A mortar composition, in particular for preparing a viscoelastic structure and/or a fire barrier, including: a) 15-50 wt.-% of a hydraulic binder, b) 5-35 wt.-% of lightweight aggregates, c) 5-25 wt. % of further aggregates which have a particle density that is higher than the particle density of the lightweight aggregates, and d) 10-50 wt.-% of a polymer.