Page 205 - New Trends in Eco efficient and Recycled Concrete

176 New Trends in Eco-efficient and Recycled Concrete

Figure 7.7 Detail of the surface shape of the slag.

The manufacture of the aggregate from EAFS can be performed by screening, or
by crushing and screening. It has been observed that EAFS aggregates from a previ-
ous crushing present mechanical characteristics which are slightly higher than those
of aggregates obtained from simple separation by size, or screening. Likewise, it
has also been verified that the use of water for the slow cooling of EAFS also pro-
vides aggregates of better mechanical performance because, as in the previous case,
the weak fractions of the aggregate are eliminated during crushing and, on the other
hand, in the thermal shock breakage. However, it is important to point out that
EAFS cooled with water has not been rapidly cooled and, consequently, its struc-
ture is mostly crystalline.
EAFS incorporation into concrete and asphalt mixtures improves the resis-
tance to wear, rigidity and compressive strength when compared with natural
aggregates.

7.2.2 Specific gravity and absorption

EAFS presents a high content of Fe oxides and even metallic Fe. The amount of
metallic iron is minimised by crushing and is then recovered using electromagnetic
fields. The density of the aggregate decreases in size due to the presence of Fe.
3
Coarse EAFS aggregate presents an average density of 3.5 g/cm . It is a very dense
material and very cavernous. Fig. 7.8 shows the cavernous shape of the EAFS due
to the retained gas during cooling. Due to the high density, it is recommended to
design consistent cement pastes in order to avoid aggregate segregation.
The absorption of EAFS is between 1 wt% and 4 wt%. However, due to the
high density of the solid fraction of these materials, it is not convenient to use this
factor to describe the porosity of the material since, given the volume of pores, it
will have a significantly lower absorption than natural aggregates. This is because
the calculation is made by taking into account the weight of the sample.
Therefore, it is recommended to use the pore volume or porosity as a characteris-
tic parameter.