Metal Ions as Probes for Characterization of Geopolymer Materials

Ion exchange of Na + ions in a geopolymer by Cs + , NH 4 + , and Co 2+ was used to characterize the accessibility of the inner volume of a Na‐geopolymer (Si/Al 1.8) and the size of the rings controlling the mobility of cations in the geopolymer matrix. Ultraviolet (UV)—visible (vis) spectroscopy of Co 2+ ions in the Co 2+ ‐exchanged geopolymer was used to characterize the local geometry of the Co 2+ ions and the regularity and shape of local structures accommodating these Co 2+ ions in the geopolymer matrix. Na‐geopolymers exhibit ion‐exchange properties similar to those of zeolites. All Na + ions balancing Al atoms of the network of the as‐prepared geopolymer could be replaced (ion‐exchanged) by NH 4 + or Co 2+ ions. By analogy to zeolites, this indicates that all the Al atoms are accessible through rings containing at least eight Al and Si atoms. Co 2+ ions in the geopolymer network are accommodated in two regular types of deformed six‐membered or eight‐membered rings. As indicated by the lower extent of Cs + exchange, only one‐half of the Al atoms are simultaneously accessible through 10‐membered or larger rings. This indicates that neither regular alumosilicate six‐membered rings nor the hexagonal prisms typical of some zeolitic structures are present in the geopolymer network.