Structural Performance of Masonry Columns Confined with PBO-FRCM Systems Incorporating Alkali-Activated Mortar Matrices

Sustainable retrofit strategies for masonry structures require strengthening systems that combine mechanical efficiency, substrate compatibility, and reduced environmental impact. Fabric-Reinforced Cementitious Matrix (FRCM) composites meet many of these requirements and are widely adopted in both modern and historic masonry applications. Nevertheless, the binder used in the inorganic matrix remains a critical factor in determining the overall sustainability of these systems. This study examines the structural feasibility of alkali-activated mortars (AAMs) as alternative matrices for polyparaphenylene benzobisoxazole (PBO)-FRCM confinement of masonry columns. An experimental programme was carried out on clay brick masonry columns subjected to monotonic axial compression. Unconfined specimens were compared with columns strengthened using the same PBO fibre mesh embedded in either a conventional lime-based mortar or an AAM. By keeping column geometry, masonry substrate, reinforcement layout, and fibre architecture constant, the influence of matrix type on the confinement response was directly assessed. The experimental results show that both FRCM systems markedly improve axial load-bearing capacity and deformation performance compared with unconfined masonry. Although the peak strength enhancement is mainly governed by the fibre reinforcement, the tensile behaviour, cracking pattern, and post-cracking stability of the matrix play a crucial role in confinement activation and post-peak response. The findings confirm the potential of alkali-activated binders as more sustainable matrices for FRCM strengthening, while identifying key mechanical parameters requiring further optimisation.