*1.2. Oil Refinery Waste Recycling*

Recently, various waste materials derived from petrochemical industry, i.e., oil refinery wastes (ORW) have caught researchers' attention in terms of being investigated as a

potential green resource for the construction sector. One considerable type of solid ORW is spent catalysts; they are generally used to promote the conversion processes of crude oil into petrochemical products. These materials become deactivated after a period of time being used in the refining processes, due to the mechanical degradation and most notably contamination with various pollutants from processing resources which, while adsorbed on their structure, reduce their specific surface area and thus their catalytic activity. Consequently, spent catalysts are withdrawn from petrochemical installations of single-complex oil refinery companies in an estimated annual amount of 5515 tonnes [5]. It should also be pointed out that almost 10% of that quantity is represented itself by spent catalysts withdrawn from fluidized catalytic cracking units, which makes them the largest group of catalytic wastes in the worldwide petrochemical industry [5]. Unfortunately, the majority of these waste materials are consequently sent to landfills, thereby providing economical losses for companies, and serious health risks related to the release of chemical pollutants into the natural environment [2] (pp. 169–174). Apart from organic compounds, spent FCC catalysts might also contain heavy metals, such Cr, Pb, and Ni [6] (pp. 111–118). Therefore, finding a newer, safer application for these waste materials is necessary. A reasonable research direction for this purpose seems to be the use of ORW as partial cement substitutes in cementitious composites. Such a procedure, apart from reducing cement usage, might potentially allow the binding of chemical pollutants in their hardened state to the composite structure, thereby minimizing the risk of their release to the environment [6] (pp. 111–118). Moreover, as waste recycling is the most preferable option for managing waste materials and moving towards a circular economy, the suitability of such direction is clearly underlined.
