Page 384 - New Trends in Eco efficient and Recycled Concrete

Leaching issues in recycled aggregate concrete 339

hydration of magnesium occurs much more slowly. The leachate of these slags can
have a pH higher than 11 and, therefore, present corrosion problems in aluminium
and steel pipes that are placed in direct contact with them. The leaching of Mo, V,
Se, Cd and fluorides is especially recommended. In general, the leaching phenome-
non in normal concrete and concrete with black steel slag is similar for all metals
except chlorine.

12.2.4 Applications based on environmental behaviour

Two important limitations for the use of industrial by-products and waste in con-
struction applications are the technical feasibility and in respect with the environ-
ment, which have to be demonstrated. This means that the content of heavy metals
and toxic organic substances must be low or bound to the material so that their
migration to the environment is minimal.
Industrial by-products may contain different contaminants that can potentially
filter through and contaminate the underlying soil and groundwater. Currently there
are no universal specifications to address the environmental impact of the reuse of
by-products in construction.
In the case of concrete, leaching by water can severely damage structures like
dams, buildings and pavements. The kinetics of leaching are very slow, but can be
a risk in the long term.
When a material such as concrete is in contact with water, ionic transfers occur
between the external water and the pore solution of the material. This causes a
modification in the state of chemical equilibrium, specifically that of hydrates.
Usually, the equilibrium is restored whenever possible by the precipitation and/or
dissolution of hydrates. However, the porosity in the degraded areas increases and,
consequently, the mechanical behaviour and physical-chemical properties decrease.
It has been shown that leaching depends on the material, the environmental con-
ditions and the type of cement. That is to say, the greater the porosity, the pH and
the temperature between the pore solution and the water, the more accelerated the
ionic diffusion. This results in greater leaching.
An adequate solution for the use of industrial by-products, is that they comply
with the physical, chemical, mechanical and environmental requirements for those
that are going to be used.
Next are some pilot tests where all these requirements are studied:

Study of application of mixed recycled aggregates for pervious concrete and
backfilling trenches at Barcelona city.
Etxeberria et al. (2016).

This study presents a pilot test carried out as an innovative initiative in the reuse
of construction and demolition wastes (CDW) in the city of Barcelona. The use of
mixed recycled aggregate (ARM) in the manufacture of porous concrete as a base
for drainage paving was studied.