Crystallisation of nepheline-based glass frits through fast-firing process

Abstract

The aim of this research was to study the feasibility of producing sintered nepheline glass-ceramic through a fast firing route. The thermal behaviour of the original glass was analyzed by mean of differential scanning calorimetry (DSC) and X-ray diffractometry (XRD). The microstructural analysis of glass-ceramic materials was carried out by field-emission scanning electron microscopy (FE-SEM). The results showed that the studied wastes are able to produce a glass and glass-ceramic materials through a sinter-crystallization process, 100 µm being the critical glass particle size.Glass-ceramics are composed of residual glassy phase and crystalline phases such as nepheline (NaAlSiO 4 ), augite (Ca, Na)(Mg, Fe, Al)( Si, Al) 2 O 4 and a solid solution belonging to the melilite group (Ca, Na)(Al, Mg, Fe,)( Si, Al) 2 O 7 . In a first evaluation, water absorption (0.02%) and bending strength (71MPa) of glass-ceramic achieved after thermal treatment at 1100ºC/5min suggest that sintered glass-ceramics can be easily produced from coal fly ash and metallurgical slag wastes by a fast-firing cycle and they are extremely serviceable for outdoor flooring and wall cladding.

Kinetic study of the transformation of sodalite to nepheline

“…These curves comply with the principle of non‐isothermal process, that is, the temperature of the maximum of the exothermic peak increases with the increase of the heating rate, which allows the study of the kinetic mechanism by DTA. Thus, T p increases from 827°C to 877°C when the heating rate increases from 10°C min −1 to 50°C min −1 ; this result is in good agreement with that reported in other studies 21,46,47 . According to Bai et al., 46 the increase in temperature with heating rate is related to the fact that heat transfer becomes more difficult as the heating rate increases.…”

“…Differential thermal analysis (DTA) is extensively used as a rapid and convenient instrument to study the crystallization kinetics in glasses, to determine homogeneous crystal nucleation rates and, to obtain the activation energy for glass crystallization assuming that the crystallization process is a first‐order reaction 17–19 . In the case of nepheline, thermal analysis has been used to determine the activation energy for crystallization of glasses prepared from rice husks ash 20 and from fly ash and metallurgical slag 21 . However, to the authors' knowledge the kinetics of the thermal transformation of sodalite to nepheline using DTA technique has not been previously reported in the literature.…”

Formation of one-part-mixing geopolymers and geopolymer ceramics from geopolymer powder

“…when sintered up to 1000°C (1 MPa for metakaolin geopolymers and 4 MPa for fly ash geopolymers) even respectively containing predominant nepheline and albite crystalline phases. Meanwhile, the flexural strength was also greater than the nepheline ceramics (in the range of 14 MPa to 71 MPa) produced using fast-firing process [27]. On the other hand, using the similar processing method (i.e.…”

“…The green body was then sintered at varying sintering temperatures (900°C, 1000°C, 1100°C and 1200°C) in a furnace at heating rate of 5°C/min and soaking time of 3 hours to produce geopolymer ceramics. The range of sintering temperature was selected through trial-and-error and based on the temperature range used by previous researchers [25][26][27] in the formation of nepheline using different methods. The details of mixtures for the formation of KGC are tabulated in Table 3.…”

“…For sodiumbased geopolymers, nepheline-predominated crystallites formed when sintered at elevated temperatures [23,24]. Nepheline ceramics have hitherto been produced through sintering-crystallization, controlled devitrification and vitrification process of zeolites [25,26], fly ash and slag [27] in the temperature range of 600 -1200°C. In their study, the mechanical strength of the prepared nepheline ceramics was not studied.…”