The composition and properties of ash in the context of the modernisation of oil shale industry Mai Uibu (a)* , Kadriann Tamm (a) , Regiina Viires (a) , Janek Reinik (b) , Peeter Somelar (c) , Lembi Raado (d) , Tiina Hain (d) , Rein Kuusik (a) , Andres Trikkel (a)