*3.4. Microstructure of Cement Paste Samples*

The microstructures of the cement paste samples have been presented in Figure 6 in the form of SEM images at ×200 magnification. On the basis of the greyscale of the images, the components and pores were identified. The light grey shapes correspond to the unhydrated cement grains, whilst the coarser spherical shapes in darker color are attributed to unreacted spent catalyst particles. Accordingly, the pores, which might be seen as a black pixels, are mainly identified in the interphase transition zone (ITZ) between spent catalyst and CSH gel. Therefore, their content was found to increase with the increasing cement substitution level. As such, the matrix of the 0.30 ref was found to the be more homogenous and uncracked when compared to those matrices of the samples with spent catalyst addition.

**Figure 6.** SEM images of microstructure of the respective cement paste samples: (**a**) 0.30 ref; (**b**) 0.30 5%; (**c**) 0.30 10%; (**d**) 0.30 15%; (**e**) 0.30 20%.

### **4. Discussion**

The negative effect of the addition of spent FCC catalysts on the slump flow and rheological properties of cement paste mixes was found to be in accordance with the literature results [8–10]. This finding might be generally attributed to the very high specific surface area of spent catalysts, which is provided by highly porous zeolite structure and identified in [8] (pp. 1773–1783). Thereby, spent FCC catalysts are a water-demanding material which contribute to the reduction of available water in the system and hence the workability of fresh mixes. The slump flow value of the 0.30 5% mix was found to be in the scope of standard deviation thresholds of 0.30 ref, and thus the slight increase in slump flow between those mixes was assumed to be the result of a measurement statistic.

The development of the compressive strength of the cement paste samples with the addition of spent FCC catalysts has been strengthened by the number of works, such as [7] (pp. 103–108). This phenomenon might be attributed to the vital pozzolanic activity of spent catalysts, identified in [13]. Pozzolanic activity is the ability of a material to react with Ca(OH)2 in the presence of moisture to form additional CSH gel in the system, thus contributing to compressive strength gain. Such behavior is promoted by the typical pozzolanic properties of spent FCC catalysts, i.e., aluminosilicate composition and high specific surface area.

The deterioration of the flexural strength of cement paste samples in relation to the increasing spent catalyst addition seems to be caused by the porous structure of the respective samples, as has been investigated through SEM analysis. Accordingly, the interphase transition zone (ITZ) reveals a lack of adhesion between the spent catalyst particles and CSH gel. This might be associated with the poor binding provided by the significant consumption of water by spent catalyst.

The results of water absorptivity of cement pastes indicated that spent FCC catalysts, while substituting 10% and more of cement, generally promote the water absorptivity of the respective paste samples. This finding is in accordance with [6] (pp. 111–118), and might be attributed to the fact that those samples tend to dry up, again as the catalyst provides high

water absorption and thus also prompts water content reduction in the system. Moreover, such behavior seems to be strengthen by the low w/b ratio used to prepare the mixes.

Overall, based on the preliminary exploration of the possibility of using spent FCC catalysts as a pozzolanic additive to cementitious composites, this waste material was found to be a promising substitute for up to 20% of cement in primary cementitious composites, i.e., cement pastes, with the slight decrease in workability and increase in absorptivity acceptable from a construction materials quality viewpoint. It is vital that their addition results in higher compressive strength of hardened samples, whilst simultaneously decreasing the cost and carbon footprint arising from the cement industry. Therefore, the obtained results, combined with the conducted research into the applicability of spent FCC catalysts in cement mortars described in [14], allow us to conclude on the rightness of further research into using spent FCC catalysts from the Polish petrochemical industry in cementitious composites. The authors have to perform planned future studies in terms of investigating the impact of the addition of spent FCC catalysts on the properties of cement mortars. The research will include purification of spent catalysts from pollutants, and toxicity analyses of the resulting materials. The pozzolanic activity of spent catalysts and the setting time of related binders will be also measured. Moreover, other spent catalysts and petrochemical wastes, i.e., molecular sieves, will be investigated regarding such applicability.
