A two-pack cementitious composition consisting of iii) an alkaline component comprising gypsum, an alkaline polymer, water and a hydration inhibitor, which is stable in a fluid state and which can be set and hardened upon the addition of an hydration activator, and iv) an acidic component comprising an acid soluble hydration activator, is suitable for forming a self-levelling floor composition. The self-levelling floor composition may be used in a method of levelling a floor.