Globally, large quantities of oily sludge are produced in petroleum refineries as wastes from petroleum refining processes. Petroleum refinery oily sludge (PROS) is a major by-product of the processes and a major contributor to pollution in the oil and gas industry. In this study, Response Surface Methodology (RSM) was used for optimising and modelling experimental work. Thermally treated PROS replaced fly ash (FA) at 5�20 in geopolymer mortar mixes at a fixed combination of sodium silicate (Na2SiO3) and sodium hydroxide (NaOH). The visual observations and effects of PROS on the density and compressive strength of PROS geopolymer mortar (PGM) were studied. PGM with 10 replacement of PROS had the maximum compressive strength of 38.17 MPa after 28 days. P-values obtained from the quadratic models developed for the synergistic effect of FA-PROS on density and compressive strength were less than 0.005. Optimisation of the synergistic effect of FA-PROS binder produced an optimal combination of both materials for maximum compressive strength and density of 2200 kg/m3 with desirability factor of 0.981. This investigation shows that replacing PROS with FA in geopolymer mortar can result in a new supply chain for greener binder materials in geopolymer mortar. © 2023 The Authors