Synthesis and Characterization of Mullites From Silicoaluminous Fly Ash Waste

1. Introduction

Mullite is an exceptionally advanced solid solution of aluminum silicate (3Al₂O. 2SiO₂), which is developed by the sintering of rare raw minerals that consists of alumino-silicate under extreme temperature and low pressure (Wang & Sacks, 2005). Mullites are crystalline compositions prominently, comprising of elements like Al, Si, and O (H. Schneider, R. Fischer, & J. Schreuer, 2015a). They are non-stoichiometric compounds structurally similar to impure magnetite which belong to the compositional series of orthorhombic alumino-silicates with the general composition Al₂(Al_2+2xSi_2‐2x)O_10‐x (Li & Thomson, 1991)(Fischer, Gaede-Köhler, Birkenstock, & Schneider, 2012). They usually exhibit two stoichiometric forms, i.e. 3Al₂O₃2SiO₂ or 2Al₂O₃SiO₂ and can be characterized by using high-end techniques like X-ray diffraction (XRD) and Scanning electron microscope (SEM) in the fly ash (Y. Gong, Sun, Sun, Lu, & Zhang, 2019). Mullites are found in high temperature metamorphosed rocks of the sanidinite and clay (Laita, Bauluz, & Yuste, 2019). They are also known as “porcelainite” as they are moulded under extremely high temperature using clay (Tripathi, Ghosh, Halder, Mukherjee, & Maiti, 2012). Among the minerals, mullites are also found in hornfels rock (porcellanite) (Searle, 1962), e.g., at the point of contact of bauxites with olivine dolerite intrusions. Special and rare occurrences of mullite are in alumino silicate lechatelerite glasses produced by lightening impact in sandstones (Pasek, Block, & Pasek, 2012). In addition to this, mullites presence can also be found in small druses of volcanic rocks (e.g., in the Eifel mountain, Western Germany), where it probably grew under moderate hydrothermal conditions (H. Schneider, Schreuer, & Hildmann, 2008).

They own magnificent and unique properties in the form of needles in porcelain (Martin-Marquez & Romero, 2010) like low thermal expansion (Oikonomou, Dedeloudis, Stournaras, & Ftikos, 2007), low thermal conductivity (L. Gong, Wang, Cheng, Zhang, & Zhang, 2014), high thermal and corrosion stability (Baspinar & Kara, 2009), high strength (Liu, 2011), high fracture toughness (Santos & Rodrigues, 2003), excellent creep resistance (Torrecillas et al., 1999), good thermal shock and stress resistance (Uribe, Moreno, & Baudín, 2001), good strength, wear-resistant and useable to high temperatures (H. Schneider, Schmüker, & MacKenzie, 2005). In addition to this, mullite is the only stable binary phase existing system of the A1₂O₃-SiO₂ under ambient conditions (Martin-Marquez & Romero, 2010). Its chemical configurations empirically include 71.8 wt.% A1₂O₃ and 28.2 wt.% SiO₂, which is designated as 3/2- mullite (3A1₂Oy₂SiO₂) (Ohtake et al., 1991). Moreover, mullite has no charge balancing cations present in them (O'Connor, Mackenzie, Smith, & Hanna, 2010). As a result, there are three different aluminium sites: two distorted tetrahedral and one octahedral (H. Schneider, R. X. Fischer, & J. Schreuer, 2015b). They have two common morphologies: platelet shaped and needle shaped. Platelet shape particles have low aspect ratio while needle shape particles has high aspect ratio. If it forms during the process of sintering, then it provides increased mechanical strength and thermal shock resistance (Chen, Lan, & Tuan, 2000).