Abstract: The present invention comprises the incorporation of organic cycloaliphatic type products, such as alkylcyclohexanes, in polysiloxane base compositions hardenable at room temperature. These alkylcyclohexanes have an exceptionally good compatibility with polysiloxanes and can be incorporated in significant quantities as a permanent plasticiser improving transparency.Compounds of the alkylcyclohexane type with a molecular weight in excess of 220 are used which belong in the group having the formula ##STR1## where group R.sub.1 is a linear or branched alkyl radical of formula C.sub.n H.sub.2n+1 where n is greater than or equal to 1 and smaller than 25, and where groups R.sub.2, R.sub.3 can be equal to or different from each other in a same molecule and can be H and/or linear or branched alkyl radicals of formula C.sub.n H.sub.2n+1 where n is greater than or equal to 1 and smaller than 25.

Abstract: Silicon-based curable compositions containing a polysiloxane and an alkyl-aromatic compound, having a molecular weight greater than 200, which can be used to replace all or part of the non-reactive silicon fluids normally employed in preparing such compositions.