Expansion joints silicone sealants used in high speed railway construction suffer from ultraviolet radiation(UV),high temperature combined with the alkaline environment.To evaluate the durability and analyse the ageing mechanism,six one-component silicone sealants from different companies were selected and subjected to accelerated ageing tests including UV,thermal and alkali ageing treatments.The ageing effects on the performance of the sealants were evaluated via the appearance and the mechanical property changes.The changes in molecular structure were studied by means of Fourier transform infrared spectroscopy(FTIR),differential scanning calorimetry(DSC)and thermogravimetry(TG).This study revealed that different materials displayed different sensibilities to the ageing methods,in which 15%-20%decreases of mechanical properties could be observed under UV radiation test,owning the most significant effects.Structure analysis showed that the physical changes of aggregative states were the principal factors to the performance,along with the chemical slight changes.The glossiness dropped significantly in ageing test,which could be used as one of the effective evaluation parameters for ageing conditions in the field.