###### Abstract
The availability of aluminosilicate by-products such as fly ash, ground-granulated blast-furnace slag (GGBS), red mud, and metakaolin is dependent on local resources [10]. Recently, the supply of fly ash (one of the most commonly used and cost-effective binder materials for geopolymers) has decreased due to a reduction in coal-fired power plants related to increasing uses of renewable energy sources such as wind and solar [11]. Regional shortages have been reported in North America and European countries [12]. For instance, the fly ash shortage in the U.K. is anticipated to be approximately 2 million tons in 2030 due to the planned closure of all coal-fired power plants by 2025 [12; 13]. Against this background, by-products from fluid catalytic cracking (FCC) processes within the oil refining industry are proven to offer a potential alternative to conventional aluminosilicate fly ash [14; 15; 16; 17]. The silica and alumina content of FCC by-products is typically greater than 90% by mass [14], a feature that has been successfully exploited in their use as supplementary cementitious materials in conventional Portland cement concrete binders [17]. Contrary to the downward trend of coal-fired power plants described above, production rates from the petroleum industry have remained steady in oil-producing countries such as Vietnam, the United Kingdom, and the United States [18; 19; 20]. For example, production of crude oil in the U.K. increased from 12 million tons in 2017 to over 13 million tons in the second quarter of 2020 [19]. The corresponding amount of FCC by-product sent to landfills each year was reported to be approximately 840 thousand tons [21]. According to [PERSON] et al. [15], the spent FCC catalyst showed great practical potential as an aluminosilicate precursor for geopolymer production. In this work, its high reactivity was established from observations from X-ray diffraction, Fourier transform infrared spectroscopy, solid-state \({}^{29}\)Si and \({}^{27}\)Al magic angle spinning nuclear magnetic resonance, and scanning electron microscopy (SEM). [PERSON] et al. [16] synthesized geopolymer binders using spent an FCC catalyst and alkali activator comprising sodium silicate and sodium hydroxide (NaOH) with various levels of NaOH concentration and silica modulus. The resulting geopolymer binders achieved a compressive strength of 25 MPa after 7 days. In a study by [PERSON] et al. [17], FCC residue-based geopolymer mortar specimens with ratios of SiO\({}_{2}\)/Na\({}_{2}\)O ranging from 0-1.46 were cured under high-humidity conditions (relative humidity of 100%) at 65 \({}^{\circ}\)C for three days. The resulting compressive and flexural strengths varied from 8.5-68 MPa and 2.5-11.5 MPa, respectively. Microstructurally, the FCC geopolymer mortar specimens were observed to be in a dense-compact amorphous state via SEM analysis. In addition to geopolymer precursor applications, FCC residue and slurry oil have been used as the main constituent materials in conventional concrete [22; 23], soil stabilization [24], ceramic [25], and paving asphalt [26]. Thorough research of relevant literature yielded that minimal comprehensive research has been undertaken to investigate the effects of fly ash-FCC combinations on geopolymerization processes and the mechanical properties of geopolymer concrete. As such, this study aims to assess the rheological and hardened properties of geopolymer concrete containing FCC residue as a full or partial replacement of fly ash. Effects of FCC residue content (0, 20, 40, 60, 80, and 100% by mass) on yield stress, plastic viscosity, compressive strength, elastic modulus, and flexural strength of geopolymer concrete were investigated using standard mechanical testing. Additionally, SEM and X-ray diffraction (XRD) techniques were employed to evaluate the roles of FCC residue on microstructural properties of geopolymer concrete. Image processing using ImageJ software was used to determine void volume percentage in binary SEM photos.