# Fracture Behaviour of Concrete with Reactive Magnesium Oxide as Alternative Binder
## 1 Introduction
The results for compressive strength were about twice those of MgO mixes without FA. This response was attributed to the higher porosity of the FA mixes, leaving more space for the formulation of hydrated magnesium carbonate and strength development. Mixes with higher w/b ratios (0.6 and 0.9) showed poor strength due to low compaction and presence of saturated pores preventing CO\({}_{2}\) and hydrated magnesium carbonate formulation, respectively. The maximum strength was obtained in MgO mixes with w/b ratio of 0.7, with and without FA. The strength of the mixes increased gradually by increasing CO\({}_{2}\) concentration from 0% to 20%. The strength development from one day to seven days decreased by increasing CO\({}_{2}\) concentration. Tensile strength is also influenced by the use of MgO. [PERSON] et al. [18] studied the tensile strength of concrete mixes produced with different contents of reactive MgO, FA, and metakaolin. The authors found that the tensile strength slightly decreased (less than 10%) with the use of 10% of MgO and 20% of FA. The modulus of elasticity is also influenced by the incorporation of MgO. [PERSON] et al. [17] studied the modulus of elasticity after freeze-thaw cycles in concrete mixes produced with 20% FA, and different w/b ratios of 0.48 and 0.65, cured with water for 28 days or 360 days. The use of MgO led to a slight influence (4%) of the modulus of elasticity of the concrete mixes after 100, 200, and 300 freeze-thaw cycles. The existing references show that the use of reactive MgO in concrete leads to higher initial expansion and lower shrinkage [7; 26]. However, the MgO incorporation effect is dependent on the MgO reactivity [27] and MgO content [13]. When MgO hydration takes place, the final product is magnesium hydroxide, which has a larger volume than its constituents, decreasing the shrinkage of concrete with MgO [28]. However, concrete made with high reactive MgO may exhibit dimensional stability problems, as the expansive reactions of this material may overcome those due to cement paste shrinkage [29]. The literature does not contain many studies of the fracture properties in concrete with MgO. This property is important to predict structural behavior and to design structural concrete. [PERSON] et al. [30] studied concrete with reactive MgO and fly ash cured with an accelerated carbonating process for one, three, and seven days. The authors observed that the carbonation curing densities the bonding system, leading to an increase of the retarded tensile strength at first cracking. The changes in volume and microstructure of concrete have great influence on fracture properties of concrete. As a result, the use of an expansive agent like MgO has some influence on the fracture performance of concrete [31]. [PERSON] et al. [31] studied the effects of expansive agents (EA) and MgO on the fracture properties. The results indicated that the trends in the development of concrete fracture parameters varied with curing ages at different proportions of EA, MgO, and FA. Expansive agents generally decrease toughness at initial fracture and toughness against unstable concrete fracture. [PERSON] et al. [31] observed that, when the amount of MgO is increased, the area under the splitting force-crack mouth opening displacement (CMOD) curve becomes relatively large. However, when the amount of MgO is 5%, the curve is relatively flat. In this study, the influence of the incorporation of MgO at replacement ratios of 5%, 10%, and 20% of cement on the mechanical and fracture behavior of concrete was studied. The relevance of this paper consists in that in the literature there are not enough studies presenting the fracture behavior in concrete with different types of MgO's. This relevance increases with the analysis of three different MgO replacement ratios (5%, 10%, and 20%). Moreover, reactive MgO for concrete is not a commercial product and no standards for its production, properties, and/or specification exist.