Alkali sulphates are chemical products that can be used to activate and improve the performance of mineral additions, like Blast-Furnace Slag (BFS) and Fly Ash (FA). This study investigates the effect of two alkali sulphates (sodium sulphate and potassium sulphate) on delayed deformations of mortars containing a high content of BFS. Autogenous shrinkage is monitored after 2 days and basic creep tests are conducted on 28-day-old specimens loaded at 30% of their compressive strength. Their microstructure is characterized by using several experimentations, such as internal relative humidity measurements, thermogravimetry analysis and a water porosity test. A numerical model in terms of C-(A)-S-H content evolution is applied to quantify the degree of hydration advancement. The results show that the use of sodium sulphate leads to a higher autogenous shrinkage, but to a lower specific creep. This is linked to the fact that the mortars activated with sodium sulphate show a faster hydration rate and a lower porosity, which results in higher capillary pressures. However, this also limits the movement of water required for creep development. © 2023, The Author(s), under exclusive license to Springer Nature Switzerland AG.