Significant efforts have been made in revealing the mechanism of thaumasite formation in concrete, which continues to be fraught with ambiguities and dissension. Chemical method was employed to synthesize pure C_3S, C_2S, C_3A, ettringite, and thaumasite, and X-ray diffraction(XRD), Raman spectroscopy and infrared spectroscopy(IR) were used to identify thaumasite from other hydrates. To investigate the direct route of thaumasite formation, C_3S, C_2S, gypsum, and calcium carbonate were used to prepare a cement paste so that the interference of ettringite formation can be avoided. The indirect route of thaumasite formation was also studied by considering the effect of C_3A or ettringite content on thaumasite formation. Results show that thaumasite can be definitely generated in the absence of aluminium-bearing minerals or ettringite under appropriate conditions, while the ettringite presence promotes the thaumasite formation. No evidences support the heterogeneous nucleation route in this work. It is concluded that the method mentioned in this work can surely be used to investigate the mechanism of thaumasite formation, and thaumasite can form by both direct and woodfordite routes. Significant efforts have been made in revealing the mechanism of thaumasite formation in concrete, which continues to be fraught with ambiguities and dissension. Chemical method was employed to synthesize pure C_3S, C_2S, C_3A, ettringite, and thaumasite, and X-ray diffraction (XRD Raman spectroscopy and infrared spectroscopy (IR) were used to identify thaumasite from other hydrates. To investigate the direct route of thaumasite formation, C_3S, C_2S, gypsum, and calcium carbonate were used to prepare a cement paste so that the interference of ettringite formation can be avoided. The indirect route of thaumasite formation was also studied by considering the effect of C_3A or ettringite content on thaumasite formation. Results show that thaumasite can be definitely generated in the absence of aluminum-bearing minerals or ettringite under appropriate conditions, while the ettringite presence promotes the thaumasite formation. No evidences support the heterogeneous nucleation route in th It is said that the method mentioned in this work can surely be used to investigate the mechanism of thaumasite formation, and thaumasite can form by both direct and woodfordite routes.