Microstructural evolution of accelerated-carbonated MgO-based fiber cement reinforced with lignocellulosic and recycled textile fibers

Accelerated carbonation has emerged as a promising post-curing strategy for modifying the microstructure and improving the performance of MgO-based cementitious materials. This study investigates its influence on the microstructure, physical properties, and mechanical performance of magnesium oxysulfate (MOS)-based fiber cement composites reinforced with bleached eucalyptus pulp (EC), pine fibers (PI), and recycled cotton-based textile fibers (RF). After 48 h of hydration, the composites were subjected to accelerated carbonation and characterized by flexural testing, physical-property measurements, mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TG/DTG), scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM/EDS), and instrumented indentation testing. Accelerated carbonation increased the mean modulus of rupture (MOR) by 20.5%, 3.6%, and 28.7% for EC, PI, and RF, respectively, with a statistically significant within-fiber increase observed for RF. The RF composite also showed a 79.2% increase in specific energy, indicating an enhanced post-cracking response after carbonation. Carbonation significantly reduced apparent void volume, while water absorption decreased and bulk density increased numerically across the three fiber systems. MIP further showed lower accessible pore volumes and refinement of the pore structure after carbonation. Instrumented indentation revealed higher mean indentation elastic modulus (EIT) and hardness (HIT), together with lower maximum penetration depth (hmax), for all carbonated composites, with RF showing a particularly marked increase in local stiffness and hardness. Microstructural analyses supported the formation of hydrated magnesium carbonates (HMCs) after carbonation, while XRD indicated preservation of the primary 5-1-7 MOS hydration phase without distinct new crystalline magnesium carbonate phases. Semi-quantitative DTG deconvolution estimated HMC contents of 4.67%, 3.49%, and 4.65% for EC, PI, and RF, respectively. SEM observations showed a more continuous fiber–matrix interface after carbonation, with HMC-like products observed within pores and interfacial regions. The combined results indicate that the enhanced response of carbonated MOS-based fiber cement is associated with HMC formation, pore refinement, and changes in the local mechanical and fiber–matrix interfacial characteristics.