Functional and microstructural alterations in hydrated and freeze–thawed cement-oil shale ash composites

The oil shale industry generates a large amount of solid waste, the finer fraction of which can be utilized in civil engineering projects. However, for environmental, economic or technological reasons, oil shale is blended with other fuels, resulting in significant changes in the chemical composition of the residual ash. In this laboratory work, the fly ash from the combustion of oil shale with and without biomass in an industrial power plant was used as a binder substitute in cement-based materials to assess the potential of reuse of this ash. The hydration and pore structure of blended cementitious material subjected to long-term hydration and freeze–thawing has been characterized with a range of techniques, such as XRD, TGA and N2 physisorption. The results revealed that the ash type influences the phase assemblage related to the content of ettringite and monocarboaluminate, significantly affecting the volume of gel (1-6 nm) pores and small (10-30 nm) capillary pores. These microstructural changes impact the mechanical strength and water sorptivity properties in an opposite way after both long-term hydration and freeze–thawing. The results showed that the incorporation of ash obtained after combustion with biomass possesses higher mesoporosity and delays the hydration of cement, increases the total volume of gel (<6 nm) and small capillary pores (20-30 nm) of the composition, and deteriorates the functional properties after freeze–thawing.