Microstructural and multiscale performance regulation of UHPC by plant ash-modified bamboo fibers: A green interfacial strategy

In this study, the effects of NaOH and plant ash treatments on the surface properties of bamboo fibers and their interfacial performance in ultra-high-performance concrete (UHPC) were systematically investigated. Alkaline modification altered fiber morphology, chemistry, and wettability, thereby influencing workability, mechanical strength, and microstructure of UHPC. Excessive NaOH (10 %) caused surface erosion, functional group loss, and reduced mechanical performance. In contrast, plant ash at 10–20 % concentrations preserved fiber roughness, enhanced polar group exposure, improved hydrophobicity, and increased compressive (up to 132.37 MPa) and flexural (+29.7 %) strengths. Microstructural analysis confirmed that plant ash promoted stable Ca/Si-rich interfacial layers, balanced hydration products, and refined pore structures. SEM-EDS mapping revealed a continuous Si- and Ca-rich reaction zone with stable Ca/Si ratios (1.2–1.6) in the ash-treated groups. TG analysis indicated increased CH content (8.28 % for 20 % Ash) and stable bound water retention. FTIR spectra showed preservation of the main Si–O–Si peak at 1083 cm⁻¹ and an increased proportion of Q¹ structures, reflecting enhanced silicate polymerization. MIP results demonstrated reduced total porosity (20 % Ash group, 6.04 %) and a higher fine pore (<50 nm) fraction (51.7 %), indicating improved compactness and homogeneity. These improvements are attributed to mild surface etching and in-situ mineral deposition, enhancing fiber–matrix bonding and hydration stability. Overall, plant ash treatment offers a green and effective interfacial engineering strategy for incorporating biomass fibers in high-performance cementitious composites.