Influence of fibre treatments on early-age carbonation in yarn-cement paste composites under enforced carbonation

This study examines whether yarn moisture state and surface treatment can control CO2 transport, carbonation extent, and early-age performance in yarn–cement paste composites under enforced carbonation curing. Oven-dried, saturated, cement-slurry-coated, and epoxy-coated jute yarns were embedded in Portland cement paste (w/c = 0.40) and compared with synthetic yarns (acrylic, nylon, and polypropylene). Carbonation was assessed along sample depth using phenolphthalein image analysis, thermogravimetry, quantitative X-ray diffraction, SEM-BSE porosity mapping, and compressive strength testing. Saturated jute yarn produced the most carbonation, forming interconnected carbonated belts due to its porous structure and internal moisture retention, whereas epoxy-coated jute and synthetic yarns showed limited carbonation. Effective carbonation radius correlated strongly with TGA-measured CO2 uptake (R2 = 0.962) and 24 h water absorption (R2 = 0.872). After 7 days, carbonated cement pastes with 0.76 vol% saturated jute yarn reached compressive strengths of 80.1 MPa, comparable to the cement paste control (77.2 MPa). These results show that simple pre-saturation improves carbonation efficiency, while early-age compressive strength is maintained only at low yarn volume where carbonation-induced densification compensates for fibre-related matrix disruption.