Editorial for Special Issue “From Clay Minerals to Ceramics: Progress and Challenges”

Clay has been used in the manufacture of pottery since ancient times. Human civilization has evolved alongside the production of pottery, and pottery shards are often studied to learn about the history and traditions of ancient civilizations [1]. The unique properties of clays [2] have made them ideal for manufacturing materials for a wide variety of uses [3].

Throughout history, humans have studied the properties of different types of clay, as well as the technologies for purifying and processing them, leading to an increasing variety of applications. Among the processes studied are the pretreatment of natural clay to purify it and extract its finest components, the selection of clay types based on the desired characteristics of the ceramic to be produced, the addition of degreasing agents to control tensions and plasticity, the use of fluxing minerals, and the control of the firing temperature and the porosity of the final product [4].

Currently, the field is moving toward the sustainable use of raw materials. This is driving experimentation in the production of ceramic materials that incorporate waste into their formulations, as well as research into the manufacture of glass–ceramics and geopolymers. Using waste prevents its accumulation in the environment, promotes a circular economy, and reduces the need to extract raw materials from nature. In this regard, clays are emerging as a promising raw material for enhancing the reactivity of industrial, mining, and construction and demolition wastes, enabling their use in the production of new materials, such as geopolymers or supplementary cementitious materials [5]. This application is of vital importance in the battle against climate change, as it reduces the CO₂ emissions generated during the manufacture of Portland cement. In light of the above, there is still much research to be done in the field of clays and their ceramic applications.

This Special Issue includes eight articles that cover topics ranging from the use of clay in archeology to the improvement of clay properties and the use of waste materials in ceramic production. In the first of these, Šatavičė (Contribution 1) studied Neolithic pottery from two key sites in western Lithuania, examining thirty ceramic pieces from the fourth to third millennium BCE. They analyzed clay sources, pottery paste recipes, and the technological choices employed. This study allowed researchers to examine how environmental conditions influenced technological decisions and, conversely, how different cultural practices are expressed within a shared ecological context. The mineralogy and petrography of the samples indicate that the examined pottery was made from local Quaternary glacial sediments, and cultural traditions and the environmental context influenced the selection and manipulation of the clay. The results of the study suggest technological continuity and selective hybridization within the contact zone between hunter-gatherer communities and early agricultural communities. Potters preserved local ceramic traditions while gradually incorporating new materials and techniques brought by groups arriving from elsewhere.

Among the different types of clay, kaolinite is one of the most widely studied due to its many applications. Two contributions of the Special Issue focused on this mineral. Polcowñuk Iriarte et al. (Contribution 2) studied the transformation of kaolinite into metakaolin through experiments and calculations based on density functional theory. In the experiments, using thermogravimetric techniques, they employed a well-crystallized reference sample and an industrial kaolin. The study demonstrated the behavior not only of pure kaolinite but also of the industrial material, reflecting the heterogeneous nature of kaolin found in practical applications.

Cordeiro et al. (Contribution 3) studied the combinations of TiO₂ supported on kaolinite and of TiO₂ doped with Nb₂O₅ on kaolinite, which were applied to raw ceramic tiles and subjected to a standard firing cycle, in order to obtain a functional and cost-effective product. A photocatalytic kaolinite layer was obtained, onto which TiO₂ was deposited by adding a glass frit to produce a surface texture characteristic of ceramic tiles. The addition of Nb₂O₅ did not improve the photocatalytic performance due to increased susceptibility to removal during surface polishing. This study demonstrates the potential of TiO₂ photocatalysts supported on kaolinite for the production of self-cleaning ceramic tiles using standard single-firing industrial processes.

Durgut (Contribution 4) addresses a key aspect of clay manufacture—the grinding process. This is of great economic and technological importance. The particle size distribution of the raw materials used in the shaping stage prior to the sintering process define the microstructure of the ceramic piece and its properties. In this article, the author evaluated both batch and continuous grinding methods and analyzed the effects of the type of grinding on the body of ceramic tiles in terms of cost, capacity, and technical aspects. In this study, an analysis was conducted of the technical performance of batch and continuous mills used in the ceramics industry, and they were evaluated based on energy efficiency, capacity utilization rate, and product characteristics in a wet environment. Specific capacity and energy consumption values were compared at different mill rotation speeds. Grinding results under different conditions were compared in terms of particle size distribution, chemical composition, and sintering properties using various methods. The overall conclusion drawn by the author is that, apart from the initial investment cost, choosing a continuous mill results in a more efficient ceramic preparation process.

Candeais et al. (Contribution 5) investigate the use of red clays from the Taveiro area in Coimbra in Portugal for use in ceramics. They emphasize the value of locally sourced clay resources in supporting local economies and reducing the carbon footprint associated with transportation. Analysis of the samples and their performance confirmed that these materials are suitable for ceramic production, while also promoting environmentally responsible resource management that benefits the local community.

Karamahmut Mermer (Contribution 6) explores the use of ceramic waste powder as a supplementary cementitious material in mortar production. The author found that a disadvantage of its use was its high water absorption, so they added nanosilica to improve its performance. A silane-derived chemical was used for hydrophobization, with varying amounts of nanosilica. The addition of nanosilica and ceramic waste powder significantly improved the mechanical and durability properties of the manufactured blocks: by adding nanosilica, compressive and flexural strengths increased by up to 57% and 43%, respectively, while hydrophobic treatment reduced capillary water absorption by up to 76%. Therefore, the application of recycled ceramic waste powder in cementitious systems in small, controlled quantities is beneficial both for the environment and for the technology.

Vigneron and Holanda (Contribution 7) evaluated the effects of incorporating chicken eggshell waste in the production of low-water-absorption bi-layered red ceramic tiles. The results showed that chicken eggshell waste, when incorporated at a rate of up to 15 wt.%, can be used as an efficient source of pore-forming carbonate for the manufacture of these ceramic tiles, making them suitable for use in ventilated facades. This application makes use of eggshell waste, thereby promoting the circular economy.

Finally, Areias et al. (Contribution 8) analyzed the potential for using sludge from a municipal wastewater treatment plant as an alternative raw material to limestone in red tile formulations. The results showed that the sludge from the wastewater treatment plant exhibited good chemical compatibility for use in red brick formulations. These findings demonstrate the potential for utilizing sludge in red brick production, making this a viable recycling option for the sanitation sector.

In summary, the articles presented in this Special Issue demonstrate the diverse applications of clays in ceramics and how these applications have evolved over time. In this regard, they also highlight the contribution of sustainability aspects, such as the circular economy and the reduction in greenhouse gas emissions, in current ceramic production.