Abstract: Device for making of porous concrete intended for casting of e.g. elements of buildings, by which mainly cement, water and a frothing agent are mixed at the same time causing air pores in the mixture to be clasted, consisting of a cylindrical mixing-tank (1) in which an impeller (3) is arranged with a rotation axis which coincides with the center axis of the mixing-tank (1) and by which the impeller (3) includes blades (31) that extend helicoidally along the mixing tank (1) by which the helicoidally of each blade is less than 90.degree. over the length of the impeller (3) and by which the blades (31) extend a bit against the center axis of the impeller (3). Each blade (31) consists of two, in direction to the center axis of the impeller (3), reciprocally flat strips (32), which incline against the envelope to the mixing tank (1) so that the front edge in the moving direction of the outer strip (32) is located next to the envelope.