Power generation and other industries using solid fossil fuels like coal, lignite, oil shale and peat are responsible for producing large quantities of solid residues that are often chemically reactive and/or unstable and are disposed in holding ponds and deposition sites. Stability and long-term behaviour of such deposits are typically studied in short-term laboratory experiments that cannot describe nor predict long-term changes taking place in these materials. Here, we study long-term (>40 years) transformations, in highly alkaline conditions, of the Ca-rich ash deposit in Estonia composed of oil shale processing residues from the Eesti power plant. Detailed mineralogical,

Achieving sustainable zero-waste and carbon neutral solutions that contribute to a circular economy is critically important for the long-term prosperity and continuity of traditional carbon-based energy industries. The Estonian oil shale (OS) sector is an example where such solutions are more than welcome. The combustion of OS generates a continuous flow of ashes destined to landfills. In this study, the technical feasibility of producing monolith building materials incorporating different OS ashes from Estonia was evaluated. Three binder systems were studied: self-cementation of the ashes, ceramic sintering in clay brick production and accelerated carbonation of OS ash (OSA) compacts. Results showed that most of the OSAs studied have low self-cementitious properties and these properties were affected by ash fineness and mineralogical composition. In case of clay bricks, OSA addition resulted in a higher porosity and improved insulation properties. The carbonated OSA compacts showed promising compressive strength. Accelerated carbonation of compacted samples was found to be the most promising way for the future utilization of OSAs as sustainable zero-waste and carbon neutral solution.

Mechanical activation of the calcium-rich fly ash formed in circulating fluidized bed combustion (CFBC) boilers was investigated to enhance the compressive strength performance of the pastes. We studied the effect of the activation on the physical, chemical, and mineral characteristics of fly ash and its pastes. Our study shows that already a short mechanical activation yields a 10-fold improvement in the compressive strength of the pastes, reaching 60 MPa after 90 days of curing without any chemical activation or blending. Mechanical activation caused fragmentation of large porous aggregates in the raw ash enhancing the physical properties and reactivity of fly ash particles. Similarly to calcium sulfoaluminate cements, the mechanical strength was provided by the formation of abundant ettringite and possibly C-(A)-S-H gel-like phase that created a highly compact microstructure. Our findings suggest that mechanically activated Ca-rich CFBC fly ash can be successfully used as an alternative binder.

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In this paper, the geopolymerization of the solid heat carrier (SHC) ash waste produced at Petroter shale oil plants, Viru Keemia Grupp (VKG), Estonia was studied. Different mixtures were prepared to study and evaluate the potential use of this SHC ash for geopolymer-type mortar and cement production and to compare alkali-activated black ash with the material having self-cemented upon hydration with plain water. Mixtures prepared with plain water and NaOH solution show comparable compressive strength development, but the mixture with NaOH affords significantly lower compressive strength values, which can be explained by the absence of an ettringite/monosulphate phase in the NaOH-activated samples. Hydrocalumite precipitated instead of ettringite in the NaOH-activated mixture does not provide the interlocking structure that is found in water mixtures, though the formation of an amorphous geopolymer phase is possibly observed in the NaOH-activated sample after 90 days of curing. Sodium silicate-and Na-silicate/NaOH-activated samples show a strong geopolymerization and development of Ca-Na-Al-silicate gel formed in the pore space of the ash aggregate. However, due to strong shrinkage upon drying, the compressive strength obtained after 7 days of curing is lost in the prolonged curing process, and further research into the causes and prevention of Ca-Na-Al-silicate gel shrinkage is needed.