannot be denied that the processing of
CDW also generates an environmental burden since it implies an important energy
consumption. CDW treatment plants, which could be fixed or mobile, classify and
valorise the different waste fractions through a mechanical processing that could be
summarised in waste reception, screening, primary crushing, conveying and mag-
netic separation, secondary crushing, sorting (wind-shifting and manual triage), sec-
ondary screening, material transport and storage. The final processing intensity
would vary depending on the level of contamination and the expected application
(Hansen, 1992).
A common practice to calculate the environmental impact of the recycling pro-
cess is to consider the consumption of fuel and electricity per tonne of CDW treated
(Blengini and Garbarino, 2010; Estanqueiro et al., 2018; Lo ´pez Gayarre et al.,
2016; Mah et al., 2017; Marinkovi´ c et al., 2010; Mercante et al., 2012; Turk et al.,
2015). Results vary greatly among researchers as the values provided are case sensi-
tive (type of CDW plant, equipment employed and process implanted—dry or wet).
For instance, according to Lo ´pez Gayarre et al. (2016), a stationary CDW plant
needs 25.8 MJ/t (gasoil powered) and 10.59 MJ/t (wet processing) or 8.60 MJ/t (dry
processing, electricity powered), whereas a mobile CDW plant requires 25.96 MJ/t
(gasoil powered), which are among the highest values found in the literature.
In addition, the crushing and handling operations and the wind action produce
10
2.5
dust (coarse, PM ; D # 10 μm; fine, PM ; D # 2.5 μm; and ultrafine particles;
D , 100 nm) which can be potentially damaging for the human health, especially
for the workers at recycling plants. Research by Kumar and Morawska (2014),
which simulated the environment of a concrete management plant, found that 93%
of the particles were ultrafine and presented a slow decay rate (62% more distance
to reach 10% of their initial concentration) and a high deposition rate within 10 m
of the source. The authors stated that the exposure at operational sites could be
reduced by half when using dust respirators. Noise and vibration are also generated
by processing operations, especially from the engines, crushing and screening
equipment and vehicles. Visual and aesthetic impacts of the recycling plant also
should be taken into account. Finally, the exposure of recycled aggregates to
rainwater may result in leaching that could pollute the surrounding soil and ground-
water (Galvı ´n et al., 2014; Schwab et al., 2014).