Oil shale ash based backfilling concrete - Strength development, mineral transformations and leachability

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 < 11.5; EC < 1000 μs/cm). In order to utilize OSA on a large scale, the composite blends we developed should be further optimized by adding ground high-calcium fly ashes, Portland cement, or other components.