This work investigates the reuse of natural (SCB) and aminopropyltriethoxysilane-modified (MSCB) sugarcane bagasse fibres in cementitious composites. Sugarcane bagasse fibres are pre-used in the treatment of motor oil contaminated effluents. A full factorial design is used to identify the effects of fibre type (SCB and MSCB), fibre length (0.6 and 1.2 mm), fibre amount (1 and 2 wt%) and fibre condition (before and after oil filtration) on apparent density, water absorption, apparent porosity, ultra-pulse velocity, dynamic modulus, flexural strength and modulus. SCB fibres lead to increased apparent density compared to MSCB fibre reinforced composites. MSCB fibres contribute to reduce composite porosity, leading to higher mechanical properties. The smaller area of MSCB fibres promotes a larger amount of cementitious phase per unit volume, thus increasing the strength of the sample. Longer sugarcane fibres (1.2 mm) have a larger surface area, leading to a higher fibre concentration per unit volume, which increases water absorption. The amount of fibre has no significant effect on mechanical and physical responses. Composites made with 2 wt% 0.6 mm long MSCB fibres achieve promising results for non-structural civil engineering applications.