Metal ions as probes for characterization of geopolymer materials

Ion exchange of Na+ ions in a geopolymer by Cs+, NH4+, and Co2+ 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 Co2+ ions in the Co2+-exchanged geopolymer was used to characterize the local geometry of the Co2+ ions and the regularity and shape of local structures accommodating these Co2+ 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 NH4+ or Co2+ ions. By analogy to zeolites, this indicates that all the Al atoms are accessible through rings containing at least eight Al and Si atoms. Co2+ 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.