Page 394 - New Trends in Eco efficient and Recycled Concrete

Leaching issues in recycled aggregate concrete 349

reduction in the calcium content (20 μm) identified by scanning electron micros-
copy. This process did not occur in the samples that were manufactured with high
slag.
Galvı ´n et al. (2014) studied the leaching assessment of hardened concrete made
of recycled coarse aggregate. The authors analysed the potential reuse of construc-
tion waste in concrete manufacturing by replacing the natural aggregate with
recycled concrete coarse aggregate. They observed that the incorporation of alterna-
tive materials in concrete manufacturing could increase the potential pollutant of
the product, presenting an environmental risk via groundwater contamination. They
tested two types of concrete batches that were manufactured with different replace-
ment percentages. Leaching behaviour was evaluated based on the availability test
performed to three aggregates (raw materials of the concrete batches) and on the
diffusion test performed to all concrete. In this case EA NEN 7375 was applied.
The analysis of concentration levels allowed the establishment of different groups
of metals according to their observed behaviour, the analysis of the role of pH and
the identification of the main release mechanisms. The authors confirmed the differ-
ent leaching behaviour of the more mobile metal cations (Zn and Cu) and the oxya-
nions (Cr and Mo). The levels of Mo and Cr were very close to the threshold of
maximum leachability.

12.4 pH-dependent leaching of constituents in concrete

The analysis of environmental behaviour by testing materials under controlled pH
conditions has increased in recent years due to the fact that pH is one of the most
important parameters that govern the release of components from the solid phase to
the solution (Van der Sloot, 1997).
The leaching assessment by means of pH dependence tests has been extended in
construction materials, mainly those prepared from a mixture of residues and
cement-based products (Van der Sloot, 2000; Peyronnard et al., 2009; Van der
Sloot, 2002).
The increase in leachability when the initial pH of the cementitious products is
no longer kept constant due to carbonation is a relevant aspect that must be taken
into account in the long-term evaluation of the environmental impact of mortars,
concrete and stabilised products (see Fig. 12.8).
Geochemical modelling assists in interpreting solubility control in a primarily
inorganic matrix, as the result of leaching patterns as a function of pH can be
explained. For construction materials, understanding these relationships allows pre-
dicting the leaching behaviour under different exposure conditions and improving
the quality of the products which is a key aspect when they are applied in works or
engineering infrastructures.
The pH dependence test allows the understanding of the chemical behaviour of
the contaminant, assesses its behaviour under exposure conditions other than those
imposed by the material and checks the neutralising capacity of the material.