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 collecting systems of both boiler types consist of bottom dusters, cyclones and electrostatic precipitators. The corresponding ash types differ in specific surface 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.
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.
Oil shale ash (OSA) as a binder has air, pozzolanic or latent hydraulic properties depending on the combustion temperature and type of ash collection equipment. This paper focuses on the use of OSA as the main binder for low strength concrete. Impact of hardening conditions on the strength development and soundness of various concrete mixes made with two main types of OSA and their mixes was tested. Crushed limestone was used as aggregate. Concrete mixes were designed at an OSA:aggregate ratio of 3:1 and 1:1, using fresh concretes with the same workability. The results revealed differences in the strength development, 28-day compressive strength and durability properties between hardened concretes made with various OSA binders. The compressive strength of concretes made with various OSA was tested in different curing conditions. The durability properties of OSA based hardened concrete such as water absorption and resistance were tested. The results of expansion and water resistance tests indicated that by increasing the content of CFB ash in OSA binders, water resistance was improved and expansion diminished.
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 + , SO 4 2-. The leachable content of soluble components in PF ash mortars was higher in comparison 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.
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