The purpose of the article was to conduct an in-depth literature review on the possibilities of managing combustion by-products (mainly fly ash) in the context of a closed-loop economy. First, information on the chemical composition of fly ash in Poland was collected and compared with the composition of fly ash in other European countries. The authors concentrated on describing methods for synthesizing geopolymers and zeolites using fly ash as a substrate. By-products of zeolite synthesis, which are strongly alkaline solutions, can be used as a substrate in the synthesis of geopolymers. A concept has been proposed to combine the synthesis of zeolites and geopolymers into a single process to close the material loop. The search for comprehensive technological solutions that take into account the ideas of a closed-loop economy is essential in an era of resource depletion, and this literature review encapsulates this topic area.

This study examines the utilisation of fly ash from the energy sector as a secondary raw material in cement composites, with the aim of improving sustainability while maintaining high mechanical performance. By partially replacing Portland cement with industrial by-products, the proposed approach supports resource efficiency and aligns cement composite production with circular economy principles. Three formulations were tested: a reference mix and mixes with 25% and 50% cement reduction. Compressive strength reached 41 MPa, confirming suitability for construction use. Chemical and textural properties were analysed using XRD, FTIR, TGA, and nitrogen adsorption (BET, BJH). The results showed structural modifications, including new crystalline phases and changes in porosity. XRD confirmed newly formed phases, while FTIR identified Si-O-Si and Al-O-Si bonds, indicating effective activation of fly ash. Reducing cement content increased surface area and mesoporosity, enhancing performance. The findings demonstrate that fly ash can serve as a sustainable substitute for Portland cement within a circular economy framework, supporting CO₂ emission reduction and resource conservation while enabling the production of durable and environmentally responsible cement composites.