Investigation of the durability of hemp yarns used in textile reinforced mortar (TRM)

The rehabilitation of historical masonry structures requires reinforcement systems that are durable, compatible with mineral substrates, and environmentally sustainable. This study investigates the durability of textile reinforced mortar (TRM) composites manufactured with hemp yarns and two different matrices: an ettringitic mortar (pH 10.5) and a natural hydraulic lime mortar (pH 12.5). The tensile behavior of hemp yarns was evaluated before and after accelerated aging in alkaline solutions (pH 7, 10.5, 12.5) at 23, 40, and 60 °C for up to 90 days. Hemp-based TRMs were also tested in tension after 28 days of curing. Unaged yarns showed an average tensile strength of 269 MPa and modulus of 4.2 GPa. After 90 days of immersion, severe degradation occurred in pH 7 and pH 10.5 at 23 °C. In contrast, yarns immersed in pH 12.5 solutions at 40 and 60 °C retained over 90% of their initial strength, suggesting an unexpected stabilizing effect of the most alkaline environment at elevated temperatures. TRM specimens exhibited a two-stage tensile response (matrix cracking followed by yarn slippage), differing from the typical three-stage behavior of TRMs. This was attributed to the high water absorption of hemp yarns and the weak textile–matrix bond. Thus, hemp yarns possess favorable short-term mechanical properties, but their use in TRM systems requires surface treatments to improve durability and fiber–matrix adhesion. The findings provide new insights into the interaction between vegetal fibers and mineral matrices, and highlight the need for optimized treatments to enable hemp TRMs as sustainable alternatives in structural rehabilitation. • The effect of different accelerating aging conditions on hemp yarns performance is investigated. • Before aging, the hemp yarns show good mechanical performances. • The pH and the temperature influence the durability of dry hemp yarns in alkaline solutions. • The tensile behavior of dry hemp based TRM is investigated. • The experimental results show that a protection of the hemp yarns is needed in TRM applications.