The composition and cementitious properties upon hydration and alkali activation of the solid heat carrier (SHC) ash produced in a new Enefit280 retort was studied. The Engfit280 waste heat boiler (WHB) ash is different from other SHC solid residues. It does not contain residual organics, but is characterized by a low content of reactive Ca-phases and the soluble amorphous (aluminium)-silicate phase/glass material. Cementation of Eneit280 ash upon hydration with plain water is limited and its uniaxial compressive strength stays < 4 MPa after 28 days of curing. Ash mixtures activated with sodium silicate based mixtures show higher compressive strength values, reaching > 10 MPa after 28 days of curing. The Engfit280 ash, compared to other ash types forming in the Estonian oil shale processing industry, has significantly poorer self-cementing properties. This needs to be taken into account when designing waste depositories, if. other types of ash with better self-cementing properties are not co-deposited with this ash.