Performance enhancement and microstructural characterization of pine fiber-reinforced rammed earth stabilized with limestone powder and LC³

The renewed interest in rammed earth (RE) as a sustainable construction material requires addressing its inherent limitations related to moisture sensitivity, low tensile capacity, and variability in mechanical performance. This study investigates a stepwise stabilization strategy combining pine fibers (PF) as discrete reinforcement, limestone dust powder (LSP) as a mineral filler, and limestone calcined clay cement (LC3) as a low-carbon binder to improve mechanical performance, short-term moisture resistance, and microstructural characteristics of RE composites. This approach also supports sustainability by valorizing forest-derived pine biomass associated with wildfire fuel loads and quarry fines, while reducing clinker content through LC3 incorporation. Rammed earth blocks were produced with 1% PF and 10–25% LSP, followed by the introduction of a fixed 10% LC3 dosage into the optimized PF-LSP composition. Performance was evaluated through compaction characteristics, dry and wet compressive strength, flexural strength, ultrasonic pulse velocity (UPV), and 24 h water absorption. Microstructural evolution was examined using FESEM-EDS, XRD, and TGA. Among the tested formulations, SREPF1LS20LC10 exhibited the best overall performance, achieving a dry compressive strength of 5.47 MPa, flexural strength of 1.47 MPa, UPV of 2858 m/s, water absorption of 10.96%, and a wet-to-dry strength ratio of 0.51. Microstructural analyses provided evidence consistent with matrix densification, pore refinement, and the presence of poorly crystalline hydration products and secondary carbonate phases, while fiber-matrix interaction remained predominantly mechanical. Within the investigated design space, the proposed system demonstrates a technically viable and low-carbon pathway for enhancing rammed earth performance, with durability claims limited to short-term moisture resistance indicators.