Screening coal combustion fly ashes for application in geopolymers

Driven by cost and sustainability, secondary resource materials such as fly ash, blast furnace slag, and bottom ash are increasingly used for alternative types of concrete binders, such as geopolymers. Because secondary resources may be highly variable from the perspective of geopolymers, it is often a challenge to upscale these binder types to an industrial scale. This paper describes the testing of a screening method – feature sizing and chemical typing (FS&CT) using an electron microscope – in order to capture the heterogeneity of secondary resources in a quantitative manner. This automated technique is able to simultaneously measure inter particle variations in chemistry (energy dispersive X-ray spectra) and size (shape). Two key variables for application in geopolymers, Si:Al ratio and size, are measured using FS&CT for coal combustion fly ash and its fraction of potentially reactive aluminium-silicate particles. These measurements have been preliminary related to the reactivity of the fly ash in NaOH-solutions with high liquid/solid ratios as well as low liquid/solid ratios (geopolymers). As such the FS&CT method is found to be a useful alternative to commonly used bulk methods such as X-ray fluorescence (XRF) or manually operated electron microscopy that gives just an indication of local heterogeneity.