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Slags from the nonferrous metals industry have great potential to be used as feedstocks for the production of alkali-activated materials. Until now, however, only very limited information has been available about the structural characteristics of these materials. In the work presented herein, synthetic slags in the CaO–FeOx–SiO2 system, representing typical compositions of Fe-rich slags, and inorganic polymers (IPs) produced from the synthetic slags by activation with alkali Silicate solutions have been studied by means of X-ray absorption near-edge structure (XANES) spectroscopy at the Fe K-edge. The iron in the slags was largely Fe2+, with an average coordination number of approximately 5 for the iron in the amorphous fraction. The increase in average oxidation number after alkali-activation was conceptualized as the consequence of slag dissolution and IP precipitation, and employed to calculate the degrees of reaction of the slags. The degree of reaction of the slags increased with increasing amorphous fraction. The iron in the IPs had an average coordination number of approximately 5; thus, IPs produced from the Fe-rich slags studied here are not Fe-analogs of aluminosilicate geopolymers, but differ significantly in terms of structure from the latter.
Additive manufacturing (3D printing) of ceramics and other materials offers significant advantages compared to conventional production processes for several applications. While ceramics have been extensively investigated in this regard, additive manufacturing of geopolymers have received much less attention to date. In the present contribution we study a ‘standard’ metakaolin-based geopolymer, a fly ash-based geopolymer and a silica-based one-part geopolymer regarding their suitability for additive manufacturing via selective laser curing. Model geometries such as bars and cuboids could be produced by this route. After selective laser curing the specimens were additionally cured at 80 °C for 24 h. The specimens were studied by means of scanning electron microscopy (SEM) and powder X-ray diffraction (XRD). SEM showed that the precursors in all geopolymers had reacted partially and geopolymeric gel had formed. XRD confirmed these results and additionally revealed that the crystalline byproducts (zeolites) in the one-part geopolymer differed from the byproducts observed in conventionally produced samples. This indicates that also the geopolymerization reactions differ between the two synthesis routes. The mechanical strength after selective laser curing and 80 °C-curing appeared to be highest for the metakaolin-based geopolymer. However, SEM also showed that a significant volume of macropores remained in most regions of all specimens, while some regions in the metakaolin-based geopolymer appeared to be significantly denser. These preliminary results demonstrate that selective laser curing offers potential for the production of geopolymers, but more research has to be undertaken to optimize the process.
Topical issue: Geopolymers
(2017)
Geopolymers have gained very much momentum in recent years, in science and in industry. These highly promising and versatile materials have been researched with a view to applications as diverse as fire-resistant/refractory materials, as precursors for ceramics, in waste stabilization/immobilization, and as novel building materials. The great interest in these materials is reflected, inter alia, by the installment of a session on geopolymers at the 92nd Annual Meeting of the Deutsche Keramische Gesellschaft (DKG; German Ceramic Society) in March 2017. Therefore this Special Issue is dedicated to disseminating some of the recent findings on these materials. In addition, it contains reviews by highly experienced researchers in the field, highlighting very different aspects such as fiber reinforcement and synchrotron-based characterization methods.
Partly due to the interest from very different fields, the term „geopolymer“ is used with two different meanings. Sometimes it is used in the sense to denote the ceramic-like materials produced by activation of metakaolin with alkali silicate solutions, and closely related materials. On the other hand, sometimes it is used in a much broader sense to refer to any material that is produced by alkali-activation of many different aluminosilicate precursors, such as fly ash and granulated blast furnace slag waste materials, often serving as a cementitious binder. The present issue focusses on applications and materials that may be classified as ceramic, but also contains a review on the use of alkali-activated materials as construction materials to make the reader familiar with this approach too. Additionally, some of the materials described in this issue may be regarded as being „on the borderline“ between ceramics and cements, which is something that can be said about many studies in the geopolymer field.
The editors are very grateful to each of the authors of the articles in the present issue, as all of them have contributed high quality research based on their extensive experience. In particular, the historial perpectives on the origins and developments of geopolymers and of alkali activated materials have been authoritatively documented in contributions, by Joseph Davidovits and Pavel Krivenko. Dr. Krivenko was the Ph.D. student of Viktor D. Glukhovsky who first introduced alkali activated cements in 1957. It is hoped that the readers of this issue will enjoy reading it as much as the editors enjoyed putting it together, and that the issue will stimulate more high quality studies of geopolymers, perhaps even from research groups that have not yet been active in this area.