## 3 Results and Discussion
### Hardened Properties of OMGPC
#### 3.2.2 Compressive Strength of OMGPC  
Figure 9 illustrates the compressive strength characteristic of OMGPC, investigating the influence of RESIN8 and FWG aggregates at various percentage contents. The compressive strength of OMGPC containing RESIN8 decreases as the RESIN8 increases; however, the strength increases with curing age. There is an increase in the compressive strength of OMGPC with increasing FWG and curing age. The compressive strength of the control specimen cured at 28 days is 28.6 MPa, while the strength decreased by 15.7% and 24.1% for R5% and R10%, respectively. The decrease in the OMGPC containing RESIN8 is attributed to the lower stiffness and SG of plastic, the high porosity, and the weak interfacial transition zone (ITZ) between the RESIN8 aggregate and the matrix, which is less stiff compared to the ITZ between natural sand and FWG aggregate-containing GPC [66; 67; 28; 63; 68]. [PERSON] et al. [69] reported that the hydrophobic nature and smooth surface of RESIN8 induce weak interfacial bonding between the RSINE8 and other aggregates, leading to strength reduction of OMGPC. In addition, the high compressibility or deformability of RESIN8 results in more cracks around the aggregate when subjected to compression, hence the reason for strength decrease with the inclusion of RESIN8 [59; 70]. An increase in RESIN8 content in OMGPC increases the porosity and reduces the weight of the matrix, thereby resulting in strength reduction. The strength of OMGPC containing G5% increases by 3.5%, and the OMGPC containing G10% has a similar compressive strength to the control. [PERSON] et al. [34] also reported an enhanced concrete strength with the substitution of sand with FWG aggregate and strength reduction when substituted with recycled waste plastic aggregate in conventional concrete. This phenomenon can be attributed to the higher stiffness of FWG particles compared to RESIN8 and the participation of silicate oxide from the FWG aggregate in the polymerisation reaction, which further elevates the SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) ratio of the mixture. The presence of FWG aggregates also enhances the alkalinity of the matrix, promoting greater reactivity and dissolution around the aggregates. It was reported by [PERSON] et al. [32] that as the geopolymer growth progresses, the amorphous FWG content actively participates in the reaction, resulting in a robust, stronger, and compact matrix formation. This explains the reason for the improved strength of OMGPC with FWG aggregate. The additionalsilicate supplied by FWG further reacts with the Ca(OH)\({}_{2}\) reagent to produce more C-S-H gel, which results in a compressive strength increment of the OMGPC [71]. According to the XRD analysis performed, the C-S-H gel increases with an increase of up to 20% of FWG in the matrix [71]. The polymerisation reaction of SiO\({}_{2}\) in FWG with alkaline reagent Ca(OH)\({}_{2}\) improves the compactness and the microscopic pore structure within the matrix [72]. [PERSON] et al. [33] and [PERSON] et al. [57] obtained GPC with an improved compressive strength at the full substitution of sand with FWG compared to the control, signifying the participation of the chemical composition of FWG in the polymerisation reaction. Hence, the reason for the increased strength is the inclusion of FWG in OMGPC. However, their report revealed that incorporating about 25% of FWG in GPC can decrease strength due to the weakened ITZ between the FWG aggregate and the sand. Hence, the ITZ weakens as the FWG content increases from G5% to G10%, leading to a slight strength reduction at G10% inclusion. This phenomenon explains the incompatibility of the ITZ of natural sand and FWG due to the smooth surface of the glass aggregates. However, their result shows that the ITZ can be improved by incorporating a high quantity of FWG (about 50% above) in GPC. The improved ITZ is evident at the full substitution of sand with FWG by [PERSON] et al. [33], leading to enhanced strength.