Page 256 - New Trends in Eco efficient and Recycled Concrete

224 New Trends in Eco-efficient and Recycled Concrete

the recycled aggregate supply distances could be compensated by using photovol-
taic energy to power the recycling plant (with a 45 km limit distance) or an increase
in the production rate of the recycled materials.
Hossain et al. (2016) stated that the production of 1 t of natural aggregates emits
23 33 kg CO 2-eq , respectively for river sand and crushed stone; while the produc-
tion of 1 t of fine recycled aggregates only generates 12 kg CO 2-eq . Similarly, the
production of 1 t of gravel emits 32 kg CO 2 equivalents and the production of 1 t of
coarse recycled aggregates generates 11 kg CO 2 equivalents. Therefore, coarse
recycled aggregates from CDW generated 26%, 38%, 30%, 66% and 57% lower
impacts than natural aggregates in terms of acidification (both terrestrial and
aquatic), human health, ecosystem quality, climate change and resources, respec-
tively, which represents a total environmental gain of 51%. Regarding fine recycled
aggregates from CDW, reductions about 25%, 36% and 45% were observed for
acidification potential (both terrestrial and aquatic), climate change impact and
resource consumption compared to natural aggregates, which entails a 49% environ-
mental gain. Moreover, the sensitivity analysis showed that the results remained
beneficial (with changes lower than 12%) for the variation of transport distances up
to 20%.
Rosado et al. (2017) found that the major comparative environmental gains
between recycled and natural aggregates were linked to terrestrial ecotoxicity
(78%), nonrenewable energy consumption (68%), respiratory inorganics (61%) and
global warming potential (43%). Thus, the production of recycled aggregates from
CDW environmentally was considered preferable (except for the impact category
covering noncarcinogens) to the use of natural basalt aggregates for road applica-
tions, when the distance from the recycling plant to the construction site was lower
than 20 tkm.
Based solely on energy consumption, Wijayasundara et al. (2017) observed that
the management of recycled aggregates presented a worse environmental balance
than the production of natural aggregates. Despite the direct energy from natural
aggregate extraction being greater than the energy required for sorting and jaw-
crushing CDW, the secondary crushing of the recycled aggregates is 32% more
demanding than the one employed for natural rocks. This is attributed to plant spe-
cific conditions and the higher hardness associated concrete waste in comparison
with natural aggregate rock. In addition, the need for transporting the CDW from
the building site to the recycling plant was greater than the haulage of natural
aggregates, which resulted in increased energy consumption for the recycled materi-
als. So, at the end of the management process, recycled aggregates involved 28%
more direct energy.
Finally, in terms of the trade-offs experienced due to the different alternatives of
utilisation, the concepts of substitution ratio and down-cycling play an important
role. In some cases, the recycled material cannot replace the primary material in a
1:1 substitution ratio, that is, a greater amount of a secondary material should be
used to achieve the same function. In other cases, despite a 1:1 substitution ratio,
several replacing opportunities exist for the same pair of primary/recycled materi-
als. Under this scenario, the preferable option lies within the reuse of the secondary