Utilization of coal wastes for the production of ceramic materials: A review

This literature review provides an overview of the methods and patterns observed in the usage of coal ashes and refuse to produce sintered ceramics, glass ceramics, concrete and geopolymers. Coal refuse and combustion residuals contain valuable aluminosilicates and oxides that align with the feedstocks traditionally used in ceramic production. The use of coal waste in the production of ceramic and related materials offers significant financial and environmental benefits, aligning with the growing demand for sustainable materials in construction and manufacturing while reducing the need for the coal industry to construct and maintain impoundments. The fine particle size, reactivity and high metal oxide content in fly ash improve particle packing and facilitate sintering or alkaline reactions, making it suitable for diverse ceramic applications. Bottom ash and coal refuse are coarser and less reactive, generally requiring additional processing such as milling, magnetic separation or calcination to enhance their suitability. Introduction More efficient coal cleaning technologies combined with the development of less accessible coal reserves have resulted in an increase in the production of coal refuse. Coal combustion residuals (CCRs), including fly ash and bottom ash, are one of the largest industrial waste streams in the world and will continue to grow with increasing global coal consumption. These coal-related wastes are often stored in impoundments, which require capital investment to develop, involve long-term maintenance and liability, and can present environmental issues if not effectively managed. The documented increase in both the frequency and severity of impoundment failures since 2000, coupled with regional trends toward more frequent and intense precipitation events that may contribute to failures and mass flows, indicates the need to consider alternative waste storage methods to mitigate financial and environmental risks. Fig. 1 Organization map. Purpose This review summarizes research trends on the use of coal-related wastes as feedstocks for ceramic and ceramic-like materials. The scope of the reviewed literature includes topics in traditional sintered ceramics, glass ceramics, concrete and geopolymers. Production methods, material properties and environmental implications are examined to assess the feasibility of integrating fly ash, bottom ash and coal refuse into these applications. Figure 1 depicts the organization of the literature review. Sintered ceramics. Fly ash and bottom ash are commercially used in brick production, with pilot-scale projects exploring their use in tiles. When mixed with clay, 40 percent bottom ash can produce tiles that meet ISO 13006 standards, and 20 percent bottom ash can produce ASTM-compliant bricks. Fly ash can similarly meet ISO requirements for tiles when combined with clay or used as a partial substitute for feldspar or quartz. Ash content over 50 percent generally results in low compressive strength and high porosity. Up to 60 percent coal refuse can be used with clay to produce acceptable products, though calcination or sizing can be used to improve coal refuse as a precursor material. The suitability of coal ash and refuse for sintered ceramics must be assessed on a case-by-case basis. Glass ceramics. The study of fly ash in glass ceramics has produced materials that have achieved enhanced X-ray shielding qualities, improved celsian conversion, lower-temperature cordierite synthesis, stronger chemical resistance, and lower coefficients of thermal expansion compared to glass ceramics produced with conventional feedstocks. The particle size of fly ash and the presence of glass formers, nucleating agents and oxides in the ash allow it to be used in quantities exceeding 70 percent. Calcination and addition of supplemental glass formers can be used to address high shrinkage issues that are observed. While fly ash shows significant potential in laboratory-scale settings, its heterogeneity may limit its suitability for high-precision or specialized commercial applications. Glass ceramics have also been successfully produced from bottom ash and coal refuse, though they require additional mineral processing. Cement. Fly ash is widely used as a partial replacement for cement in concrete, contributing to improved compressive strength, durability and heat resistance, and reduced spalling, cracking and leaching. However, fly ash can also increase curing times and slow strength development, inhibiting its use in specific applications. Bottom ash can be used in limited quantities as a substitute for sand in certain concrete formulations. Research on catalysts to increase the curing time of fly ash and the improved incorporation of bottom ash will increase the total volume of ash used by the concrete industry. Geopolymers. Bottom ash, refuse and other aluminosilicate-bearing waste products have been used to produce geopolymers at a laboratory scale while fly ash, especially Class F, has been adopted for some low-strength construction applications. Broadly, high-alumina ashes with a large amorphous fraction exhibit ideal reactivity and produce high-strength geopolymers. Coal refuse requires calcination to remove impurities and generate metakaolin prior to polymerization. A broader acceptance of geopolymers is hindered by a lack of research on long-term durability and the literature presents conflicting results on whether the geopolymerization mechanism immobilizes trace elements or facilitates their leaching. Conclusion At a laboratory scale, the heterogenous nature of coal wastes make them suitable for a diverse range of applications. However, at a commercial scale, this variability presents challenges for standardizing production and performance in novel materials. High-performance glass ceramics derived from fly ash, tiles produced from coal refuse, and the partial use of bottom ash in concrete formulations represent promising applications for commercial adoption. Further research on the preparation of coal refuse and bottom ash as ceramic feedstocks will lead to innovations that support circular economy strategies and minimize the need for large-scale waste management efforts in the coal industry. References A list of all references is available in the full paper.

Utilization of coal wastes for the production of ceramic materials: A review

Authored by: Hannah Chop and Barbara Arnold

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