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Steel slags 175

state and floats on top of the molten steel. The EAFS is extracted from the furnace
and is usually air-cooled (slow cooling) in order to form crystalline structures. In
modern EAFS recovery facilities, it has been seen that the water spraying of ABS
(without becoming rapid cooling) causes the slag to fracture. In this way two pur-
poses are achieved. On the one hand, the EAFS presents better mechanical proper-
ties even before being crushed. And, on the other hand, in periods of some months,
the moisture causes the hydration of the lime (CaO) and periclase (MgO) inerting
the expansiveness (Monosi et al., 2016; Pellegrino and Faleschini, 2016; Arribas
et al., 2015; Manso et al., 2004). This slag is acidic, so it is not usually cooled rap-
idly in order to achieve pozzolanicity and the main application is the use as an
aggregate for concrete, bituminous layers, bases and subbases of roads.
In the second stage of scrap-based steel production, in a reducing process to
refine the steel, the LF is used. The LFS is not suitable for use as an addition or
aggregate for concrete due to its expansiveness. However, there are instances in
which the use of the sands of LFS for the manufacture of mortars and binders has
been proposed (Shi, 2004; Tossavainen et al., 2007; Manso et al., 2013; Nicolae
et al., 2007; Qian et al., 2002; Papayianni and Anastasiou, 2006, 2010; Branca
et al., 2009; Rodriguez et al., 2009; Setie ´n et al., 2009; Montenegro et al., 2013).
The most suitable slag to be used as an aggregate in concrete is slow-cooled
slags. The purpose of this treatment is to achieve volumetric stability. However,
certain percentages of vitreous compounds do not influence the concrete properties
manufactured with them (Allen, 1948). There have, however, been some cases in
which the benefits of the EAFS for the manufacture of concrete has been demon-
strated (Shi, 2004; Tossavainen et al., 2007; Barra et al., 2001; Luxa ´n et al., 2000;
Manso et al., 2006; Tsakiridis et al., 2008), but they do not conform to the standards
and regulations as an aggregate for concrete. For example, the Spanish Structural
Concrete Instruction (Ministerio de Fomento Gobierno de Espan ˜a, 2008) only
contemplates the use of air-cooled BOFS, excluding EAFS from standardised use.
The main properties of the EAFS are summarised next.

7.2.1 Morphology and mechanical behaviour
EAFS present a cavernous and irregular surface shape when compared with natural
slag. Fig. 7.7 shows the surface shape of the slag. The shape of the particles will
have a significant influence on the calculation of the mix proportions, demanding
more water for the fluidity of the fresh concrete and making it more consistent. As
for the mechanical properties, these are improved in two ways. Firstly, this surface
will provide a more efficient union between mortar and aggregate (interfacial tran-
sition zone) and, secondly, the resistance of the solid fraction of the aggregate will
positively influence the compressive strength of the concrete. In this way, the
EAFS aggregates present excellent mechanical properties when compared to natural
ones. The values obtained from Los Angeles (Sosa, 2017) wear or crushing tests
are higher than those obtained using limestone, siliceous, ophitic or feldspar and
show a more rigid and resistant assembly of aggregates.