Since ultra-lightweight foam concretes were successfully pre-pared, the fundamental properties of the foam concretes were characterized. Fig. 10 shows that the compressive strength of ultra-lightweight foam concretes decreased sharply with the decrease of apparent density, when the apparent density was in the range of 150-250 kg/m3, while tensile strength of foam con- cretes decreased slowly when the apparent density was higher than 150 kg/m3