**1. Introduction**

Fly ash (FA) is waste material generated by coal combustion in power plants. It is a fine aluminosilicate powder and owing to these properties, FA has been used for decades as cement and/or a concrete component. However, when FA is used as the major component in high volume FA–Portland cement (PC) blends, it causes prolonged setting time and delayed strength development of a binder [1]. Although the reactivity of FA and early age properties of high volume FA binders can be improved by using chemical activators [1,2], it often results in decreased strength of a binder at later ages.

Spent fluid catalytic cracking catalyst (sFCCC) is a waste material from petrol refineries that consists mostly of zeolite and an amorphous aluminosilicate matrix. Due to relatively small quantities of sFCCC produced (200,000–400,000 tons per year globally [3]), landfilling is often considered as the most economical option for its disposal. This waste material can be used as a cement additive, although its amount in binder is limited by the fact that it can cause short setting time and increased water demand [2,4,5].

Previous studies demonstrated that using sFCCC as a component of blended cements (70–80% of cement in the binder) had a positive impact on the binder properties [6–8]. However, little work has been done so far on investigating the possibilities of using sFCCC as part of a binder in which higher amounts of cement were replaced by waste materials, such as FA. Calorimetric and thermal analyses of very high volume FA binders (20% of cement in the binder) showed that using sFCCC as a FA replacement in the binder accelerated both cement hydration and the pozzolanic reaction [4].

The aim of this work was to further investigate the effects of replacing part of FA in the high volume FA binder (70 mass %) with sFCCC. It was expected that the use of sFCCC as the binder component would shorten setting times and increase early compressive strength of the binder. In order to evaluate the effects of using sFCCC as the binder component,

**Citation:** Raki´c, J.; Bašˇcarevi´c, Z. Influence of Spent Fluid Catalytic Cracking Catalyst on the Properties of the New Binder Based on Fly Ash and Portland Cement. *Mater. Proc.* **2023**, *13*, 8. https://doi.org/ 10.3390/materproc2023013008

Academic Editors: Katarzyna Mróz, Tomasz Tracz, Tomasz Zdeb and Izabela Hager

Published: 14 February 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

setting time, compressive strength, mineral composition and microstructure of the new binders were analyzed.
