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, for example 1:2 with a dust poor or dust free 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 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 and the grain size distribution is around an average, so that an open structure is obtained.