Fourteen raw slag samples were collected from different steel companies from China, using which setting, mechanical and early-age microstructural behaviours of alkali-activated slag (AAS) cements were investigated. To assess the reaction process between ground granulated blast furnace slag (GGBS) and activator, isothermal calorimetry tests were conducted on three selected AAS pastes for 50 h, while TG, FTIR, and Raman tests were conducted to characterise the early microstructural changes. Main findings include that the setting behaviour of AAS pastes depends primarily on the fineness and chemical compositions of GGBS, and statistical analysis results indicate that three of the constituents namely, SiO₂, MgO and K₂O, have a strong impact compared to the rest constituents. It is also found that commonly-used parameters to describe characteristics of GGBS are not sufficient to predict setting behaviours of AAS paste made by different GGBSs. Furthermore, the 28-d compressive strength of AAS mortars cannot be considered as a reliable indicator to assess its reaction potential at early age, at least, has no relationship to its setting behaviour. The early age microstructural analysis indicates that one possible reason behind the huge different setting and early age compressive strengths of AAS pastes is the variation in the releasing speed of Ca ions.