*3.3. Flexural Performance*

Flexure strength (FS) of investigational examples was established on examples after CST. The acquired norms of strength ranged between 6.6 MPa and 3.9 MPa. The consequence of the FS of the examples is presented in Figure 7. In Figure 7, it is detected FS with variable ratios of waste glass. Related to CS, FS at the initial phase failures with the incorporation of waste glass. It was detected that with the addition of waste glass at 10%, 20%, 30%, 40%, and 50% of cement weight, the decrease in FS was 6.7%, 12.5%, and 21.1%, 46.5%, and 61.5% correspondingly in proportion to the reference sample (6.3 MPa). Figure 8 demonstrates a rectilinear relationship between the tensile FS of the example and the substance of waste glass addition. On the other hand, if fine and coarse aggregates in cement are replaced with waste glass, it was noticed that the waste glass was replaced with the cement, fine and coarse aggregate at 10%, the increase in FS was 4.7% correspondingly in proportion to the reference sample (6.3 MPa). Nevertheless, if the quantity of waste glass is occupied relative to the cement content, it may be detected that the association between FS and waste glass content is related (the slope of the curves is approximate (Figure 8)).

**Figure 7.** Results of FS.

**Figure 8.** Results of tensile flexural strength tests of concrete examples with altered waste glass substances.

#### *3.4. Comparison of Findings of this Study with Other Existing Studies*

The effect of waste glass on the structural performance of concrete was investigated by many research teams. Effects of the use of waste glass as cement replacement on some mechanical properties such as CS, STS, and FS have been investigated. In this part of the study, values of CS, STS, and FS for plain concrete and concrete produced from different glass powder amounts have been collected from the research publications [46–63]. Then, measured strength values of concrete produced with waste glass were first normalized by plain concrete strengths. These normalized strength values were plotted in Figures 9–11, respectively, as a function of the waste glass content.

**Figure 9.** Variation of the normalized CS of the concrete produced with waste glass.

**Figure 11.** Variation of the FS of the concrete produced with waste glass.

As shown in these figures, with the addition of waste glass, the strength values of concrete produced with glass powder generally decrease after a certain value. The maximum decrease in compressive strength was observed in the study by Kalakada et. al., 2022. When the cement was partially replaced with glass powder at 50%, there is a 65% reduction in compression strength of the plain concrete specimen. In the same manner, a 20% addition of the waste glass leads to a 36% reduction in the splitting tensile strength of the plain concrete specimen in the study of Abdulazeez et al., (2020). With the addition of 50% glass powder, a 38% reduction in flexural strength was observed in our experimental study. Thus, a rational design expression was developed to consider these reductions in the strength values as follows:

$$f = \left[1 + c\_1 \times \left(\text{WGPR}\right) + c\_2 \times \left(\text{WGPR}\right)^2\right] \times f' \tag{1}$$

where *f* is strength values as follows: *fc*: compressive strength *ft*: splitting tensile strength; *ff*: flexure strength; *WGPR*: waste glass power ratio (0 < *WGPR* < 50). *c1* and *c2* are coefficients given in Table 2; *f* is strength values of the plain concrete.

**Table 2.** Constants for Equation (1).


As shown in Equation (1), CS, STS, and FS values of concrete produced with waste glass were expressed as a function of the amount of the waste glass (*WGPR*). These expressions can be easily used in the design stages.

#### *3.5. Scanning Electron Microscope (SEM) Analysis*

Scanning electron microscope (SEM) analysis was performed from the sample pieces taken after the compressive strength test from the concrete samples produced with recycled glass waste powder. SEM analysis is carried out to show the typical morphology of the surface tissue of OPC and WGP. The particles of both OPC and WGP are composed of glassy structures and irregular shapes with sharp edges. OPC particles consist of sharper edges and shapes, while WGP particle appears on smoother surfaces, sharper prismatic edges, and denser content [2]. The best images were selected to observe the effect of glass dust and glass particles in OPC concrete production. Figure 12a–f contains images where WGP is replaced by cement. Images of the mix design samples from Figure 12g–j contain the details of the change in glass particles with cement and aggregates. In order for WGP to be processed as a filler by grinding in electric mills without any treatment, 74% of the particles must be passed through a 36 μm sieve [64]. The key findings of the inner image details at 500 times magnification are shown in detail in Figure 12. Judging from the general view in Figure 12a, it can be said that glass powder provides good bonding with cement and aggregates. A good interlocking is observed in terms of surface quality, but some flat zones have cracks due to fracture. In Figure 12b, the appearance of glass powder particles like sharp fibers is remarkable. Although there is a homogeneous distribution, gaps are seen in some regions. Voids can be removed by better mixing, shaking, and skewering [64–66]. The image in Figure 12c shows material connections more closely, such as a finely woven bee comb. Figure 12d shows the concrete surface bonding and spacing. Figure 12e,f shows the gelled state of the concrete surface. In terms of surface quality, it can be said that the glass powder is in a good correlation, but it has some holes and gaps. In the mixed model, glass powder was used as a binder with cement, and broken glass particles were used by replacing fine and coarse aggregates in certain proportions. Calcium silicate hydrate (C-S-H), an amorphous phase, is defined as weak crystals [67]. The fine glass particles can be seen in Figure 12g. They are seen as a honeycomb in the C-S-H phase. Since this honeycomb structure provides a good pozzolanic effect, it increases compressive strength [68,69]. Figure 12h shows how large and small glass particles are bonded with binders in concrete. It is seen in Figure 12i with good fillings of binders between large

pieces of glass. Figure 12j shows the homogeneous distribution and connection of the glass particles in the concrete.

**Figure 12.** *Cont*.

**Figure 12.** SEM micrographs of the obtained samples.
