The properties of the semi-lightweight concretes produced by using volcanic slags as coarse aggregate were investigated. The volcanic slags were brought from the quarry crushed and then classified according to their aggregate sizes of 0-8, 0-16, 0-31.5, 4-8, and 8-16 mm. The concrete series of five different volcanic slag sizes were produced by addition of a specific cement paste in volume fractions of 0.15, 0.30, 0.45 and 0.60. The cubic, cylindrical and prismatic specimens were made from each of the concrete series. The physical and mechanical properties of the concrete series were determined by conducting unit weight, slump, ultrasound velocity, Schmidt hardness, cylindrical and cubic compressive, bending and splitting tensile strength tests. The results indicated that the volcanic slags can be safely used in the production of semi lightweight concrete. Copyright (C) 1997 Elsevier Science Ltd.