# Utilization of Waste Clay-Diatomite in the Production of Durable Mullite-Based Insulating Materials
## 1 Introduction  
Guided by economic and ecological approaches, many scientists and professionals are increasingly utilizing industrial, construction and mining waste in accordance with the Sustainable Development Goals (SDGs), which are implemented through the concept of the circular economy. Since the circular economy of materials has a long-term impact on minimizing negative environmental impacts such as air, soil and groundwater pollution, the utilization of natural and waste materials is increasingly being used in the production of lightweight concrete (LWCs) as lightweight aggregates [1,2]. In recent years, significant attention has also been directed toward geopolymer concretes, which are considered a sustainable and environmentally friendly alternative to conventional Portland cement in traditional cementitious systems. Recent comprehensive studies have emphasized the potential of these environmentally friendly building materials, which are produced from extracted soils and sediments such as clays, mining waste, slags, etc. In particular, these mining wastes have proven to be effective precursors for geopolymer binders [3, 4, 5]. Given their promising properties, such materials could be used in the construction of highways, tunnels and embankments, enhancing roadway performance of roads, including improved fire and frost resistance [6, 7, 8]. Another valuable application of industrial waste and natural materials, including mining waste such as kaolin, diatomite, bauxite and other minerals that accumulate in landfills, is in the production of highly durable refractories. As already mentioned, the use of such materials for the production of high-performance technical ceramics remains a challenge. It requires innovative strategies for the management of sustainable post-mining landscapes and balanced societal decisions to promote a green economy--one that provides equitable solutions for both people and the planet. At the same time, the proper design of ceramic materials is essential to achieve the appropriate technological properties (i.e., thermal, mechanical and chemical) and performance (e.g., highly resistant mullite refractories) for various industries such as iron, steel, cement, glass, petrochemicals, etc. Solving the problem of industrial, construction and mining waste not only mitigates air and soil pollution but also enables economic savings through the production of highly durable mullite refractories [9, 10, 11]. One of the most common industrial waste materials used for the production of mullite refractories is coal fly ash, which remains one of the most significant environmental problems in today's society [12,13]. The utilization of coal fly ash, which is typically located at uncontrolled landfills, supports the implementation of a circular economy, thereby mitigating growing pollution and depletion of natural resources. In addition, natural and mining-derived materials such as kaolin, diatomaceous earth and bauxite are also used extensively in the production of lightweight bricks, panels and precast elements. These materials significantly reduce structural loads and enable the design and construction of more efficient high-rise buildings and structures. In addition, precast elements can be designed to fulfill specific project requirements, including acoustic performance, fire resistance and thermal insulation [14, 15, 16, 17]. The applicability of natural materials and waste/by-products in the production of mullite-based refractories has already been confirmed. Highly vitreous bodies offer specific advantages in processing [18]. First, the high viscosity of the glassy phase (high content of SiO\({}_{2}\)) allows the ionic diffusion of nucleating agents essential for the formation of mullite. This promotes the growth of mullite within the network and forms a loose frameworkof randomly arranged needle-like structures.