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Steel slags 177

Figure 7.8 Detail of the cavernous shape of the EAFS. EAFS, Electric arc furnace slag.

7.2.3 Concrete using EAFS aggregate
The volumetric instability of crystalline slag (ABS, BOFS and EAFS) is mainly
due to two components, alumina and periclase, which stabilise by hydration. The
hydration of the slags can be accelerated by keeping them under irrigation or sub-
merged in water. In regions where rain is abundant, about 3 months of direct
exposure to the environment could be enough to guarantee volumetric stability. In
any case, before using any type of slag in concrete it is important to verify the
volumetric stability following, for example, the standard EN 1744-1: 2010 regard-
ing the tests to determine the chemical properties of the aggregates in Part 1:
Chemical analysis.
The use of slag can affect the properties of both fresh concrete and hardened
concrete. The two main characteristics that differentiate this aggregate from natural
ones are the surface roughness and high density.
The cavernous and irregular surface shape also gives it different qualities from
the natural ones. In reference to fresh concrete, surface irregularities cause aggre-
gates to mesh with each other. This fact could lead to a loss of fluidity if the mix
proportions are not adequate. On the other hand, as regards hardened concrete, the
irregular surface causes the physical adhesion between the cement paste and the
aggregate, which is the possible cause of the increase in mechanical properties com-
pared with more-rounded aggregates.
During the mixing process, the cavernous and irregular surface shape requires
larger amounts of water because the exposed surface is greater. Also, this shape
requires a higher volume of cement paste than natural aggregates. Fig. 7.9 shows
different types of surface shapes of aggregates. Three same-size aggregates from
the point of view of grading for the calculation of mix proportions, are represented
in Fig. 7.7. However, the rounded stone aggregate occupies a larger volume than
the crushed stone aggregate and is considerably larger than the slag aggregate. The