The effect of various fiber surface modification on the microstructure and strength of sisal-reinforced mortar

This study investigates the mechanical performance, workability, and durability of sisal fibre-reinforced mortar (SFRC) modified through alkali treatment (NaOH) and combined alkali-paraffin treatment. Three mortar mixes were evaluated: untreated sisal fibres (USFRC), alkali-treated fibres (ATSFRC), and paraffin-alkali-treated fibres (PATSFRC). Mechanical properties were assessed at 3, 7, 14, 28, and 56 days, while durability was evaluated through sorptivity and chloride conductivity tests at 28 and 56 days. Microstructural characterization using SEM, FTIR, and EDX revealed treatment-dependent morphological and chemical modifications that directly influenced composite behaviour. Workability improved progressively with fibre treatment, with PATSFRC achieving the highest slump value (85 mm) compared to USFRC (65 mm). Setting times increased from USFRC (initial: 75 min, final: 135 min) to PATSFRC (initial: 165 min, final: 255 min), indicating that paraffin coating delays hydration by creating a hydrophobic barrier. Mechanical testing showed that all fibre-reinforced mixes outperformed the control mortar, with ATSFRC exhibiting the highest compressive (37 MPa), tensile (3.4 MPa), and flexural (5.3 MPa) strengths at 56 days—representing increases of 19%, 26%, and 20%, respectively, over the control. The improved mechanical performance of ATSFRC correlates with SEM-observed surface roughening and FTIR-confirmed removal of hydrophilic components, which enhanced fibre-matrix interfacial bonding. Durability assessments revealed significant treatment-dependent differences. PATSFRC exhibited the lowest sorptivity (0.08 mm/min¹/² at 28 days; 0.05 mm/min¹/² at 56 days) and chloride conductivity (760 C at 28 days; 500 C at 56 days), representing 64% and 17% reductions, respectively, compared to USFRC at 56 days. These improvements are attributed to the paraffin coating’s hydrophobic barrier effect, which limits moisture ingress and alkali degradation. Microstructural analysis confirmed that paraffin-treated fibres maintained structural integrity with minimal mineral deposition, while untreated fibres showed extensive degradation. XRD analysis revealed increased cellulose crystallinity in alkali-treated fibres, while paraffin treatment created a composite diffraction pattern indicating surface-level modification without altering the crystalline core. FTIR spectroscopy confirmed successful removal of hemicellulose and lignin in alkali-treated fibres and the presence of hydrophobic hydrocarbon chains in paraffin-coated fibers. This study establishes clear correlations between fiber surface modification, microstructural characteristics, and macroscopic performance. The results demonstrate that combined alkali-paraffin treatment provides an optimal balance between mechanical enhancement and durability, offering a sustainable reinforcement strategy for cementitious composites.