Page 297 - New Trends in Eco efficient and Recycled Concrete

Comparative studies of the life cycle analysis between conventional and recycled aggregate concrete 261

similar results: impacts were reduced to 88% and 65% of the corresponding conven-
tional concrete’s impacts in the case of RAC and RAC with fly ash, respectively. The
main reasons for such improvements in RAC environmental behaviour were avoided
burdens: avoided waste landfilling and avoided pig-iron production if steel scrap as a
co-product of recycling was recovered.
In attributional LCA studies where allocation is used instead of system expan-
sion with substitution, results are not so beneficial for RAC. The credits from recy-
cling are accounted for only on the different waste management scenarios
comparison, not on the level of the product’s life cycle comparison (Vogtl¨ ander
et al., 2001). With this approach, at best, for low cement increase in RAC, impacts
of RAC and the corresponding conventional concrete are similar (Weil et al., 2006;
Marinkovi´ cetal., 2010; Fraj and Idir, 2017; Kleijer et al., 2017). According to
Jime ´nez et al. (2015), if a mix proportioning method (called ‘equivalent mortar vol-
ume method’) (Fathifazl et al., 2009) is used in the RAC design, the RAC impacts
are lower than those of the corresponding conventional concrete, even with an attri-
butional approach.

10.3.3 Influence of CO 2 uptake during primary and secondary
life of concrete structures

One important issue often neglected in the environmental assessment of concrete
structures is the carbonation of concrete over its life cycle. This process has been
previously investigated a lot as a concrete durability aspect, but it was rarely taken
into account when assessing CO 2 emissions during the life cycle of concrete
structures.

10.3.3.1 Chemistry of carbonation process
It is a well-known fact that cement production is responsible for a large volume of
CO 2 emission, approximately 0.8 1.0 tonnes of CO 2 per tonne of cement. About
half of this amount is emitted from the calcination process of limestone and the
other half comes from burning of fossil fuels in clinker kilns since high temperature
is needed for clinker production. The calcination process is a chemical reaction in
which limestone (which mainly contains calcium carbonate) is converted to calcium
oxide and carbon dioxide at high temperatures:

(10.1)
CaCO 3 .CaO 1 CO 2
When exposed to air, concrete will, over time, reabsorb CO 2 from atmosphere in
a process called carbonation. It is a chemical process reversed to calcination in
which atmospheric CO 2 diffuses into concrete to react with hydration products (cal-
cium hydroxide and other calcium-rich hydrated oxides) and form calcium carbon-
ate again. Carbonation is an inherent process in all cement-based materials.
Therefore, over the life cycle, part of the CO 2 released from calcination process