Copper slag (unground); Inset: X-ray absorption spectra at the Fe K-edge of hardened alkali-activated binders made from synthetic non-ferrous slags
Source: Photo: BAM, graph: Division Technology of Construction Materials
In the context of the national sustainability strategy and the German resources efficiency program it is the declared goal of the German government to increase the raw materials productivity. However, in many cases the valorization of byproducts is limited to the recovery of so-called ‘technology metals’ (i.e. rare metals etc.), since at present this creates the highest added-value. However, without a valorization, as complete as possible, of the main mostly mineral material throughput of this process the declared goal cannot be fully reached. One example for this are non-ferrous slags, i.e. slags that are formed during the smelting of non-ferrous metals (copper, zinc, lead etc.). These slags are currently mainly used in low added-value applications, such as aggregates for road construction or as fill, or they are landfilled.
It is known, however, that non-ferrous slags can be used to produce binders for mortars with high strength, porous ceramics as well as fire proofing systems with excellent engineering properties, which would represent valorizations with significantly higher added-value. Nevertheless, the structure of the hardened binders is hitherto unknown; this is due to their amorphous nature and their high iron content, which precludes the application of nuclear magnetic resonance spectroscopy. To guarantee a safe use of these materials, though, their structure must be determined.
For these reasons, BAM collaborated with the KU Leuven (Belgium) to produce synthetic non-ferrous slags in the system CaO-FeOx-SiO2 and characterized these and alkali-activated materials produced from the slags (so-called inorganic polymers, IPs) for the first time with X-ray absorption near-edge spectroscopy (XANES). The results show that the iron in the slags is mainly in the reduced state and that it is coordinated by approximately five oxygens on average. In the hardened binders the five-fold coordination is retained, thus, the IPs are not simply ‘iron-analogs’ of geopolymers. During hardening the iron is oxidized to Fe(III). Based on this behavior, the degree of reaction of the slags could be determined, and the amorphous fraction of the slags, which in turn is influenced by its CaO and FeO contents, was identified as an important factor for the degree of reaction.
The fate of iron during the alkali-activation of synthetic (CaO-) FeOx-SiO2 slags: An Fe K-edge XANES study
Sebastian Simon, Gregor Gluth, A. Peys, S. Onisei, D. Banerjee, Y. Pontikes
Journal of the American Ceramic Society, Volume101, Issue5, Pages 2107-2118
BAM Department Safety of Structures, Division Building Materials and Division Technology of Construction Materials