In this study, the effects of mechanical activation on the potential pozzolanic activity, hydration properties of iron ore tailings (10Ts), and their application as supplementary cementitious materials in cement were studied. It was found that structural degradation of the 10Ts occurred with prolonged grinding time and the mechanically activated 10Ts possessed cementitious properties in the alkaline environment provided by calcium oxide. Fibre-like ettringite and amorphous C-S-H gel were the major hydration products of the ground IOTs-calcium oxideanhydrite system. With increasing curing time, the composition of the C-S-H gel shifted towards a lower atomic ratio of Ca/Si, reaching 0.84 after curing for 28 days. The activity index of mechanical activated lOTs was qualified, and the developed cements blended with 10 wt%, 20 wt%, and 30 wt% activated 10Ts were prepared, and their mechanical and physical properties met the design requirements for 32.5 composite Portland cement. (C) 2019 Elsevier B.V. All rights reserved.