The authors of the paper “Characterization of the microstructure and mineral phases of slags from German iron and steel plants”  investigated the mineral composition of metallurgical slags and their use in the building materials industry, also against the backdrop of an alkali-silica-reaction in the concrete. The FEhS-Institut für Baustoff-Forschung (Institute for Building Materials Research of the Metallurgical Slags Research Association) has been dealing with this topic for a long time and was interested in getting some more answers as regards some of the facts. Dr.-Ing. Peter Drissen from the FEhS-Institut für Baustoff-Forschung, Duisburg/Germany, had a corresponding conversation with Dr. Zhao from the University of Technology in Wuhan/China.
Peter Drissen: Mrs Zhao, you and your co-authors deal with the mineral composition of metallurgical slags in your article, i.e. the slags from iron and steel making, and their use in the building materials industry. In Europe, and particularly in Germany, metallurgical slags have been used according to the corresponding standards, rules and regulations already for a long time. The level of utilization of the slags amounts to about 95 % as an average value for many years,whereof about 85 % are used in the building materials industry. What is the background of your investigations?
Qinglin Zhao: The high level of utilization means that a lot of positive experience has been gathered, in particular in Germany, as regards the use of these materials. For this reason I would like to find out, within the framework of a research stay in Weimar, what are the qualities for which fields of application. I think that this experience could be very useful for us in China. In the meantime there are similar approaches in China to make use of industrial by-products. However, these approaches have not progressed as far as here in Germany. On the other hand, this gives us the opportunity to pursue new approaches, which would not be possible in Germany without further difficulties. For instance, first experience has been gathered in China as regards the use of ground LD slag in concrete or bricks. Therefore, I am also interested in the behaviour with regard to a potential alkali-silica-reaction.
Peter Drissen: The alkali-silica-reaction is a reaction of alkalis with reactive, siliceous mineral components. It is just this point where I have difficulty in understanding some of the statements in your article. For example, the separate listing of “silica/silicic acid” as an amorphous constituent of various slags is extremely confusing for me. With the given chemical compositions it is not possible that quartz will crystallize out from molten metallurgical slags as can clearly be seen the corresponding phase diagrams. Furthermore, free silicic acid should be separated as christobalite at these temperatures.
Qinglin Zhao: In this case silica/silicic acid was only mathematically determined from the element Si based on the SEM/EDX investigations. As an amorphous constituent this is a positive assessment for the ASR since Si as very fine particles may have a buffering effect on the alkalis.
Peter Drissen: As regards the effect of finely distributed silicic acid, I agree. If, however, these are particles that correspond to a great extent to an SiO2 according to the SEM/EDX investigations, I assume they are rather grains of sand, i. e. foreign matter due to storage or preparation.
Qinglin Zhao: In the investigated lumpy blastfurnace slag 1.3 % of quartz were detected as crystalline constituent. Christobalite could not be detected. Figure 1 shows the diagram of the X-ray crystal analysis of a lumpy blastfurnace slag as an example. It clearly shows the main peak of quartz, even if the amount of quartz is very small. The main peak of christobalite does not exist.
Peter Drissen: You mention a total of 23.9 % of amorphous constituents for the mineral composition of the crystalline lumpy blastfurnace slag. Today the solidification of blastfurnace slag is controlled in the plants in such a way that it is completely solidified to a crystalline lumpy blastfurnace slag or almost completely to a vitreous granulated blastfurnace slag.
Qinglin Zhao: The connection between the vitreous portion and the cooling rate is generally known. Therefore, as I see it, lumpy blastfurnace slag may also contain amorphous portions. The crystalline portions of the slag mentioned in the paper were determined by means of a quantitative X-ray crystal analysis, which is very exact. The analytical balance, as opposed to the chemical analysis, i. e. the proximate composition, was then assigned to the “X-ray amorphous” or vitreous portion, respectively. It resulted in an amorphous portion of approximately 24 % for the lumpy blastfurnace slag investigated in this case. Figure 2 shows the microstructure of blastfurnace slag under the microscope. It can clearly be seen that the blastfurnace slag has already been well crystallized.
Peter Drissen: In Figure 2 I see large, platy crystals, probably melilites, and a finely crystalline interstitial mass. When looking at your SEM images (Fig. 3) I come to the conclusion that they are not amorphous constituents. In the SEM images exactly those ranges, which you call “amorphous”, have the typical house of cards structure of hydration or carbonatization products, respectively (Fig. 4). The detection of sulphates and gypsum, respectively, in places in your published images also argue in favour of older, deposited samples. Since hydration and carbonatization products have a great number of different mineral phases, as a rule, it is not possible or only very difficult to identify them by means of X-ray photography. Furthermore, the thin-walled structures are mostly smaller than the diameter of the electron ray used for the SEM/EDX analyses. Due to the inexact analytical chemistry these ranges cannot be assigned to any mineral phase, particularly as the portions of water of hydration and CO2, respectively, are not identified either.
Qinglin Zhao: Referring to the properties of old, deposited samples it is certainly important and helpful for the interpretation of the test results. We will take that into account for future explanations.
Peter Drissen: For LD slag and slag from electric furnaces you also write about amorphous portions. This is in contradiction to all our experience. Under technical conditions it is almost impossible to cool down slags from steel works, which are rich in lime and iron compared to blastfurnace slag, so quickly that an amorphous vitreous structure is generated. However, the very quick cooling, as opposed to geological processes, results in the partial presence of very small, intercrystallized crystallites. Figure 5 (from Figure 6 in ) shows such an intercrystallized microstructure in the µm range. Additionally taking into account the partially very complex chemical composition of the different minerals, the SEM/EDX analysis may give the impression that there is a non-crystalline, i.e. amorphous phase.
Qinglin Zhao: The amorphous portion of the investigated samples was also determined with internal standard via XRD. I conclude from your explanations that it certainly would have been clearer to list the portions specified in the tables separately as amorphous silicic acid and calcium dioxide as constituent of a mixed phase.
Peter Drissen: In your paper you also speak about iron and heamatite in the steelworks slag. When being formed steelworks slags are in contact with metallic iron. For this reason, wüstite and magnetite are the stable oxidic components in steelworks slags. In my opinion, if heamatite exists in the slags, it is oxidized iron or steel pellets generated during storage. Iron and steel pellets are not original constituents of the slags but residues of the iron or steel melt of the metallurgical process.
Qinglin Zhao: Yes, it is known that wüstite and iron may exist as constituents in LD or electric furnace slags. The presence of heamatite in these slags is really not very typical. However, the formation of heamatite is possible during cooling and also during storage in the open air in the presence of humidity. This is shown very clearly in Figure 4 in . However, the heamatite content is clearly below 3 %.
Peter Drissen: Thank you very much for the discussion.