*1.3. Recent State of the Art Knowledge*

In the literature, spent FCC catalysts are predominantly investigated in ORW material, regarding their applicability in construction sector. Accordingly, the substitution of up to 25–30% of cement, by mass, with spent FCC catalysts derived from, e.g., Lithuanian [7] (pp. 103–108), Chinese [8] (pp. 1773–1783), Spanish [9] (pp. 12–17) Portuguese [10] (pp. 109–121) and Oman [11] (pp. 77–81) oil refineries contributed to the mechanical strength enhancement of hardened state cementitious composites. However, even though the incorporation of this waste material typically contributes to rheological properties' deterioration of cementitious composites mixes, which has been confirmed in many works such as [8–10], this negative effect might be reduced by using plasticizing admixtures. Nonetheless, it should be pointed out that the oil-refining catalysts are manufactured individually for each oil refinery company to obtain the most favorable properties, i.e., activity and selectivity, tailored to the specific conditions in the petrochemical installations and depending on many different factors such as type of processing crude oil resources and oil refinery complexity. Therefore, the specific properties, chemical composition and potential pollutants of spent FCC catalysts, as well as of other ORW, might be significantly variable between the oil refineries from which they are withdrawn. This might be considered a key limitation in terms of freely using these materials in construction practice. As such, an individual approach is required for assessing the feasibility of recycling ORW in cementitious composites [2] (pp. 169–174).

#### *1.4. Aim of the Research*

The aim of the conducted research was to assess the potential of spent FCC catalysts derived from a Polish oil refinery company, to be recycled as a partial cement substitute for the production of sustainable cementitious composites. As such, the work consists of preliminary research carried out on basic cementitious composites, i.e., cement pastes, with the addition of a spent FCC catalyst used as a partial cement substitute within the range of 0 to 20%, by mass. The cement pastes have been prepared with water to a binding (cement + spent catalyst) ratio of 0.3. The influence of spent catalysts on their selected properties was investigated through the measurements of slump flow, compressive

strength, flexural strength, water absorptivity and microstructure. For the purpose of future investigations, we plan to expand the scope of the current research by investigating cement pastes prepared with different w/b ratios (e.g., 0.35, 0.40). Moreover, tests will also be conducted on cement mortars. Apart from spent FCC catalysts, we plan also to investigate a few other types of ORW.
