## 1 Introduction  
The level of industrialization is considered to be an important reflection of the growing economic power of a nation [1]. However, although it presents a vital indicator of technological development, the rapid industrial revolution has a major harmful impact on the environment due to millions of tons of potentially hazardous and toxic waste materials, which are being generated in industrial processes each year on a global scale [1; 2; 3; 4]. Waste sludge from industrial wastewater treatment plants is one such ecologically undesirable by-product, and is also regarded as one of the largest solid waste streams originating from wastewater-related technological operations [5]. Owing to the rather problematic chemical composition, industrial wastewater sludge has long been considered unsafe for the environment [5; 6; 7]. Hence, very stringent environmental regulations and high-cost legal practices are established for the purpose of avoiding the endangerment of the environment due to the production of such a widely presented waste product [8; 9]. Nowadays, there is a tendency in modern society, especially among the scientific community, towards contributing to the sustainability and the transition of the current linear economy to a circular economy [10; 11; 12; 13]. In this context, the fundamental goal is based on the transformation of recyclable wastes into valuable raw materials, which may be effectively used for different industrial applications. Accordingly, the reuse of industrial by-products and other waste streams as alternative Supplementary materials can promote sustainable development in different industrial fields and be beneficial from both economic and environmental points of view [6]. In general, an extensive literature survey evidenced that wastewater sludge is being widely investigated for a broad range of effective applications in numerous engineering fields. It has been reported to possess great potential to be applied as a component of construction materials [14; 15; 16; 17], as an adsorbent for the removal of different pollutants present in soil and water [13], as fertilizers and ameliorants for an enhancement of the structure and properties of soil [13; 18], and even as a potential sustainable biomass resource for energy production [19; 20; 21]. However, most of the mentioned research studies have been focused namely on municipal (sewage) wastewater sludge. Therefore, there is so far a lack of scientific work aimed toward the potential application of sludge generated in plants where wastewater from different industries is being processed. With the recent advancements in the energy industry, the demand for materials that can effectively operate under high temperatures has dramatically increased [22]. To the best of our knowledge, there are no literature reports on such refractories, which can be successfully manufactured with a certain addition of waste materials. In order to cover this gap, for the first time, in this study, we suggest the utilization of waste sludge generated by industrial wastewater treatment processes as a partial substitution for raw materials in the production of unconventional ceramic adhesives or sealants for various commercial applications. More specifically, a major focus is put on application as sealants used in Intermediate-Temperature Solid-Oxide Fuel Cell (IT-SOFC) technology and as high-temperature-resistant adhesives for providing connections in the structure of different heating devices, such as furnaces and dryers. The effects of waste sludge addition were thoroughly investigated in terms of the overall performance and commercial viability of the obtained materials, depending on the intended purpose, through a determination of the mechanical, thermal, and morphological properties.