## 4 Results and Discussion
### Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX) Analysis  
Three samples of TBM were analyzed using SEM (at different scales) as shown in Figures 4-6. The SEM images were taken at 50, 30, 10 and 5 \(\upmu\)m scales in order to fully understand the muck morphology (surface texture, shape, surface pores, etc.). Moreover, digital images at larger scales were taken to observe the morphology of the larger particles as shown in Figure 7. The EDX analyses of each sample corresponding to the SEM images are shown in Figures 8-10  
Figure 5: SEM images of TBM muck Sample #2 at different scales. Figure 6: Cont. Figure 6: SEM images of TBM muck Sample #3 at different scales. Figure 7: Digital images of TBM muck samples retained on each sieve during sieve analysis. The SEM images shown in Figures 4-6 may reflect the morphological characteristics of the TBM muck particles. It could be noted in the SEM images that the particles with clear irregular contours in the SEM images do not exceed a maximum of 50 \(\upmu\)m; however, the gradation results presented in Table 1 show that 99.4% of the particles are larger than 63 \(\upmu\)m. Digital images at a larger scale were taken for each portion of the muck sample retained on each sieve during the gradation process in order to observe the morphological properties of the larger particles, as shown in Figure 7. It can be seen in Figure 7 that the TBM muck particles could be considered a mixture of rounded to angular and elongated shaped particles. This might affect the workability of the concrete mixture when the muck is utilized as a coarse aggregate replacement. However, this issue could be overcome by incorporating chemical superplasticizers to enhance the workability of the concrete mixture. Moreover, it was observed that the TBM muck has a relatively large number of voids, which explains the high-water absorption value of the muck (which is presented in Figure 7). Figure 8: EDX analysis of TBM muck Sample #1. Figure 10: EDX analysis of TBM muck Sample #3. Figure 9: EDX analysis of TBM muck Sample #2.  
the following sections). However, the high-water demand of the mixture containing TBM muck could be tackled by using high-range water reducers (HRWRs). By decreasing the SEM images scales to 10 and 5 \(\upmu\)m, it could be clearly noted that the muck particles have a rough surface texture which may enhance the mechanical bond with cement paste in concrete mixtures, especially in high-strength concrete, as reported by [PERSON] et al. [37]. Other researchers also suggested that, in high-strength concrete, the bond strength between the cement and aggregates is a limiting factor for the overall mixture strength rather than the strength of the aggregate itself [38; 39]. This suggests that that the use of TBM muck in high-strength concrete may result in a significant improvement of the mechanical properties of the mix due to the rough surface texture of the muck particles. This conclusion should be demonstrated by further testing of concrete mixtures prepared using TBM muck as a partial or full aggregate replacement. In correspondence with the muck examination using SEM, its elemental composition was determined by EDX. As shown in Figures 8-10, the major components of the TBM muck are oxygen (O), carbon (C), magnesium (Mg), silicon (Si) and calcium (Ca). [PERSON] et al. [40] investigated the chemical composition of different coarse and fine aggregates and studied the effect of that composition on the Coefficient of Thermal Expansion (CTE) of concrete mixture. They reported that the silicon contents in dolomite, granite, manufactured sand and natural sand were 0.12%, 18.59%, 11.49% and 14.54% by weight of the aggregates, respectively. The authors demonstrated that the CTE increases with the increase in the aggregates' silicon content. They also found that concrete mixtures with higher CTE are expected to be more susceptible to thermal stresses, which may result in lowering the compressive strength [40].