The effects of chemical (30% sulphidity and 30% alkali active) and mechanical treatments (1500, 2000, 7500 and 10,000 revolutions for different refining and beating rates) on Guadua angustifolia Kunth (Bamboo) fibres are investigated for the first time. The morphological, chemical and physical changes of treated and untreated fibres after refining were characterised and analysed. The macro mechanical and structural properties at 7 and 28 days of curing of fibre-cement composites reinforced with treated fibres are evaluated. Fourier Transformed Infrared (FT-IR) and X-Ray Diffraction (XRD) spectra are performed to investigate structural compound changes. The MOR (Modulus of Rupture), fracture surface and Scanning Electron Microscopy (SEM) images were determined and analysed. The evaluation of manufactured cement composites showed that flexural strength increased by approximately 15% when composites were reinforced with alkali-treated fibres compared to untreated fibres. However, in the case of alkali fibres combined with mechanical refining, the increase reaches 57%. The specific energy (SE) of cement composites is not considerably different between untreated and alkali-treated fibres. Nevertheless, the combination of alkali with mechanical refining results in an increase in SE of more than 7 times. Although the combination of alkali treatment and mechanical refining improves the flexural strength and specific energy of cement composites, the high refining rates have a negative effect on these properties owing to an increase in the fibre fibrillation and a decrease in the swelling capacity, which reduce the fibre–matrix bonding.