# Effect of Metallic Waste Addition on the Physical and Mechanical Properties of Cement-Based Mortars
## 3 Results and Discussion
### Effect of Metallic Waste on the Mechanical Strength of the Test Specimens  
Figure 5 shows the average results of the flexural (Figure 5a) and compressive (Figure 5b) tests. It can be observed in Figure 5 that the addition of shavings increased the average values of flexural strength compared with reference mortars, while the addition of fibres caused slight variations of this property, reducing and increasing average flexural strength at 7 and 28 days, respectively. Moreover, compressive strength was increased at 7 days, both with fibres and with shavings. In contrast, at 28 days, the addition of metallic waste did not improve the compressive strength of specimens. In summary, (i) overall, the mechanical resistance of mortar specimens with metallic waste proportionally increased with the increase of the waste content and (ii) test specimens with shavings presented higher average values than specimens with fibres, both in the case of flexural and compressive strength. This difference in the obtained results was attributed to the morphology and elemental composition of the metallic waste added to the mortar that presented a different mechanical behaviour depending on their distribution inside the specimens [18]. Thus, depending on the position and orientation of the metallic waste inside the specimens, they can contribute in greater or lesser degree to the reduction in mortar specimen cracking due to the flexural or compressive efforts. To better illustrate this, Figure 6 shows a schematic representation of the theoretical cracking process produced during the flexural and compressive tests on cement-mortar specimens with shavings and steel wool fibres, respectively. In this illustration, it can be observed that metallic waste particles experienced shear and tensile efforts during the flexural and compressive tests, which mainly depended on the orientation of the waste and on their morphological properties, such as the surface morphology and aspect ratio. Additionally, SEM images shown in Figure 7 confirmed that the metallic shavings have a rougher surface compared with the fibres which presented a smooth surface. The default geometrical characteristics of steel shavings provided a larger surface area on which the hydration products could proliferate. Indeed, this is what Figure 7 verifies. The rough surface of the steel shavings facilitated the adherence between the reinforcement material and the mortar matrix, thus allowing more hydration products to bond onto the shavings surface. This bonding ability between shavings and mortar increased their mechanical interlock and subsequently the probability of shavings failing under shear when they were submitted to tensile loading, consequently avoiding slipping on the shavings' edges (Figure 6a). In contrast, due to the high average aspect ratio of metallic fibres and to their smooth surface, see Figure 7b, they were more susceptible to failure by pull-out slipping, before reaching their shear strength (Figure 6b). This behaviour was mainly due to the smooth surface of the fibres that reduced the adherence between fibres and the mortar matrix, thus facilitating the slipping of the  
Figure 5: Average results of mechanical strength: (**a**) flexural and (**b**) compressive strength of the mortar specimens with different waste type, content, and curing times (s: shavings; _f_: steel wool fibres). fibres (Figure 7b). Likewise, it was also shown that metallic fibres presented shear failures due to their ductility caused by the low aspect ratio, see Figure 6b. Moreover, comparing the average values of flexural and compressive strength at the two curing times in Figure 5, a significant increase of the strength with the curing time was not observed, which could be due to the addition of the superplasticiser that contributed to homogenising the behaviour between the two curing ages evaluated.