Dry mortar mixture characterized by a glass mixture of expanded glass beads with a grain size d/D 0/8, mixed in a ratio of between 1:1 and 1:3, with a binding mixture of hydraulic binders and stone granules in the weight ratio of 1:2 to 1:4. The glass has a discontinuous grain distribution. For the glass mixture the fractions 0.5/1.0 and 2.0/4.0 are present while a fraction intermediate other fractions are absent. Preferably the fractions 0.25/0.5 and 1.0/2.0 are absent. For the glass mixture preferably all grain sizes between 1.0 and 2.0 mm are absent. The grain size distribution is around an average, so that an open structure is obtained.