The initial setting time, fluidity and rheological properties of compound paste systems containing ultra-fine GGBS (UFS) were studied. The replacement of UFS into cement results in fast setting. An increase in the dosage of UFS shortened the initial setting time gradually due to the shortening of the induction period. Under the same UFS dosage, the induction period and initial setting time of the pastes was shorter in proportion to the fineness of the particles. The UFS showed a negative effect on the fluidity, and increasing the dosage of UFS gradually reduced the initial fluidity of the pastes. However, an interesting phenomenon was observed: for the same dosage of polycarboxylate superplasticizer (PCE), the initial fluidity was approximately the same as the increasing dosage of UFS. These results indicate that a possible positive synergistic effect on the initial fluidity was exerted due to the combination of UFS and the PCE, indicating that the UFS benefits the dispersing efficiency of PCE and the fluidity preservation of the pastes. The cement paste with UFS showed adverse effects on the rheological behaviour. The yield stress (tau(o)) and the plastic viscosity (eta(p)) increased with the rising dosage of UFS. Under the period of steady-state shearing (1800 s), the initial shear stress (tau(i)) and minimum shear stress (tau(min)) increased consistently as the UFS content increased. However, adding PCE to the cement-UFS system resulted in rheological properties opposite to those of the pastes without PCE; the tau(i) and tau(min) decreased as the UFS content increased. The results also confirmed the positive synergy effects between the UFS and the PCE. (C) 2019 Elsevier B.V. All rights reserved.