Characterization of the microstructure and
mineral phases of German iron and steel slags

1 Introduction
The recycling of iron and steel slag is currently an urgent and important problem for all scientists throughout the world, but particularly for Chinese scientists. In Germany there is already a great deal of experience with the recycling of iron and steel slag. The tests to characterize the microstructure and mineral phases of German iron and steel slag from different production processes have provided practical guidelines for improving the quality of iron and steel slags in other countries, especially China. Good quality is an essential requirement for effective and comprehensive use of iron and steel slag in its various applications.
 
The term iron and steel slag is derived from metallurgy [1]. Iron and steel slag is a non-metallic melt that is obtained during the production of pig iron and steel. After cooling slowly in air it is obtained as a synthetic, sometimes crystalline, rock. Iron and steel slags include granulated blastfurnace slag (GBFS), air-cooled blastfurnace slag (ACBFS), and steel slag etc. Depending on the steel-making process used a distinction is made between basic oxygen furnace slag (BOFS) from the Linz-Donawitz process, Siemens-Martin slag (this slag is no longer found in Germany as the method is no longer used) and electric arc furnace slags (EAFS).
  2    Origin and chemical composition
of the iron and steel slags
BOFS, EAFS, GBFS and ACBFS were selected as typical starting materials for the investigations described here. The chemical compositions as well as the microstructures and properties of the slags are closely related to one another. The chemical compositions and true densities of the slags under investigation are shown in Table 1.
 
All the slags are lime silicate melts that exhibit differences in basicity – the (CaO + MgO): (SiO2+Al2O3) ratio. The basicity of blastfurnace lump slag is normally low. The oxides CaO and MgO, which come from the limestone and dolomite, combine with SiO2 to form melilite or merwinite during the slow cooling of the slags.
Basicity is also an important characteristic value for steel slags. According to MASON [2] the steel slags can be classified by their CaO : SiO2 ratios. The following main mineral phases are formed in the slags depending on the basicity:
­–    monticellite slag with a basicity of 0.9 – 1.4
­–    merwinite slag with a basicity of 1.4 – 1.5
­–    belite slag with a basicity of 1.6 – 2.4
­–    alite slag with a basicity of > 2.4
 
3    Mineralogical compositions
of the iron and steel slags
The mineralogical compositions of the slags are crucially important for their working properties. The mineral phases of the slags under investigation are described below.
 
3.1 Granulated blastfurnace slag
Granulated blastfurnace slag is a latent hydraulic material and is completely amorphous. Two different samples were chosen to provide better characterization of the microstructure of granulated blastfurnace slag. One sample was taken from the part of the slag that represents the main constituent – this constituent is light grey (GBFS1). The other sample was taken from a secondary constituent of the slag that is only present in small ­
quantities – these are black slag constituents (granules) (GBFS2). Figure 1 shows the microstructural features of these samples (Fig. 1a: GBFS1; Figs. 1b and 1c: GBFS2). GBFS1 is amorphous while GBFS2 also shows crystalline fractions of melilite and calcite. The main elements were determined by EDX analysis.
 
3.2 Air-cooled blastfurnace slag
The mineral phases melilite and pseudowollastonite separate out when the liquid blastfurnace slag is cooled slowly in beds. Figure 2 shows the microstructure of ACBFS. The constituents of the ACBFS determined by X-ray diffractometry and the ­Rietveld refinement are shown in Table 2 and the ­constituents also determined by SEM and EDX are shown in Figure 2. There are also constituents that occur in small quantities and cannot be determined by XRD, such as gypsum and barytes.
 
The XRD and SEM/EDX investigations show that melilite can be regarded as the basis of ACBFS. Pseudowollastonite, with its fishbone structure, was also detected. Small quantities of ingot-shaped gypsum as well as petal-shaped barytes and calcite form on this lattice-like or tetragonal base. Certain quantities of amorphous silica and calcium oxide resembling cotton wool were also found.
 
3.3 Basic oxygen furnace slag
Air-cooled basic oxygen furnace slag often contains free oxides, in particular free CaO and periclase. As a rule the content of free CaO and periclase is higher in BOFS than in EFS. The mineralogical composition of the BOFS under investigation is given in Table 3. Significantly rounded formations of belite and free lime can be seen (Fig. 3). The volume stability of BOFS is linked closely with the levels of free lime and free magnesium oxide. On admission of water these can hydrate and double in volume. Portlandite (Ca(OH)2) can be seen as a leaf-like hydration product of free lime (Fig. 3).
 
