Oil refinery sludges from tank bottoms, oil-water-solid separators and dissolved air-filtration range around 0.2 w% of crude oil processed and are classified as hazardous wastes. Worldwide data manifest large disparities in composition (10-20% hydrocarbons, 5-20% solids -including heavy metals- and water) and heating values. Treatment by landfarming/bioremediation, hazardous waste incineration, anaerobic digestion or disposal in hazardous waste landfills bears high costs and environmental impacts. Substitution of fossil fuels (petcoke) by refinery sludges in cement plants is investigated. Main impacts are quantified: energy efficiency and clinker production level, plant operation and capacity constraints, halogen presence in plant circuits, inclusion of alkalis and heavy metals in cement and atmospheric emissions. It is found that clinker losses may exceed one tonne clinker/tonne sludge. Environmental concerns, mainly increased NOx and Hg emissions, may restrict substitution. Guidelines are obtained for identifying efficient blends with renewable biogenic fuels (urban or agro-industrial biowaste). Blends of sludge from local refineries with renewable fuels are assessed for a local cement plant using about 1.65 x 10(5) tonnes fossil fuels/year (70% petcoke, 30% coal) to produce 1.5 x 10(6) tonnes clinker/year. Superior customer-tailored blends are identified that avert clinker losses, maintain operability and tolerable inclusions, reduce greenhouse gas emissions and improve atmospheric pollutant emissions. (C) 2020 Published by Elsevier Ltd.