Study on hydraulic characteristics of ashes from various thermal processes and the enhancement of the reactivity for their use as an immobilization additive

Fly ashes, bottom ashes, cyclone, and bag filter ashes represent the by‐products generated by the air pollution control equipment in high‐temperature processes in industry and energetics. With the gradual reduction of the use of fossil fuels (Integrated National Energy and Climate Plan paradigm), the quantities of ash generated during coal combustion decreases, while the quantities of ashes from co‐combustion and ashes from biomass combustion increases. The ideal sector for the recycling of the ash is the construction industry. In the construction sector two synergetic effects are achievable: large amounts of material can be consumed and, at the same time, in the case of earthworks (bonded composites) the harmful elements can be permanently immobilized through the immobilization process. Specific types of ashes can be successfully recycled by their usage as an efficient immobilization additive of potentially toxic elements which parallel also enhance the geotechnical characteristics of remediated materials to be used for earthworks. Since different ashes possess a high usable value, and on the other hand, ashes represent one of the more complex anthropogenic materials, the need for a detailed characterization of each individual ash before use is essential. To assess the recycling potential of ash, in addition to microstructure, mineral and chemical composition, and environmental impact, it is necessary to precisely define their hydraulic reactivity. An important characteristic of most of the ashes is that they possess (latent) hydraulic characteristics, which mainly accounts for the glassy phase in their complex composition. It has been shown that the reactivity of ashes cannot be determined solely on the basis of their overall chemical and phase composition. Analyses of ashes with a very similar chemical and mineral composition showed large differences in the properties/behaviour of the composites in which they were incorporated. The reactivity of the ashes depends mainly on the characteristics of the glassy phase. The aim of the proposed project is to characterize in detail the ashes from various thermal processes. One of the key parts of this project is the detailed characterization of the glassy phase of the ashes which represent the major component of the ashes. However, there is a lack of experimental evidence, which could directly link the hydraulic activity of ashes, their bulk composition and physical properties to the characteristics (type) of amorphous phase. On the basis of detailed characterization of ashes, the potential for increasing the reactivity (activation) of ashes with lower reactivity (and those identified as non‐reactive) in a two‐component system, using an additional recycled material will be investigated. The realization of the proposed research work would enable to define the fundamentals based on which the hydration process of the investigated ashes at two different amounts of water addition would be defined and correlations between the level of hydration, types of hydration products, type of amorphous phase and mechanical and physical properties of ashes composites would be explained on the phenomenological level. The methodology will be based on broad range of advanced complementary methods, including the analysis prescribed in relevant standards. Research work described in the project would promote the recycling rate and the added value of ashes, which would have a significantly positive impact on the sustainable use of natural resources, the reduction of the carbon footprint and the development of circular economy. Gained knowledge would enable the prediction of (long‐term) characteristics of ash composites, which would not be based solely on the use of empirically determined amounts of ash needed on a case‐by‐case basis but on the carefully tailored mixing of various residues.