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This study investigated the effect of alkalis on C3S hydration with and without incorporating aluminate phases. Pastes of 100% C3S and 90% C3S+10% metakaolin (MK) were prepared at 1%, 2%, and 3% Na2O content. X-ray diffraction (XRD) and thermogravimetric analysis (TGA) were used to characterize the solid phase assemblages, and then the hydration degree of C3S. Inductively coupled plasma optical emission spectroscopy (ICP-OES) was used to analyze the aqueous composition of pore solution. Mercury intrusion porosimetry (MIP) was used to measure the porosity of the hydrated pastes. The results highlighted the promoting effect of Na and the inhibiting effect of Al in pore solution on C3S hydration after a period of curing (90 days). Without incorporating metakaolin, the hydration degree of C3S continuously increased with the increase of Na concentration. Incorporating metakaolin into C3S can generate aluminate hydrates (carboaluminates, in this study), and the increased Na concentration can increase the dissolution or instability of carboaluminates and elevate the equilibrium Al concentration. Under this condition, the hydration degree of C3S fluctuated depending on the combined effect of Na and Al concentrations in pore solution on C3S hydration. [ABSTRACT FROM AUTHOR]