## 3 Results and Discussion
### Compressive Strength  
Compressive strength is one of the significant properties of concrete, which is linked with other properties as well. It is generally used to ascertain the mechanical strength and durability-related characteristics of concrete structural elements. The compressive strength tests were carried out on cubical specimens (150 \(\times\) 150 \(\times\) 150 mm) at the ages of 7, 28, and 56 days using a universal testing machine of 4000 kN capacity following BS EN 12390-3: 2019 [39]. From Figure 4, it can be observed that there was an increase in the compressive strength in all the mix samples from 7 to 28 and 56 days. The replacement of QRD with SG up to 15% (GPC-D) has shown the maximum overall strength of 33.55 MPa at 56 days. However, the compressive strength was decreased when the QRD content was further increased from 15% to 20%, i.e., in the GPC-E mix, which showed 22.77 MPa strength at 56 days, nearly 24% and 32% less than the control (OPC) and GPC-D mixes, respectively. For the control (OPC) mix, increments of 23% and 19% in the compressive strength were observed from 7 to 28 and 28 to 56 days, respectively, with an overall increase in the strength of 46% from 7 to 56 days. Meanwhile, in GPC-A, GPC-B, GPC-C, GPC-D, and GPC-E, increments of 40%, 32%, 33%, 23%, and 34%, respectively, were observed in the strength from 7 to 28 days, with an overall increase of 49%, 48%, 47%, 31%, and 57%, respectively, from 7 to 56 days under ambient curing conditions. The target compressive strength of 30 MPa (of control OPC concrete specimen) was accomplished by GPC-B (30.44 MPa), GPC-C (32.22 MPa), and GPC-D (33.55 MPa) with 50% FA and SG/QRD contents of 45/5%, 40/10%, and 35/15%, respectively. It can be observed that the replacement of SG beyond 15% with QRD has a negative effect on the compressive strength of QFS-GPC. Several additions/replacements in geopolymer concrete can be used to enhance its mechanical properties. Melamine fibers were used in the literature shows around 10% enhancement in strength from 5% fiber addition [43].  
Figure 3: The slump values of control (OPC) and QFS-GPC mixes to the target slump (medium workable) after the addition of the varying amount of SP.  
The increase in the strength can be attributed to the content of calcium oxide [44]. The contents of CaO in FA, SG, and QRD used in present research study are 2%, 37.33%, and 47.13%, respectively (Table 1). There is a significant role of CaO content in the polymerization process of GPC mixtures. The CaO content aids in accelerating the polymerization process at ambient as well as at low temperatures [45; 46]. In the present study, the CaO contents were increased with the replacement of SG by QRD, which has the maximum CaO content (47.13%) among all the source or binder materials. Therefore, the addition of CaO (by replacing SG with QRD) in the GPC matrix increased the compressive strength [46] due to the pore size reduction of specimens. However, beyond the 15% replacement of SG by QRD, a significant reduction in strength was observed. This reduction in the compressive strength can be due to excess of CaO contents in QRD and the partial hydration of CaO in the alkaline medium to form Ca(OH)\({}_{2}\)[45] and incomplete geopolymerization [46; 47]. This trend of decrease in strength can be due to the addition of extra water and superplasticizer to maintain workability.