This study investigates the impact of different temperatures and initial Mg²⁺/Ca²⁺ molar ratios in the solution on the wet-accelerated carbonation of β-dicalcium silicate (β-C₂S). The x-ray diffraction (XRD), fourier transform infrared spectroscopy (FT-IR), thermogravimetric analyzer (TGA), and field emission scanning electron microscopy (FE-SEM) analysis results indicated that temperature and the Mg²⁺/Ca²⁺ molar ratio are key factors in the nucleation of aragonite. Aragonite formed at a temperature above 60 °C, and the high temperature promoted the crystallinity of needle-like aragonite with a length of 1–6 μm and a diameter of ~1 μm. Moreover, 80 °C was the most favorable temperature for the formation of aragonite with a large aspect ratio in the carbonation system of β-C₂S. Mg²⁺ had a significant effect on inhibiting the transformation of aragonite to calcite and promoting the stability of aragonite. Aragonite became the dominant CaCO₃ polymorph instead of calcite when the Mg²⁺/Ca²⁺ molar ratio was above 1.0, and pure aragonite-style calcium carbonate was formed at a Mg²⁺/Ca²⁺ molar ratio of 1.5.