From waste to strength: surface modification of açaí fibers using tannic acid and NaOH for cement composites

Natural lignocellulosic fibers are promising reinforcements for cementitious composites; however, their application is limited by low durability and weak fiber–matrix interaction in alkaline environments. This study investigates the effect of surface treatments on açaí fibers to improve their performance in cement–lime mortars, addressing the use of agro-industrial residues in construction materials. Untreated fibers and fibers treated with sodium hydroxide (NaOH) and tannic acid were incorporated at 1.5%, 3.0%, and 5.0% by mass into mortars (1:1:6, cement: hydrated lime: sand by mass). Mixing was carried out following ABNT NBR 16,541, using a standardized two-speed mechanical procedure with an intermediate resting period. The mortars were cast, demolded after 24 h, and cured at room temperature for 28 days prior to testing. Fiber characterization included SEM, FTIR, XRD, TGA, and contact angle, while mortars were evaluated for workability, mechanical strength, and carbonation resistance. NaOH treatment increased surface roughness, enhancing mechanical interlocking, whereas tannic acid formed a hydrophobic layer, improving fiber stability. Mortars with tannic acid-treated fibers showed superior performance, with increases of up to 32% in flexural strength and 18% in compressive strength, along with reduced carbonation depth (up to 25%).