J1-3026 Applicability of the cold sintering process to clay minerals
Clay minerals are formed through chemical weathering of silicate-bearing rocks and aggregated in sediments. Sedimentary rocks then form through cementation processes as part of the diagenesis or compaction of sediments over millions of years. On the other hand, man-made products are formed in few days’ time. However, the related energy needs and CO2 emissions contribute to global warming. Efficient use of energy and raw materials at all stages is the key for decarbonization of construction sector. Clay sediments are an abundant but yet finite source of raw material for brick production. Fired clay bricks are one of the oldest materials produced for building purposes that remain to be preferred due to their strength and durability. A major decrease in the carbon footprint of the brick industry would be an important step towards the sustainability of the construction sector. Such a revolutionary achievement could be based on a cold sintering process (CSP), a new sintering concept for ceramic materials enabling efficient sintering at low temperatures (from room temperature to 300 °C) and mild pressures (<500 MPa). In contrast to the main research on the cold sintering focused on synthetic phase-pure materials CSP4clays puts in focus the clay mineral, abundantly available in nature, for the exploitation of mechanical properties. The aim of the CSP4clays project is to unveil the applicability of the CSP principle to clays and broaden the knowledge on the CSP principle. The project will uncover the cold sintering mechanisms during densification of selected clay materials, where complex phase evolution is anticipated. The project is initial evaluation of the low temperature process for fabrication of structural clay-based elements with large environmental benefits. Based on the comprehensive literature review and expertise of the project participants we elaborate four main objectives: 1. To research the CSP principle and underlying mechanisms on pure clay minerals kaolinite, and illite as well as anhydrous metakaolin, 2. To reveal the role of physically and chemically bounded water in the CSP of clays and track the paths of water elimination from the clay structure and the microstructure evolution of the sintered body, 3. To demonstrate the applicability of CSP to naturally occurring clay material as used in clay brick production, and 4. To compare the properties of cold sintered samples to conventionally sintered samples and literature data on geologic compaction of clay-bearing sediments. In order to assess the applicability of CSP to clays, the research will first focus on pure clay minerals and the role of physically and chemically bonded water, also by cold sintering anhydrous kaolinite (metakaolin). Incongruent dissolution of clay mineral, hindering the CSP, will be compensated by adjusting composition and pH of the liquid phase. The characterization will be focused on the changes on grain boundaries related to the mass transport occurring during CSP, providing the required mechanical performance. The next objective is to apply CSP to naturally occurring clay material as used in clay brick production. As in conventional processing, we anticipate the mechanical properties will be provided by the clay matter, which can be amorphized and re-crystallize during CSP. We will consider several strategies to aid the sintering process of natural clay material, containing more than 50% foreign phases, including addition of alkaline activators. The reactions occurring under hydrothermal conditions of CSP will be contrasted to the complex behavior of clay minerals during conventional firing. Moreover, the properties of cold-sintered samples will be contrasted to the literature data on the compaction of siliciclastic sediments with similar clay content.
2021/10/01 - 2024/09/30
Slovenian Research Agency