# Fracture Behaviour of Concrete with Reactive Magnesium Oxide as Alternative Binder
## 4 Conclusions  
From the analysis of the main mechanical properties and of the fracture energy of concrete mixes with partial replacement of cement with reactive MgO, some conclusions were obtained. The incorporation of MgO usually reflects negatively on the mechanical properties. This is due to the fact that the formation of the hydration product Mg(OH)\({}_{2}\) increases and, consequently, there is a weakness of the interfacial transition zone (ITZ), mainly responsible for the mechanical response. The greatest reductions were found in mixes with MgO from Canada (MgO-C), with a maximum reduction of 36% of the splitting tensile strength. As previously explained, this was because this mix contains a higher w/b ratio, generating greater porosity. Regarding the treatment of MgO from Spain, it was possible to conclude that the mixes with MgO S1 had the worst results. This behavior is because the grinding of MgO from Spain decreased the surface area of the particle, not allowing complete hydration of the MgO particles. The modulus of elasticity did not decrease with the use of reactive MgO. In some mixes, there were improvements of up to 15%. This increase can be explained by the expansive nature of reactive MgO. This characteristic of MgO leads to a decrease of shrinkage and micro-cracks and, consequently, to an increase of the modulus of elasticity. Regarding G\({}_{\rm F}\), it is possible to conclude that toughness is reduced to 53% for the greatest replacement of MgO (20%). However, the splitting force-CMOD response has a softened behavior because MgO is an expansive agent reducing micro-cracks. The best results were observed for MgO's from Spain, with a reduction between 15% and 20% (for 20% MgO). For substitutions between 5% and 10%, all mixes presented similar values of 13-20%, except the one with MgO from Australia, which had major reductions. K\({}_{\rm I}\), being related to G\({}_{\rm F}\) and the modulus of elasticity, showed the same trend, with a good correlation with these two properties (R\({}^{2}\) of 0.96). Finally, with these results, it was possible to establish the necessary parameters to characterize a useful stress-strain curve for structural design and numerical modeling. Figure 6: Relationship between fracture energy (G\({}_{\rm F}\)) versus strength and fracture toughness (K\({}_{\rm I}\)) (**a**) and compressive strength versus fracture energy (G\({}_{\rm F}\)) (**b**). **Author Contributions:** Conceptualization, M.B., J.d.B., L.E. and J.P.; Methodology, M.B., J.d.B., L.E. and J.P.; Formal analysis, J.A.F., M.B., J.d.B. and L.E. ; Data curation, J.A.F., M.B., J.d.B. and L.E. ; Writing--original draft, J.A.F. ; Writing--review and editing, M.B., J.P., J.d.B. and L.E. All authors have read and agreed to the published version of the manuscript. **Funding:** The authors wish to thank CERIS (Civil Engineering Research and Innovation for Sustainability) research centre and FCT (Foundation for Science and Technology). **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** No new data were created or analyzed in this study. Data sharing is not applicable to this article. **Conflicts of Interest:** The authors declare no conflict of interest. * [PERSON] et al. (2013) [PERSON] [PERSON]; [PERSON] [PERSON]; [PERSON] [PERSON]; [PERSON] [PERSON] The effects of nanoscale expansive agents on the mechanical properties of non-shrink cement-based composites: The influence of nano-MgO addition. _Compos. Part B Eng._**2013**, _55_, 193-202. [CrossRef]
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