The phase changes of gypsum were considered to be sufficiently understood for a long time. However, due to the increasing importance of gypsum recalcination (connected with the planned closure of thermal power plants resulting in a lack of FGD gypsum) and new detailed research associated with it, gaps in the current knowledge have been discovered. This paper clarifies phenomena occurring during the production of gypsum binder, anhydrite, and potentially eco-friendly calcium sulfate-based ceramics at 22–1300 °C by in-situ XRD, thermal analysis, and SEM. It was found that undesirable insoluble anhydrite II arises even in the temperature range from 40 to 90 °C despite the current state-of-the-art. New knowledge about the existence of the individual phases, microstructure (cracking of crystals; fragmentation and sintering of anhydrite AII at temperatures above 700 °C when a ceramic-like material is formed), and their external consequences (changes in moisture and heat transport properties) has been gained.

The thermally activated clay minerals are well-known as perspective supplementary cementing materials (SCMs) able to reduce the Portland clinker consumption and thus also the related CO₂ emissions. The best SCM performance among clay minerals is provided by thermally activated kaolin (metakaolin). Nevertheless, kaolinitic clays are not available everywhere, while varying mixed clays can be considered as globally affordable raw materials. The present work deals with the thermal activation of four illitic-kaolinitic clays with varied content of clay minerals, quartz and calcite, available locally in the Czech Republic. The attainment of activation was evaluated with the help of mechanical strength and the saturated lime test. There is no doubt that kaolin is the best clay for SCM production. However, the activation of mixed illitic-kaolinitic clays at 600 °C provided comparable and sufficient performance, despite that the crystal structure of illite was not completely decomposed. The non-clay minerals presence did not reduce the activity either; even the clay containing just about 50% of clay minerals (mostly illite) treated at 650 °C provided sufficient mechanical performance. As the drawback of activated illitic clays must be considered their higher sensitivity to the proper calcination temperature compared to metakaolin since illite (and other 2:1 clay minerals) does not form any metastable dehydroxylated phase.

The rationalization of material flows, together with the utilization of waste raw materials for the production of alternative binders, became a very attractive topic during the last decades. However, the majority of designed materials can be used as a replacement for low-performance products. In this work, the waste materials (brick powder and blast furnace slag) are valorized through geopolymerization to design high-performance material as an alternative to high-performance concrete. Designed mixtures activated by sodium silicate and waste-originated alkali solution are characterized by the meaning of the chemical and mineralogical composition, evolution of hydration heat, and mechanical strength test. To contribute to the understanding of the environmental consequences and potential benefits, the carbon footprint and embodied energy analysis are provided. Obtained results highlight the potential of end-of-life bricks for the design of high-performance composites if mixed together with more reactive precursors. Here, even values over 60 MPa in compressive strength can be achieved with the dominant share of low-amorphous brick powder. The higher crystalline portion of brick powder may lead to the reduction of drying shrinkage and preservation of flexural strength to a greater extent compared to used slag. Performed environmental analysis confirmed the CO₂ emission savings; however, the embodied energy analysis revealed a huge impact of using alkaline activators.