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Leaching issues in recycled aggregate concrete 341

Figure 12.4 Quay of Punta Solana (Spain).

water or with aggressive components in its composition, which can lead to the dis-
solution of concrete components progressively. This degradation process is the
main effect produced by the release of contaminating components into an aqueous
medium (Duchesne and Bertron, 2013). Therefore, in principle, the agents that can
cause the degradation of concrete must be known, which favours the processes of
leaching of pollutant components into the aqueous medium.

12.3.1 Concrete degradation by water effects
Concrete is a multiphase, porous and strong basic material. The pH of the intersti-
tial solution is around 13 due to its mineralogical and chemical nature and this fact
causes a disequilibrium with its environment (Beddoe and Dorner, 2005). Concrete
can degrade in water environments considered as pure and with much greater speed
in aggressive environments of acidic water.
Two mechanisms are involved in the degradation of concrete, permeability and
diffusion. The concrete components and the matrix-base of the concrete influence
the degradation process of the concrete.
The chemical composition of the hydrated phases and their proportions within
the matrix, which depends on the composition of the binder, have a clear influence
on the degradation process of the concrete (Duchesne and Bertron, 2013).
Water causes chemical reactions in the concrete, mainly through two types: the
dissolution of the hydrated and anhydrous phases, and the precipitation of the newly
formed products (Powers and Brownyard, 1947). These reactions often come from
outside the concrete and altered areas are progressively created in the concrete
matrix.
Once the degradation processes in the concrete matrix have begun, the processes
of component migration will begin to take place, that is, leaching in a more pro-
nounced way. The greater the degradation of the outer surface and of its internal
matrix, the greater will be the components leached by the concrete (Rosenqvist
et al., 2017).
The most common causes of concrete degradation, which will produce more lea-
chates are:

Decalcinisation.
Attack by sulphates.
Alkali carbonate and alkali silica reaction.