Possibilities of coal fly ash utilization in the low temperature belite cement production

Abstract

At present, belite cement synthesizing has become the hotspot in cement-concrete materials science. Many investigations have been realized in order to synthesize belite (β polymorph modification of C2S) from coal fly ash as an alternative secondary raw material. Belite cements and mortars were formed in accordance with Jiang and Roy´s method based on hydrothermal treatment of coal fly ash and subsequent calcination of formed calcium silicate and aluminate hydrates. Another mechanochemical way of low temperature belite synthesis consists in high-energy milling of reactive mixture based on coal fly ash in combination with heating. This paper summarizes the results of an extensive investigation of application of both methods for synthesizing belite cements from reactive mixtures consisting of two waste kinds (bottom ash – BA and fly ash - FA) from fluidized brown coal combustion in Slovakian power plant and CaO (analytical grade reagent) addition. Starting mixtures with CaO/SiO2 molar ratio of 2 were hydrothermally activated in rotating autoclave under optimal conditions (200°C and 4 hours) and mechanochemically treated during 2 hours in high-energy planetary mill. The starting mixtures and products of treatments were subsequently calcinated at temperatures in the range of 600 to 1200°C. Phase composition development in all products was studied by X-ray diffraction (XRD), FT infrared spectroscopy (FTIR), thermal analysis and scanning electron microscopy (SEM). Comparative study of phase composition of products won by hydrothermal and mechanochemical treatment and subsequent calcinations of reactive mixtures based on two kinds of waste with different CaO content showed that fluidized coal ash matter with low CaO content in fixed form is a suitable material for belite cement production. Any of two applied pre-treatment methods led to activation of anhydrite structure presented in bottom fly ash; therefore the low conversion degree to belite phase was reached.