3.4 Electric arc furnace slag
During the production of EAFS the most important preconditions for the mineralogical composition of the slag lie in the choice of raw materials, especially the lime and admixtures. By setting suitable process parameters during the melting process a slag can be obtained that normally has a high volume stability. The mineralogical composition of the slag under investigation is given in Table 4. The mineral phases formed in the EAFS are shown in Figure 4. Hexagonal hematite can be detected there in the pore voids; isolated points of hexagonal or bone-shaped wustite, locally distributed monticellite and magnesium-aluminium spinel, melilite and dispersed barytes can be detected in the fragment of EAFS.
 
The EAFS investigated here is a slag with a high MgO content, in which the majority of the MgO is normally combined as an RO phase (FeO ∙ MgO ∙ MnO, wustite). A high MgO content  influences the high volume stability of EAFS [3].
 
4    Usage and outlook
The characteristics and structures of the iron and steel slags investigated differ significantly from one another. These differences must be borne in mind during their use. Granulated blastfurnace slag is often used in cement and concrete. It is vitreous and has latent hydraulic properties. Granulated blastfurnace slag used in conjunction with an activator (e.  g. Ca(OH)2, CaSO4) can harden hydraulically within an industrially useful time. The hydraulic properties depend essentially on the vitreous content and chemical composition of the granulated blastfurnace slags. Certain amorphous mass fractions and (CaO + MgO): SiO2 mass ratios are required in the DIN EN 197-1 cement standard [4]. Granulated blastfurnace slag is also used as an admixture for reducing a harmful alkali-silica reaction. A lower basicity is an advantage here.
 
Air-cooled blastfurnace slag is normally used in the construction of roads and railway tracks, etc. The higher the crystalline content of the blastfurnace slag the greater is its strength and durability either in the basecourse in road building (ACBFS as a mineral mix) or in concrete (ACBFS as aggregate).
 
Steel slag is often also used as a building material in road construction, soil engineering, hydraulic engineering, etc. The volume stability of the slag is the most important criterion for its use. Periclase and free lime are therefore dangerous constituents in BOFS. BOFS does in fact have a high activity at high basicity, but a high basicity can also lead to high levels of CaO and MgO. Belite and srebrodolskite can also occur in the BOFS so these slags are also of interest for cement production.
 
The effective rate of utilization of granulated blastfurnace slag in China is already 100  %. In contrast to this the rate of utilization of steel slag is only 50 to 60  %. At present more than 14 million t steel slag are produced annually in China and 0.18 billion t are being stored [5]. It is important to gather as much information as possible about the mineralogy and properties of iron and steel slag to achieve the highest possible recycling rate.
 
5    Conclusions
­–    The various iron and steel slags differ in their chemical and mineralogical compositions as well as in their characteristic parameters, such as basicity and proportion of amorphous constituents, and in the levels of periclase and free lime. The basicity of blastfurnace slags is normally lower than that of electric arc furnace slags, and basic oxygen furnace slag has the highest value. Granulated blastfurnace slag is typically vitreous. Air-cooled blastfurnace slag is dominated by melilite and is highly crystalline (crystalline content of 76.1 %). Pseudowollastonite, quartz and iron are found in air-cooled blastfurnace slag. Basic oxygen furnace slag has an amorphous content of 63.7 % and contains a crystalline fraction of belite and srebrodolskite. Hematite, periclase, portlandite and free lime are also contained in the crystalline fraction of basic oxygen furnace slag. Electric arc furnace slag has an amorphous content of about 50  % and a main crystalline fraction of monticellite and wustite. The other crystalline fractions in electric arc furnace slag are hematite, hercynite, melilite, quartz, barytes and magnesium-aluminium spinel.

–    ­Granulated blastfurnace slag and air-cooled blastfurnace slag contain no free MgO or free CaO and have good volumetric stability. In electric arc furnace slag there is a high MgO content (11.7 %) and a low content of free CaO. In this case it is therefore necessary to take the high MgO content of the electric arc furnace slag into account. Basic oxygen blastfurnace slag has a high content of periclase (1.9  %) and also a certain quantity of free CaO (0.6  %). These two have a great influence on the volume stability of the basic oxygen blastfurnace slag.

­–    The iron oxide content in blastfurnace slag is very low (0.5  % in granulated blastfurnace slag and 0.6  % in air-cooled blastfurnace slag), while in steel slag the iron oxide content varies from more than 20  % to 44.2  %. Iron and hematite are therefore frequent mineral phases in steel slag. The iron oxide content is higher in electric arc furnace slag than in basic oxygen blastfurnace slag.

­–    Iron and steel slags are used in a variety of areas depending on their characteristics, so potential problems that could affect the particular usage have to be borne in mind.
 
Acknowledgements
We gratefully acknowledge financial support from the German Academic Exchange Service (DAAD) for project A/03/14907 and the National Natural Science Foundation of China (NSFC) for project number 50678139.