The effect of sodium sulphate on stratlingite formation in the high alumina cement — silica fume —water system is described. The reaction mechanisms associated with stratlingite and hydrogarnet formation are discussed. The effect of silica fume addition on the microstructure of HAC paste is also reported. Sodium ions have a significant influence on stratlingite formation in these systems. A mechanism for stratlingite formation is postulated. It involves reactions between CAH 10 or C 2AH 8 and dissolved silica. Presence of sodium ions accelerates dissolution of silica required for stratlingite formation.