A hydraulic binder containing a glassy bulky slag milled to a fineness above 5000 cm.sub.2. Blaine and wherein the latently hydraulic blast furnace slag has a glassiness above 93%, contains 34 to 40% SiO.sub.2, 34 to 37% CaO and above 9% and previously below 13% Al.sub.2 O.sub.3. The (CaO+MgO)/(Al.sub.2 O.sub.3 +SiO.sub.2) ratio ranges from 0.88 to 0.98. Preferably the binder contains 0.5 to 5% porcelain cement clinker or porcelain cement. The above conditions can be met by mixing several slags.
