*2.1. Materials Characteristic*

The spent FCC catalyst used in this research was supplied by a Polish oil refinery company. The role of FCC catalysts in the petrochemical industry is to promote the cracking reaction of high-boiling oil fractions into lower-boiling hydrocarbons for blending commercial fuels. From the physicochemical viewpoint, this waste material might be characterized as a white-grey powder. In the research, it was used as a substitute of cement (CEM I 52.5R). The microstructures of these materials have been presented in Figure 1 in the form of SEM images at ×100 magnification. Their chemical compositions, obtained using X-ray fluorescence spectrometer analysis (XRF) and from [12] for the spent catalyst and cement, respectively, have been presented in Table 1. Accordingly, the mean particle size of cement grains is finer than in case of the spent catalyst which, as a typical pozzolanic material, is mainly composed of SiO2 and Al2O3.

**Figure 1.** SEM images of microstructure of the materials: (**a**) cement; (**b**) spent FCC catalyst.


**Table 1.** Chemical composition of the materials, wt.%.

### *2.2. Formulations of Cement Paste Mixes*

The cement paste mixes were prepared by mixing of ingredients in the laboratory bowl. The spent FCC catalyst was used as a substitute of 0%, 5%, 10%, 15% and 20% of cement. The water to binder (w/b) ratio of 0.3 was kept constant for all of the formulations of mixes. Accordingly, the reference (plain cement) mix is described as 0.30 ref, while subsequent substituted mixes are described as 0.30 5%, 0.30 10%, 0.30 15% and 0.30 20%, corresponding to the cement substitution level with the spent catalyst, i.e., 5%, 10%, 15% and 20%, by mass, respectively. The complete formulations of paste mixes have been listed in Table 2.


**Table 2.** Formulations of 1 dm3 of cement paste mixes.
