Page 198 - New Trends in Eco efficient and Recycled Concrete

170 New Trends in Eco-efficient and Recycled Concrete

the World Steel Association (2016, 2018), more than 400 million tonnes of slag are
produced annually worldwide by the steel or iron industry.

7.1.2 Types and classification of steel slags

Steel slags (SSs) are usually classified according to the type of furnace in which
they are produced. The properties of the slag depend on the type of process used to
produce the crude steel, the cooling conditions of the slag and the valorisation
process.
In the primary process, crude steel is produced in two ways. In the first method,
the iron is produced from ore in the BF, thus, generating BF slag (BFS). BBOF slag
(BOFS) is produced in the steelmaking process by using the molten iron coming
from the BF. In the second method, slags are generated in the scrap-based steel
industry. The first stage of the scrap-based steel industry production generates EAF
slag (EAFS) and a second stage is performed to refine the molten steel. The slag
produced in the LF (LFS) is the result from steel refining and, therefore, it is gener-
ally a heavy metal carrier such as chrome, lead or zinc (CEDEX Ministerio de
Fomento, 2018). Table 7.1 shows the characteristics of the main slag types gener-
ated by the crude steel industry.
Another aspect that determines the physical and chemical properties of the slag
is the type and speed of cooling. If the cooling of the molten slag is performed
slowly, the components crystallise into stable structures producing, in most cases, a
dense and inert crystalline material. However, if the cooling is rapid, its compo-
nents are fixed in an amorphous structure and, therefore, the slag is unstable or
active in the presence of certain substances. This is the case of the granular BF slag
(GBS) which presents hydraulic properties if it is rapidly cooled and is widely used
as a partial substitute of clinker in Portland cements.
There are two types of BFS, see Fig. 7.1, depending on the speed of cooling.
The air-cooled slags (ABS) are subjected to a slow reduction in temperature (gener-
ally to air) and, therefore, have a crystalline structure and physico-chemical stabil-
ity. Once cooled, the ABS is crushed and sieved according to the geometric
characteristics required by the application. The ABS has a density of around 2.5 g/
3
cm and is used as a concrete aggregate, for bases, subbases and road layers
(EUROSLAG; Liu et al., 2013; Nippon Slag Association). The Spanish Instruction
for Structural Concrete (EHE08) (Ministerio de Fomento Gobierno de Espan ˜a,
2008; Martı ´n-Morales et al., 2011) only allows the use of BF ABS as aggregates,
provided that the requirements regarding sulphate, sulphur and volumetric stability
are met.
GBS, see Fig. 7.2, present hydraulic capacity due to its amorphous or vitreous
structure. Its chemical composition is the same as that of ABS, but the structure
and the stability are different. To obtain rapid cooling so as to obtain a vitreous
structure, the slag is granulated. After that, the ABS is ground to sizes below
100 μm. ABS exhibits cementitious properties and so it is highly appreciated and
used in the manufacturing of Portland cements (EUROSLAG; Liu et al., 2013;