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This work focuses on the local alkaline wastes of Estonia where different waste streams were studied for the performance testing of the fly ash based building blocks. Fly ash from oil shale direct combustion and wood fly ash from district heating plant were considered in the applications for utilizing both CO2 and combustion residues. These types of ashes contain Ca/Mg-oxides, silicates, or other metal oxides as candidate precursor materials for CO2 mineralization. Based on the results obtained from the performance testing of all compacted and carbonated (both with model flue gas and 100%CO2) samples, it can be concluded that flue gas curing can be an effective method for direct mineral carbonation of compacted fly ash blocks. The formation of carbonate phases in compact bodies lead to an increase in compressive strength. The rate limiting impact due to the low CO2 concentration in flue gas curing was negligible on the CO2 uptake level when elevated curing pressures were applied to the fly ash based compacts.

The main aim of the study is recycling of niobium slag, separating valuable metallic Nb and mineral ballast – calcium aluminate. Niobium slag is a waste product from Nb production at NPM Silmet OÜ. For the retreatment of slag, an efficient disintegrator milling technology was used. Laboratory grindability tests of slags were carried out. We propose a potential principal scheme of slag treatment, consisting of preliminary size reduction by impact milling at direct mode and final at separative mode using a disintegrator milling system DSL-115. As a result of slag pretreatment, pure Nb from a coarse fraction was separated. To elaborate an industrial technology, semi-industrial tests for specifying recycling modes and finding optimal milling parameters were performed. Granulometry studies of milled products and chemical composition as of Nb as well as that of remained products were conducted. Yield of Nb with purity 90–95% was in the range of 0.4–1.0%. Preliminary cement tests for using the main product as a binder or a substitute of cement confirmed its potential for use in building materials production.

In this study, fly ash (FA) compacts were prepared by accelerated carbonation as a potential sustainable building material application with the locally available ashes (oil shale ash (OSA), wood ash (WA) and land filled oil shale ash (LFA)) of Estonia. The carbonation behaviour of FAs and the performance of 100% FA based compacts were evaluated based on the obtained values of CO2 uptake and compressive strength. The influence of different variables (compaction pressure, curing temperature, CO2 concentration, and pressure) on the CO2 uptake and strength development of FA compacts were investigated and the reaction kinetics of the carbonation process were tested by different reaction-order models. A reasonable relation was noted between the CO2 uptake and compressive strength of the compacts. The porous surface structure of the hydrated OSA and WA compacts was changed after carbonation due to the calcite formations (being the primary carbonation product), especially on portlandite crystals. The increase of temperature, gas pressure, and CO2 concentration improved the CO2 uptake levels of compacts. However, the positive effect of increasing compaction pressure was more apparent on the final strength of the compacts. The obtained compressive strength and CO2 uptake values of FA compacts were between 10 and 36 MPa and 11 and 13 wt%, respectively, under various operation conditions. Moreover, compacts with mixed design (OSA/LFA and WA/LFA) resulted in low-strength and density compared to the single behaviour of OSA and WA compacts, yet a higher CO2 uptake was achieved (approximately 15% mass) with mixed design. The conformity of Jander equation (3D-diffusion-limited reaction model) was higher compared to other tested reaction order models for the representation of the carbonation reaction mechanism of OSA and WA. The activation energy for OSA compact was calculated as 3.55 kJ/mol and for WA as 17.06 kJ/mol.

The combustion of oil shale (OS) in electric power plants is accompanied by the generation of vast amounts of waste ash. One promising idea to maximize the recovery of oil shale from mines is to backfill them with oil shale ash (OSA)-based concrete. However, the properties of this concrete have yet to be analyzed in detail and this approach also raises concerns about the risk of polluting both surface and groundwater. To address these concerns we developed different types of OSA-based concretes and characterized their structure and leaching characteristics. This information enables us to predict the type and durability of each respective ash stone. A compressive strength of 1–5 MPa was achieved after 7 days (maximum after 28 days > 25 MPa). During the early stages of curing, the pH and electrical conductivity (EC) of the leachates exceeded or were close to the limits set for general wastes, however, both properties decreased considerably after 28 days (pH

The focus of the current study is to characterise the leaching behaviour of polycyclic aromatic hydrocarbons (PAHs) from oil shale ashes (OSAs) of pulverised firing (PF) and circulating fluidised-bed (CFB) boilers from Estonian Thermal Power Plant (Estonia) as well as from mortars and concrete based on OSAs. The target substances were 16 PAHs from the EPA priority pollutant list. OSA samples and OSA-based mortars were tested for leaching, according to European standard EN 12457-2 (2002). European standard CEN/TC 15862(2012) for monolithic matter was used for OSA-based concrete. Water extracts were analysed by GC–MS for the concentration of PAHs. Naphthalene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene and pyrene were detected. Still, the release of PAHs was below the threshold limit value for inert waste. The amount of the finest fraction (particle size

To estimate the environmental properties of oil shale ash-based mortars the leaching of harmful components was studied. The leachates were highly alkaline. The predominant ions were Ca 2+ , K + , Na + , SO4 2- . The leach- able content of soluble components in PF ash mortars was higher in com- parison with that in CFB ash mortars. Results indicated that over curing time the fraction of readily soluble inorganic components decreased and the mobility of potentially hazardous Cd and Zn did not increase. Addition of bypass dust could affect the content of leachable ions. Results give new knowledge about the environmental properties of oil shale ash-based materials, including backfilling composites for underground mining technology.

Combustion of low calorific fuel - oil shale - in industrial-scale pulverized firing and circulating fluidized bed combustion boilers produces large amounts of ash. Estonian oil shale ash is characterized by a high content of free CaO as compared to those listed in the European Standard EN 450. The main alternatives to oil shale ash utilization include its use as a lime replacement in mineral binders or as a constituent of Portland cement. The pulverized firing ash formed at 1400 °C has been effectively used as a second main constituent of Portland cement during the last fifty years. Further utilization of the low-temperature circulating fluidized bed ash (formed at 800 °C) depends on its composition and properties. Dust collect- ing systems of both boiler types consist of bottom dusters, cyclones and electrostatic precipitators. The corresponding ash types differ in specific sur- face area, grain size and mineral composition. The structure and composition of the dry ash and ash based stone were studied using chemical, XRD and SEM analysis. The results indicated that hydration type, as well as the setting and hardening course of the selected ash type are determined by the firing temperature of oil shale.