USE OF ADVANCED MICROSCOPICAL TECHNIQUES FOR THE CHARACTERISATION OF INORGANIC POLYMERS SYNTHESIZED FROM Fe-RICH METALLURGICAL SLAGS

摘要

The materials under investigation were a series of inorganic polymer samples prepared from fayalite slag from the non-ferrous metal processing industry. Inorganic polymers are materials consisting of a polymer or polymer network which does not include carbon in the skeletal structure. They are processed by mixing a solid precursor and an alkaline activating solution. The gel phase, which is formed through the dissolution of the solid species in the alkaline solution, consequently transforms into a solid binder through a condensation reaction. As the processing has a large effect on the microstructure and thereby on the final mechanical, physical and chemical characteristics of the inorganic polymer, an accurate and detailed characterisation of the microstructure is of the utmost importance, including knowledge on whether ferrous or ferric Fe is present in the binding phase. This paper presents a summary of the microscopical characterisation techniques applied to these inorganic materials at the Department of Materials Engineering at the KU Leuven. As the binder phase of the inorganic polymer material is a relatively soft material compared to the embedded particles, sample preparation using only mechanical grinding and polishing resulted in too low surface quality for the envisioned electron probe microanalysis (EPMA) and electron backscatter diffraction (EBSD) analyses. The use of a cross-section polisher (JEOL IB-09010CP, Ar flow of 3.5 L/min, 6 kV, 8 hours) improved the surface quality, and thereby enabled the envisioned analyses. Figure 1 provides a summary of the microscopical techniques applied to a series of geopolymer samples prepared using either 30 % or 70 % sodium silicate. The difference in matrix consistency was revealed by high-resolution imaging using a Nova Nanosem 450 from FEI, operated in low vacuum mode. Using the same equipment and conditions, EBSD was used to identify the crystallographic nature of the particles and the binder phase. For the elemental distribution, wavelength-dispersive X-ray spectrometry (WDS) mapping on a JEOL JXA-8530F was performed, for which the high spectral resolution enables to distinguish the Na K-line from the Zn L-line. For in-depth understanding of the Fe behaviour during the inorganic polymer processing, the oxidation state of Fe was investigated by recording the ratio of the intensities of the Fe L_α- to Lp-peaks (based on ref. [1]) after melting the inorganic polymers and reference materials using the lithium metaborate dissolution technique. This analysis suggests the presence of rather Fe~(3+) than Fe~(2+) within the inorganic polymers (Fig. 2). Although the effect of the matrix has been considered to some extent, further elaboration is recommended to confirm the applicability of the analysis method for this purpose.