Influence of low-alkali surface modified natural fibres on the performance, microstructure and sustainability of hybrid fibre reinforced concrete

An innovative and low-environmental-impact strategy for sustainable high-performance concrete is presented through the use of Na 2 CO 3- treated natural fibres combined with steel fibres in silica fume concrete. Natural fibres require surface treatment to enhance fibre -matrix bonding in cementitious composites. In this study, Na 2 CO 3 -treated natural fibres (coir, sisal and ramie) were incorporated into hybrid steel fibre reinforced silica fume concrete, with polypropylene fibre used as a reference for comparison. Mechanical, durability, microstructural and environmental properties were evaluated and statistical analysis was performed using ANOVA-based analysis. The Na 2 CO 3 treatment improved fibre surface characteristics and enhanced fibre-matrix interaction. The incorporation of hybrid fibres significantly improved mechanical performance, with the steel-coir system (SC 1:0.5) achieving increases of 38% in compressive strength, 41% in flexural strength and 45% in split tensile strength compared to the control mix. Durability performance was also enhanced, with reductions of up to 12.9% in water absorption and 16% in sorptivity. Microstructural analysis indicates matrix densification and refinement of the Interfacial Transition Zone (ITZ), supported by increased C-S-H formation and reduced portlandite content. Statistical analysis suggested that fibre type, fibre ratio, and their interaction significantly influenced the measured mechanical properties. Additionally, CO 2 emissions were reduced by approximately 50-52 kg/m 3 . The results demonstrate that hybridisation of steel fibres with Na 2 CO 3 -treated natural fibres provides an effective and sustainable approach for enhancing the performance and durability of high-strength concrete.