Surface and bulk characterization of an ultrafine South African coal fly ash with reference to polymer applications

Abstract

South African coal-fired power stations produce about 25 million tons of fly ash per annum, of whichonly approximately 5% is currently reused. A growing concern about pollution and increasing landfillcosts stimulates research into new ways to utilize coal fly ash for economically beneficial applications.Fly ash particles may be used as inorganic filler in polymers, an application which generally requiresthe modification of their surface properties. In order to design experiments that will result in controlledchanges in surface chemistry and morphology, a detailed knowledge of the bulk chemical and miner-alogical compositions of untreated fly ash particles, as well as their morphology and surface properties,is needed. In this paper, a combination of complementary bulk and surface techniques was explored toassess the physicochemical properties of a classified, ultrafine coal fly ash sample, and the findings werediscussed in the context of polymer application as fillers. The sample was categorized as a Class F fly ash(XRF). Sixty-two percent of the sample was an amorphous glass phase, with mullite and quartz being themain identified crystalline phases (XRD, FTIR). Quantitative carbon and sulfur analysis reported a totalbulk carbon and sulfur content of 0.37% and 0.16% respectively. The spatial distribution of the phases wasdetermined by 2D mapping of Raman spectra, while TGA showed a very low weight loss for tempera-tures ranging between 25 and 1000◦C. Individual fly ash particles were characterized by a monomodalsize distribution (PSD) of spherical particles with smooth surfaces (SEM, TEM, AFM), and a mean par-ticle size of 4.6 m (PSD). The BET active surface area of this sample was 1.52 m2/g and the chemicalcomposition of the fly ash surface (AES, XPS) was significantly different from the bulk composition andvaried considerably between spheres. Many properties of the sample (e.g. spherical morphology, smallparticle size, thermal stability) appeared to be suitable for its applicability as filler in polymers, althoughthe wide variation in surface composition between individual particles may challenge the developmentof a suitable surface modification technique. The observation that the bulk and surface compositions ofthe particles were so intrinsically different, strongly suggested that surface characterization is importantwhen considering compatibility between matrices when applying fly ash as filler in polymers.