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Leaching issues in recycled aggregate concrete 343

It is necessary to verify the potential of leaching presented by concrete, espe-
cially when recycled materials are applied to the waste plants, due to a reduction in
its mechanical behaviour and durability compared to conventional materials. It is
also important to check the leached components in concrete to be used for dams,
pillars of submerged bridges and pipes, etc.
Concrete leaching tests are performed by simulating processes that occur in real
situations, where water circulates in a tank around small concrete monoliths (NEN
7345). The objective of studying leaching in concrete is usually for applications in
which it can be in contact with water. There are applications where there is no risk
of leachate problems, such as in building concrete in interior areas, or in protected
outdoor areas.
In the potential leaching studies of concrete, the components dissolved in the cir-
culating water are extracted, mainly metals, sulphates, chlorides, etc., that have
been generated after the test.
It must be considered that standardised procedures are performed, usually with
deionised water. These tests give us an idea of the possible contamination that
occurs in concrete under these conditions.
It should be noted that concrete that is exposed to aggressive agents, and will
thus be exposed to intense degradation processes which can cause a greater detach-
ment of particles and components, will produce more leachates. Therefore, concrete
with a certain potential to generate polluted leachates should not be applied in
aggressive environments, such as maritime structures, reservoirs and exterior walls,
etc.

12.3.3 Evaluation of leaching behaviour on different recycled
concrete types

Concrete leaching caused by pure water involves different types of phenomena:
1. Diffusion of ionic species (Ca and OH mainly) in the pores of the cementitious matrix
due to the concentration gradients between the highly alkaline and alkaline interstitial
solution and the aggressive external solution.
2. Dissolution of the cement phases with Ca are moving by hydrolysis to supply Ca ions and
to maintain equilibrium. This process is also known as decalcinisation or leaching by dis-
solving Ca(OH) 2 by soft water. These influence the duration of water that acts as a leach-
ing agent, the speed of water, and the components that have concrete, which is the largest
amount of pozzolanic additions and lower liberation of free Ca(OH) 2 .

One of the aspects that can influence concrete leaching is the presence of
recycled materials (as recycled aggregates from CDW with high sulphate content or
steel slag which presents variable content on metals). Thus, this type of concrete
which could contain more soluble components or cause a migration in basic media
with the circulation of water, and which can potentially cause contamination on the
environment.