Cement additive

A method of making a cement additive that has pozzolanic activity, comprising:

• sulphating a vanadium titano-magnetite (VTM) slag comprising perovskite and silica with sulphuric acid to form a sulfated material, wherein the sulphuric acid concentration is greater than 80%, and wherein the sulphation reaction is conducted at a temperature from approximately 130 degrees C. to approximately 250 degrees C.;
• and
• separating a first composition comprising calcium sulfate and silica from one or more other components contained in the sulfated material, and wherein the VTM slag comprises percent by mass from approximately 15% to approximately 55% titanium dioxide, approximately 15% to approximately 40% calcium oxide, approximately 1% to approximately 25% silica, approximately 2% to approximately 15% magnesium oxide, approximately 5% to approximately 20% aluminium oxide, approximately 0% to approximately 10% iron oxide, and approximately 0% to approximately 2% vanadium oxide, and
• wherein the cement additive comprises from approximately 40% to approximately 80% calcium sulfate and from approximately 10% to approximately 50% silica, and has pozzolanic activity.