Structural study of mullite based ceramics derived from a mica-rich kaolin waste

Highlights

• Use of kaolin waste to prepare almost pure mullite ceramics.

• Rietveld refinement analysis of kaolin waste processed ceramics.

• Effect of the glass phase content on properties of fired ceramics.

Abstract

Mullite-based ceramics have been synthesized by reactive sintering of a mixture containing kaolin and a mica-rich kaolin waste. Samples fired in the temperature range from 1300 to 1500 °C were characterized by X-ray diffraction (XRD). The quantitative phase analysis and unit cell parameters of the mullite were determined by Rietveld refinement analysis of the XRD data. Mullite-based ceramics with 1.2 wt% quartz, 56.3 wt% glass (amorphous phase), 2.64 g/cm³ of apparent density, and 35±1.2 MPa of flexural strength were obtained after firing at 1500 °C. A liquid phase sintering mechanism activated by a total mica content of 13.3 wt% allowed to increase the mullite content to 47.6 wt% (2.3 wt% quartz and 50.1 wt% glass phase) and improve the flexural strength (70±3.9 MPa) after firing at 1400 °C.

Introduction

Mullite (3Al₂O₃·2SiO₂) is one of the most important refractory ceramic materials. This material can be synthesized by reactive sintering (solid state reaction) of natural or synthetic raw materials such as clays, kaolin, alumina, aluminum isopropoxide, and fly ash. The mullitization process (mullite formation reaction) by reactive sintering is a thermally activated process that takes place through ionic diffusion of Al⁺³ and Si⁺² (in a silico-aluminous mixture) at temperatures usually above 1300 °C [1], [2], [3], [4], [5], [6]. Besides the above mentioned raw materials, kaolin waste (an aluminosilicate produced in huge amount during the mining processes of kaolin) may also be used to synthesize mullite-based ceramics by solid state reaction [7], [8], [9], [10], [11]. In a recent paper [12], we reported the preparation of single mullite after sintering pure kaolin with addition of 25–50 wt% kaolin waste. In fact, acicular mullite and glass (amorphous phase) were the main phases detected around 1500 °C. Hereby, we follow this approach evaluating the phase transformations (with a proper phase quantification) of such kaolin waste processed ceramics.

In this context, the present paper is focused on the mullitization process of mullite-based ceramics derived from a formulation with 25 wt% kaolin waste and further fired in the temperature range from 1300 to 1500 °C. A detailed structural analysis of the X-ray diffraction data by the Rietveld refinement method allowed to determine the relative fraction of phases and lattice parameters of the mullite phase as a function of the firing temperature.