Phase assemblage and microstructure of burnt oil shale-containing blended cements

Burnt oil shale (BOS), obtained from the combustion of oil shale, is a promising supplementary cementitious material (SCM) based on its chemistry and mineralogy. This paper summarizes the use of BOS and its hydration in blended cements. It presents new data on the effect of combinations of alkali activators and Ca(NO₃)₂ in blended cements containing 50 % Portland cement (OPC) where BOS is combined with limestone, fly ash and ground granulated blast furnace slag. These chemical admixtures increase the slope of the correlation between compressive strength and heat of hydration of BOS containing mixes, providing an increase in compressive strength from 1 to 7 days for similar heat release to the control system. In contrast, the slope is not affected in absence of BOS. The change is due to a higher volume of hydrates from BOS increased hydration for a given C₃S degree of hydration, likely from a less exothermic dissolution of BOS amorphous component. These admixtures increase the reactivity of both BOS and OPC at different curing times and depending on the type of alkali activator. They promote ettringite and portlandite precipitation, inducing a refinement of the microstructure, particularly around BOS particles. The information presented should pave the way to a broader and more effective use of BOS in blended cements with particularly low clinker contents.