Preparation and characterization of glass-ceramic composites from South African coal fly ash

Abstract

Though fly ash in South Africa has mainly been utilized in the construction industry, a large portion of it is still discarded. Production of glass-ceramic composites through fly ash devitrification has gradually become one of the applications being examined internationally. This study reports the investigation of producing glass-ceramic composites from fly ash and varying quantities of beverage waste glass and magnesium oxide as main raw materials. Glasses were obtained by melting fly ash and its additives using an arc furnace. Based on their behaviour when subjected to differential thermal analysis, the glasses were then crystallised through a double-staged thermal treatment to form glass-ceramic composites. The composition and behaviour of the glass-ceramic composites were investigated using X-ray diffraction, Scanning Electron Microscopy, mechanical and chemical assessments. The crystal content in the glass-ceramic composites increased as the magnesium oxide content was increased. The desired diopside phase decreased with an increased magnesium oxide content and ultimately resulted in the formation of forsterite and anorthite as the major and second major phases respectively. The glass-ceramic composites with lesser amounts of magnesium oxide were chemically and thermally resistant and they also demonstrated high cold compressive strength.

The effect of titanium slag on the properties of glass-ceramic composites from South African coal fly ash

“…As reported in the previous study by Modiga et al (2019), the fly ash was "class F" fly ash having higher quantities of SiO 2 which is a network former. Noticeable amounts of the oxides, CaO and MgO which modify the structure of the glass were also detected in the fly ash and as well as the oxides that act as intermediates which are Al 2 O 3 , TiO 2 and Fe 2 O 3 .…”

“…The titanomagnetite slag was produced from the smelting of titanomagnetite ore to recover iron and vanadium in an open slag bath furnace setup. The chemical composition and mineralogical characterization of the fly ash and the waste glass were reported in the previous study (Modiga et al, 2019). The chemical composition of the titanomagnetite slag was determined through inductively coupled plasma optical emission spectrometry (ICP-OES).…”

“…Significant concentrations of CaO and MgO were also detected at 16.1 wt% and 12.1 wt% respectively. These oxides modify the glass structure for the formation crystal phases and CaO increases the chemical and mechanical strength of the glass-ceramic composite (Modiga et al, 2019). Additionally, MgO promotes crystallization and enhances the thermal resistance of the glass-ceramic composites.…”

“…Thus the silicate wastes are at times mixed to exploit their chemical composition for enhancing the properties of the final product (Rawlings et al, 2006). Previously, fly ash and beverage waste bottles were used in the production of glass-ceramic composites whose compatibility was close to stones and granite in terms of compressive strength and the coefficient of thermal expansion (Modiga et al, 2019). Pure magnesium oxide (MgO) was added to facilitate crystallization which is key in achieving the excellent mechanical properties of the glass-ceramic composites.…”

“…Taking the study by Modiga et al (2019) as the basis, this paper reports on the study of the effects of MgO replacement with a silicate waste, a titanomagnetite slag and varying quantities of beverage waste glass on the microstructure and properties of the glass-ceramic composites.…”