This study demonstrates the potential of developing a future Raman spectroscopy-based sensor system for monitoring carbonation of concrete structures. A tailored optical fibre Raman system was developed under the classical 45° geometry with a specific optical fibre assembly acting as the laser excitation-path. This system was then employed to characterise the calcium carbonate polymorphs, i.e. calcite and/or aragonite [ʋ1 (CO₃²¯) at 1086 cm¯¹], formed in the carbonated PC paste powder. The finding shows a good potential of optical fibre Raman spectroscopy for monitoring the health condition of concrete structures in the future.