Costa, CarlaNunes, Sandra2026-02-202026-02-202026-03Costa, C., & Nunes, S. (2026). Optimisation of hydraulic lime mortars incorporating an oil-refinery catalyst by-product for sustainable building rehabilitation. Development in the Built Environment, 25, 1-13. https://doi.org/10.1016/j.dibe.2026.100860http://hdl.handle.net/10400.21/22672Abstract This investigation employs a Central Composite Design-based Design of Experiments (DoE) methodology to develop hydraulic lime mortars incorporating equilibrium catalyst (ECat), a by-product generated at the fluid catalytic cracking unit in oil refineries. The derived mathematical models describe the quantitative effects of key mixing variables, specifically ECat content, water-to-binder ratio and water repellent dosage, as well as their cross-interactions, on mortar properties, namely workability, compressive strength, ultrasound propagation velocity and dynamic modulus of elasticity. Numerical optimisation techniques enabled the identification of optimal lime mortar compositions that maximise eco-efficiency while ensuring compliance with both regulatory and technological requirements for diverse masonry applications, including the rehabilitation of ancient buildings. Results confirm the by-product upcyclability of ECat, with feasible incorporation levels up to 56.6 % by mass, yielding mortars with significant potential for reducing the environmental impact of the built environment while advancing the circular economy and fostering technological innovation in the construction sector.engSpent fluid catalytic cracking catalyst (ECat)Hydraulic lime-based mortarsBy-product upcyclingHistoric building rehabilitationSustainable construction materialDesign of experimentsMulti-objective optimisationresearch article10.1016/j.dibe.2026.1008602666-1659