Burnt shale

Burnt oil shale is a the by-product of the combustion of shale oil in power plants. But it is also a cementitious material that can be used to replace clinker in cement. 

Burnt oil shale is produced from shales with 5%-65% organic matter. It is produced by the combustion of oil shales in bottom or fluidized bed furnaces used for steam or electricity generation. The resulting ash is of highly variable composition, depending on the make-up of the original shale and the temperature at which it is produced.

In contrast to other supplementary cementitious materials (SCMs), when finely ground, burnt oil shale exhibits both hydraulic and pozzolanic reactivity, enabling it to harden without the need for activation (in contrast to purely pozzolanic materials). It can be used to replace up to 35% of clinker in Portland cement.

Challenges include high sulfur levels, which much be taken into account during cement manufacture, and high sulfate levels, which can make it difficult to comply with maximum limits set in cement standards. Some burnt shale oils may also increase water demand and impact concrete expansion, a result of their high free lime content.