Ground granulated blast-furnace slag (GGBS) can be used as the hydraulic phase in alkali-activated slag cement (AAS) or as partial replacement in blended cements, whose storage conditions need to be fully assessed before practical applications. This paper tries to address this problem by storing the GGBS at the controlled relative humidity (RH) using saturated solution methods at 20°C. Followed by performing the dissolution experiments in both deionized water and NaOH solution on the obtained GGBS, where both aqueous and solid phases are characterised. The experimental results indicate that the GGBS stored in a high RH will depress the dissolution of silicate networks, thus suppressing the formation of C-S-H phases, especially in the NaOH solution. The dormant period of GGBS hydrated in DI water is positively correlated to the storage RH. From the degree of hydration point of view, the RH less than 60% should be recommended for storing GGBS. However, if the faster hydration of GGBS is the purpose when it is used as an SCM or in neutral salt activated cements, an extreme low RH should be recommended. It is also found that the increased storage RH shows the potential for increasing the inner products of hydrated GGBS.