A few glassy slags in the system, CaO-Al2O3-SiO2 were prepared having a fixed SiO2/Al2O3 ratio of about 1.15 and different proportions of CaO in the range of 35–50 per cent by weight. The hydraulic properties of the slag powders in admixture with different proportions of hydrated lime were studied. The hydraulicity of the slags increased with increase in the proportion of CaO in their chemical compositions and also with the increase of lime in the mixture. The amount of lime necessary for the development of maximum strength was found to decrease with increasing CaO content of the slag. © 1964 Taylor & Francis Group, LLC.