## 3 Results and Discussion
### Mechanical Properties of Hardened Geopolymer Concrete  
The relationship between FCC residue content (% by binder mass) geopolymer concrete mechanical properties such as compressive strength and flexural strength after 7 curing days is shown in Figure 5. As shown in Figure 5a, geopolymer concrete containing fly ash and FCC residue possessed a wide range of mechanical properties, with compressive strength of 10-26 MPa and flexural strength of 0.9-2.8 MPa. In the case of fly ash-based geopolymer concrete without FCC residue, geopolymer strength increased with a decrease in ALGS ratio. There were two different trends presented in Figure 5, with the increased mechanical performance trend associated with increasing FCC residue content in geopolymer mixtures ALGS = 0.65 and decreasing compressive strength associated with increasing FCC residue content in geopolymer mixtures ALGS = 0.5 and ALGS = 0.4. The positive performance of geopolymer concrete mixes  
Figure 5: Relationship between FCC residue contents (% by binder mass) and mechanical properties of geopolymer concrete. (**a**) Compressive strength. (**b**) Flexural strength. AL:GS = 0.65 comprising FCC residue was highlighted, with 7-34% higher compressive strength than fly ash-based geopolymer concrete. In contrast to mixes AL:GS = 0.65, there is a negative impact of FCC residue content on geopolymer mixes AL:GS = 0.5; and AL:GS = 0.4, with maximum 17% strength reduction associated with geopolymer concrete with 100% FCC residue. Additionally, when FCC residue content increased from 0 to 100%, the flexural strength of AL:GS = 0.65 mixtures increased to 67%, while up to 14% of the flexural reduction was observed in the cases of AL:GS = 0.65 mixtures. Elastic modulus is a significant parameter of concrete for structural design, which is calculated as the secant modulus measured at the stress value equal to 40% of the average compressive strength of concrete cylinders, according to ASTM C469 [33]. Figure 6a presents the relationship between FCC residue contents (% by binder mass) and elastic modulus of geopolymer concrete. As shown in this figure, geopolymer concrete containing FCC residue possessed a modulus of elasticity ranging within 12.3-21.3 GPa. A similar trend with concrete strength obtained in Figure 5 was found where decreasing elastic modulus corresponded to increasing FCC residue content in geopolymer mixture AL:GS = 0.5 and 0.4, while increasing FCC residue content in geopolymer mixture AL:GS = 0.65 led to an improvement in elastic modulus. A comparison between elastic modulus obtained from experiments and estimated by the following equations [37; 38; 39; 40; 41] is shown in Figure 6b. \[E_{c}=3320\sqrt{f_{c}}\ +6900 \tag{4}\]  
\[E_{c}=0.85\times 2.15\times 10^{4}\times\sqrt[3]{0.1f_{c}} \tag{5}\]  
\[E_{c}=2707\sqrt{f_{c}}+5300 \tag{6}\]  
\[E_{c}=0.037\rho^{1.5}\sqrt{f_{c}} \tag{7}\]  
\[E_{c}=5300\sqrt[3]{f_{c}} \tag{8}\]  
where \(f_{c}\) is the mean compressive strength (MPa) and \(\rho\) is the unit-weight of geopolymer concrete (kg/m\({}^{3}\)). The average unit-weight of geopolymer concrete was 2415 kg/m\({}^{3}\). It is noted that Equations (3) and (4) were used to calculate the value of elastic modulus of normal concrete, while Equations (5)-(7) are designed for the determination of elastic modulus of geopolymer concrete. According to Figure 4, the value of elastic modulus estimated using equations for geopolymer concrete was lower than that of conventional concrete. The most significant  
Figure 6: Relationship between: (**a**) FCC residue contents (% by binder mass) and elastic modulus; (**b**) elastic modulus and corresponding compressive strength. differences between experimental data and predicted data using Equations (3) and (4) were 32% to 40%, respectively. Therefore, those equations were not recommended to be used to predict the value of elastic modulus with given compressive strength. Equation (6) obtained from the study by [PERSON] et al.