The reactivity of cemented paste tailings (CPT) that contains sulphide mineral-bearing tailings is a key parameter that influences its environmental performance and durability. This reactivity can be influenced by several factors, such as the initial sulphate content of the CPT. In this paper, the effect of the initial sulphate content of the CPT on its reactivity is experimentally investigated by conducting oxygen consumption (OC) tests on CPT specimens. Microstructural testing is also conducted on CPT specimens to better understand the mechanisms responsible for the changes in the reactivity of CPT. These specimens are prepared by mixing defined amounts of pyritic tailings (45 wt%), varying proportions of Portland cement type I or Portland cement partially replaced with different types and amounts of mineral admixtures, and mixing water with various sulphate contents (0, 5000,15,000 and 25,000 ppm). The samples are cured for 150 days at room temperature. The results show that regardless of the type of binder, the reactivity of the CPT specimens increases with increasing contents of sulphate except for a sulphate content of 5000 ppm Also, partial substitution of Portland cement type I with mineral admixtures, such as granulated blast furnace slag or fly ash, reduces the chemical reactivity. Regardless of the initial sulphate content, increasing the cement content and/or replacing cement with mineral admixtures leads to the reduction in the reactivity of the paste. (C) 2017 Elsevier B.V. All rights reserved.