Effect of Coconut Fiber Content on the Properties of Unfired Building Bricks Incorporating Thermal Power Plant Ashes

This study examined the effect of coconut fiber (CF) content on the properties of unfired building bricks (UBBs) using fly ash (FA) and bottom ash (BA) from the Nghi Son coal-fired thermal power plant (CTP). Cement and FA served as binders while BA replaced crushed stone as the fine aggregate. Raw brown CFs were treated with a 5% NaOH solution at 50 °C to increase their mechanical properties. The treated CFs were then cut to a short length of 15 mm and had an average aspect ratio of 103. Brick samples were cast with CF content at ratios of 0%, 3%, 6%, and 9% by binder weight (CF/B) and water-to-binder (W/B) ratios of 0.30 and 0.35. The UBBs were formed under a low pressure of 0.5 MPa and tested to evaluate the effects of CF/B and W/B ratios on brick properties. Results showed that compressive strength (CS) increased with higher CF content, with all samples exceeding 10 MPa after 28 days, classifying them as grade M10 per TCVN 6477:2016. The optimal CF content was 6%, which yielded the highest CS. Increasing CF content reduced unit weight (UW), ultrasonic pulse velocity (UPV), and thermal conductivity (TC), while water absorption (WA) increased. Conversely, a lower W/B ratio improved CS, UW, UPV, and TC, while reducing WA. Microstructural observations revealed that CF acts as a bridge connecting cracks, which helps restrain crack widening and enhance CS.