Page 383 - New Trends in Eco efficient and Recycled Concrete

338 New Trends in Eco-efficient and Recycled Concrete

The steel industry generates many by-products and some are widely used suc-
cessfully in different applications. A good example is steel slag as a concrete
aggregate.
From the environmental point of view, the results of leaching tests have shown
that, except for the pH value due to the slag’s lime content, the heavy metal content
is minimal in terms of environmental impact. The mineral component chromium is
the most conflictive element, but leachate concentrations are low because chromium
ions bind within stable crystalline phases. The other concentrations of heavy ele-
ments are insignificant. On the other hand, organic components do not exist due to
the high melting temperature of around 1600 C(Motz and Geiseler, 2000).

The elimination of municipal solid waste (MSWI) through thermal treatments
generates different types of solid waste (Vandecasteele et al., 2007). These residues
contain high concentrations of heavy metals and toxic organic substances from the
environmental point of view. There are studies for their possible application in
building materials (Mitchell et al., 1992; Chandler et al., 1997).
With the incorporation of MSWI in cement or concrete pastes, the accumulated
concentrations of all elements in the leachate are lower than the limit values.
Therefore, there is no environmental restriction to use these materials in mortar
mixtures of cement and concrete (Saikia et al., 2008).

12.2.3 Leaching behaviour of coarse recycled materials

At present, the residue most used in the manufacture of concrete is the recycled
waste from CDW, providing a high environmental and economic benefit, (Dhir
et al., 1999; Poon et al., 2002; Sagoe-Crentsil et al., 2001). Various studies have
demonstrated the suitability of coarse recycled aggregates for use in concrete
(Limbachiya, 2004; Meinhold et al., 2001).
To evaluate the environmental impact of applying RA, the availability of leach-
ing should be analysed instead of the total content, because leaching depends on the
solubility, and the solubility depends on the pH, composition of the material, con-
tact time, relationship L/S and permeability (Van der Sloot, 2004; Townsend et al.,
2003; Medina et al., 2014; Tiruta-Barna and Barna, 2013).
Regarding the heavy metal content, Romero et al. (2013) revealed that com-
pounds that have a leachability close to inert waste in accordance with the EU land-
fill directive are Ni, Cr, Sb, Zn, Cu and sulphate. Butera et al. (2014) concluded
that Se, Cr, Sb and sulphate are the most critical components. CEN/TS 15862
(2012) analysed the RA of a large number of treatment plants in Spain and detected
Cr and sulphate as the most critical components.
Aggregates from black slag from electric arc furnaces is another industrial by-
product used for the manufacture of concrete (Manso et al., 2004) as this has a high
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relative density, between 3.1 and 4.5 kg/m , which is much higher than that of natu-
ral aggregates. This difference must be taken into account in dosages and when
considering transport costs. Due to the presence of lime oxides and free magnesium
in its composition, the black steel slag has an expansive nature. Free lime hydrates
quickly and can cause large volume changes within a few weeks, while the