Page 377 - New Trends in Eco efficient and Recycled Concrete

332 New Trends in Eco-efficient and Recycled Concrete

in paving roads) implies an analysis of the leaching potential of the material in its
specific application (Puthussery et al., 2017).
Thus, previous works in field scenarios prove that leaching under field condi-
tions in general is relatively lower than controlled conditions in a laboratory. This
fact has been observed in several types of engineering applications: road infrastruc-
ture (Engelsen et al., 2009; Lidelo ¨w et al., 2017; Bestgen et al., 2016; Galvı ´n et al.,
2014; Butera et al., 2014; Engelsen et al., 2012) construction work as backfill or
embankments (Coudray et al., 2017; Cristelo et al., 2016) and hardened concrete
(Puthussery et al., 2017; Galvı ´n et al., 2014; Erdem and Blankson, 2014). The
leaching behaviour under real applications has been also analysed for surface roads
(Paulus et al., 2016), in the fine-grain portion used in the construction of geosyn-
thetic reinforced structures (Vieira and Pereira, 2016) or as alternative pipe backfill-
ing material for storm water and sewerage pipes (Rahman et al., 2014).
The effect of the pollutant potential depends on the release progress of the con-
taminant over time (Galvı ´n et al., 2014; Engelsen et al., 2017). The transfer of con-
taminants present in the solid of the material to the aqueous phase is based on
several physical and chemical factors.
In leaching processes, the interaction between the aqueous phase and the solid
phase is dominate/influenced by physical phenomena such as percolation, diffusion
or surface wash-out and chemical mechanisms such as solubility control by the dis-
solution of minerals, sorption control in consequence of adsorption processes or
availability due to the total content of an element. In granular materials percolation
plays a significant role due to the effect of the water flowing through the porous
material facilitated by gravity. However, other mechanisms influenced by physical
factors, such as particle properties, flow past particles, degree of saturation or phys-
ical changes due to ageing, can occur (Van der Sloot, 1997; Van der Sloot and
Dijkstra, 2004).
In order to predict the pollutant behaviour over the long-term situation, firstly a
characterisation of the dominant leaching mechanisms is needed to translate test
results to practice. For instance, the effect of grain size on leaching by percolation,
or the effect of and ageing on the chemistry of the material, may lead to a leaching
potential that is higher or lower than the standard laboratory column test result.
Grain size affects the reaction and transport mechanisms, being faster for fine parti-
cles than the coarser fraction. Thereby, transport tends to be limited/controlled by
diffusion in coarse grain size (as might occur in a monolithic material). In materials
with predominantly coarse grains with low permeability, the water tends to flow
around the product rather than enter it. Hence, the constituent shows diffusion-
controlled release. The pore space influences the transport rate of contaminants
towards the water phase; high porosity leads to a larger surface area and to a higher
release.
In the context of cement-based products, the ageing effect is relevant due to
strong alkalinity, which leads to progressive changes of pH values due to carbon-
ation. Carbonation is the reaction of predominantly alkaline materials, such as
demolition wastes, with carbon dioxide from the air. The initially high pH of the