The properties of slag-rich cement paste are fundamentally associated with slag chemistry. In the present research, 10 slags covering the common chemistry range, including eight synthetic slags of CaO-SiO2-Al2O3-MgO system and two commercial slags, were adopted to evaluate the influence of slag composition on the early (7 days) and later (3 months) age properties of blended paste. Mixture containing Al2O3-rich slag performed better at 7 days as it favored the formation of ettringite and/or monosulfate. The MgO-rich slag cement paste exhibited good properties at both early and later ages, and it effectively promoted the precipitation of Mg-Al hydrotalcite-like phase. It was also noted that the Mg/Al atomic ratio of hydrotalcite-like phase was positively related to the Mg/Al atomic ratio of slag itself. Conversely, with the increasing MgO content in slag, the Al/Si atomic ratio of C-S(A)-H gel phase decreased. High Al2O3 and/or MgO contents can compensate the negative effect of reduced CaO/SiO2 ratio at early age while not at later age. Overall, attention should be paid to aluminum- and sulfur-rich slags. These two elements in slag promoted the formation of ettringite and/or monosulfate at an early age; however, this positive effect disappeared at later ages.