Mechanical and durability performance of mortar containing biochar derived from pyrolyzed Posidonia oceanica leaves: a circular approach to marine biomass waste

This study investigates the feasibility of using biochar derived from pyrolyzed Posidonia oceanica leaves (PBC) as a partial cement replacement for sustainable mortar production. The research addresses two critical challenges simultaneously: valorization of marine biomass waste and reduction of the carbon footprint associated with Portland cement. Biochar produced at 400 °C was incorporated at replacement levels of 1-6% by weight of cement, and the resulting mortars were evaluated for fresh properties, mechanical performance, durability, microstructure, and environmental impact. Workability decreased with increasing PBC content, while compressive strength at 28 days improved at low replacement levels, reaching an optimum at 3% with 8.71% strength increase relative to the control mixture. Water absorption decreased marginally from 5.38% in the control to 5.31% at optimal PBC content, but increased progressively at higher replacement levels, reaching up to 7.50%, indicating improved matrix compactness. Higher dosages resulted in strength reduction due to increased porosity and interfacial defects. Thermal resistance testing showed stability of PBC3 specimens up to 600 °C, whereas higher contents led to structural degradation, showing microcracking and mass loss exceeding 7%. Microstructural analysis confirmed pore refinement at low dosages and matrix disruption at higher contents. Embodied carbon decreased linearly with increasing PBC content, achieving a 5.3% reduction at 6% replacement. One-way ANOVA confirmed that PBC dosage significantly influenced the fresh, mechanical, and durability properties of the mortar (p < 0.05). Overall, 3% PBC was identified as the optimal dosage balancing mechanical performance, durability, and sustainability. The findings position biochar as a sustainable solution for lowering cement consumption and transforming PBC waste into value-added construction materials.