<p>This study addresses the urgent need for sustainable alternatives by comparing Rat-Trap Bond (RTB) masonry constructed with Fly Ash (FA) bricks against traditional English bond clay brickwork. A three-phase experimental program was undertaken: (1) characterization of material properties (water absorption, compressive strength, and bond strength) of FA bricks and cement mortar; (2) evaluation of mechanical performance through prism testing (compressive, diagonal shear, and triplet tests) following ASTM and EN standards; and (3) in-plane quasi-static cyclic loading of masonry walls to assess lateral strength, ductility, stiffness degradation, and energy dissipation. Results reveal that RTB FA brick prisms exhibit an average compressive strength of 3.6&#xa0;MPa and a modulus of elasticity of 1.20 GPa, while shear tests yielded a cohesion of 0.09&#xa0;MPa and friction coefficient of 0.43. Quasi-static in-plane testing of Rat Trap Bond (RTB) fly ash brick masonry walls showed a sufficient elastic stiffness (15.95 kN/mm) and moderate ductility (mean ratio 4.41), as well as stable energy dissipation mechanisms. Seismic performance parameters such as stiffness, strength, and ductility were assessed using a bilinear idealization of the average envelope curve derived from the equal energy principle, backing the possible appropriateness of RTB masonry in areas vulnerable to earthquakes. A bilinear idealization of the envelope curve verifies the applicability of RTB masonry in seismic-prone regions. Economic and environmental analyses indicate an 88% reduction in carbon emissions and a 25% cost saving per cubic meter of wall. These findings substantiate RTB FA brick masonry as a viable, eco-efficient construction technique.</p>