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.
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