The pozzolanic reactivity of a recycled glass fibre reinforced polymer (GFRP)-derived supplementary cementitious material (SCM) was enhanced through mechanical treatment involving ball milling (120 rpm for 60–360 min). The nature of the amorphous phase was characterised using quantitative X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectrometry, which indicated the material to be significantly short-range disordered (>95 wt%). The pozzolanic reactivity was evaluated by determination of the bound water content in parallel with twin-phase amorphous quantification using XRD. The quantity of residual glass was evaluated using an arbitrary phase populated with a calibrated peak list following the PONKCS (partial or no known crystal structure) method, standardised to a 50 wt% corundum spike. The amorphous hydrate content was determined with a spike phase. The strength development of mortar with this new SCM was then investigated. All experimental tests were repeated at least three times to ensure statistical significance. The results indicated a matching trend correlating reactivity, strength, and fineness. The compressive strength at both 7 and 14 days increased by approximately 50 % when the milling duration was extended from 60 min to 360 min. The increase in reactivity of the milled GFRP was attributed to a combination of an increase in specific surface and the alteration to Si-O-Si sites at the reactive surface which enhanced the solubility characteristics.