Page 246 - New Trends in Eco efficient and Recycled Concrete

214 New Trends in Eco-efficient and Recycled Concrete

price dependency relates to the local availability of the material. High availability
implies low prices and low economic allocation coefficients. Under such circum-
stances, more extensive use of supplementary cementitious materials is interesting
from an environmental viewpoint. When they are locally scarce and need to be
imported, their benefit quickly disappears via economic allocation due to the
increase in price (Van den Heede, 2014).
The whole allocation issue only relates to production related impacts of the sup-
plementary materials. Apart from that use and end-of-life phase issues once incor-
porated in concrete may also exist. Unfortunately, these are usually much less
documented. For instance, some toxic metals remain also present in the eventually
obtained by-products. The presence of these metals in slags and fly ashes is not
really seen as a problem. Mehta (1998) suggested that the concrete industry offers
ideal conditions for the use of these by-products, since the metals can be immobi-
lised and safely incorporated in the hydration products of cement. Leaching tests
conforming to NEN 7345 (1995) showed that paving concretes made with CEM III/
A 42.5 LA, only leach heavy metals at very low concentrations, significantly lower
than the parametric values given in European Union (1998), which defines the qual-
ity of water intended for human consumption. Thus, the partial replacement of clin-
ker with BFS, within the limits defined in NBN EN 197-1 (2011) for CEM III/A
type cements (36% 65% of slag), has no effect on the leaching behaviour of the
concrete (Marion et al., 2005). Specific data on the leaching behaviour of fly ash
and silica fume in concrete could not be found. Another problem might be the
potential Radon exhalation. Most building materials contain naturally occurring
radioactive elements. Building inhabitants may be externally exposed to gamma
rays originating from these radio-isotopes. As the presence of Radon is responsible
for the largest fraction of the natural radiation dose to the population, the tracking
of this Radon concentration is of great importance. The Radon in fly ash originates
from the coal burned in the electrical power plants. Kovler et al. (2005) found that
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despite the higher Ra content of fly ash (more than three times, compared to
OPC), Radon emanation from cement-FA pastes is significantly lower (7.65% for
cement vs 0.52% only for fly ash). However, note that little information is currently
available on the emanation behaviour of fly ash in concrete during the use phase of
the structure and after demolition.

9.2.1.3 Admixtures
It has become a common practice to incorporate small amounts of chemical sub-
stances, known as admixtures, in the mixing process of concrete to modify its fresh
or hardened properties. Although these substances are mostly water-based solutions,
toxicological concerns from its chemical composition still arise.
The European Federation of Concrete Admixture Associations (EFCA, 2018)
has published eco-profiles on six types of admixtures.
Air entrainers are mostly surface-active substances (soaps from natural resins or synthetic
nonionic and ionic tensides) with defoaming agents (EFCA, 2015a).