Fracture Behaviour of Concrete with Reactive Magnesium Oxide as Alternative Binder

This research evaluates the fracture behavior of concrete with reactive magnesium oxide (MgO). Replacing cement with MgO is an attractive option for the concrete industry, mainly due to sustainability benefits and reduction of shrinkage. Four different MgO’s from Australia, Canada, and Spain were used in the concrete mixes, as a partial substitute of cement, at 5%, 10%, and 20% (by weight). The fracture toughness (KI) intensity factor and the stress–strain softening parameters of the wedge split test were evaluated after 28 days. The experimental results showed that the replacement of cement with MgO reduced the fracture energy between 13% and 53%. Moreover, the fracture energy was found to be correlated with both compressive strength and modulus of elasticity. A well-defined relationship between these properties is important for an adequate prediction of the non-linear behavior of reinforced concrete structures made with partial replacement of cement with MgO.


Introduction
To reduce the effect of global climate change due to greenhouse gases, many governments around the world have been striving to reduce carbon dioxide's (CO 2 ) emission rates as the main cause of this effect. One of the sectors with a large share of CO 2 emissions is the construction industry, which is responsible for 10% of the CO 2 expelled to the atmosphere each year [1], with the cement industry alone emitting about 5.7 billion tonnes of CO 2 in 2018 [2]. A path for CO 2 reduction in the construction industry is the use of supplementary cementitious materials, such as fly ash (FA), silica active, slag, metakaolin, reactive MgO, among others. Among these alternatives, reactive MgO has become a good option as a supplementary material, as non-reactive MgO is already used in the composition of cements for refractory use [3]. Yet, standards restrict the content of MgO in clinker to a maximum of 5% [4].
One of the main advantages of using reactive MgO-based cements is that the production of reactive MgO (calcinated at 700 • C to 1000 • C) requires lower temperatures in comparison to the production of clinker (>1400 • C). MgO is mainly produced by calcining mined magnesite deposits (MgCO 3 ). The temperature and calcination time produce changes in the crystalline structure of MgO. For example, with the increase of temperature and calcination time, MgO suffers a reduction in its surface area increasing the size of the crystalline structure and reducing its reactivity [5][6][7]. The calcination process in obtaining MgO can be identified according to the calcination temperature. The European Commission has defined the grades of MgO as: caustic calcined (the case of reactive MgO), 600-1300 • C; dead burnt, 1600-2200 • C; and fused, >2800 • C [8].
During mixing, the use of reactive MgO with cement (either in cement production as a small percentage of clinker, or as an addition) results in the formation of Mg(OH) 2 (Equation (1)), which subsequently carbonates to a hydrated magnesium carbonate (Equation (2)) [9][10][11].
MgO + H 2 O → Mg(OH) 2 (1) MgO can also be used as an expansive additive. This expansive character can be controlled with MgO reactivity and fineness. The higher the MgO calcination temperature, the lower the expansion at early ages, and the longer the hydration process lasts. As the hydration of MgO is a gradual and irreversible process, Mg(OH) 2 is stable, and its expansion is stable and not unlimited. The influence on the incorporation of different reactive MgO has shown that, with the use of reactive MgO as binder, there is a reduction of mechanical properties, such as compressive strength [12][13][14][15][16][17][18][19][20][21][22], flexural strength [15,16,19,[22][23][24], and splitting tensile strength [22,25].
More specifically, authors such as Mavroulidou et al. [18] showed that there is a reduction of compressive strength in ternary concrete with fly ash (FA), metakaolin, and reactive MgO. As the MgO ratio is increased, the reduction was about 30%. The water/binder (w/b) ratio was kept constant (0.55) for all the mixes. Mavroulidou et al. [18] attributed this reduction to the fact that MgO increased the porosity and production of magnesium silicate hydrates due to the lack of reaction between FA and Brucite to form the M-S-H gel. In a study by Gao et al. [19], it was observed that the type of curing influences the compressive strength. This property increased by increasing the autoclave time and decreased by increasing the autoclave temperature. The compressive strength values were higher in mixes with 4% MgO, followed by 8% and 12%.
Unluer and Al-Tabbaa [21] studied the compressive strength under conditions of accelerated carbonation (10% of CO 2 ) in concrete blocks with reactive MgO and FA. 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. Mavroulidou 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. Choi 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. Wu 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 densifies 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]. Guo 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. Guo 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. By analyzing three sources of MgO produced for other industries (e.g., agriculture), the paper analyses whether products readily available in the market suit concrete production. In order to obtain the fracture parameters, compressive strength, tensile strength, and modulus of elasticity tests were carried out, with the intention of having a more comprehensive understanding. In turn, correlations were made between the mechanical properties and fracture properties, to have a complete analysis of the influence of MgO as a partial cement substitute on the fracture behavior. In summary, all the properties analyzed in this paper will serve as a basis to establish the necessary parameters for structural use, missing until now.

Materials
The cement used as the main binder for the mix composition reference concrete (RC) was CEM I 42.5 R produced by Secil (Lisboa, Portugal). In this study, two types of natural aggregates were used: coarse aggregates (with commercial grading conforming with the designations 2/6, 6/12 and 12/20 of EN 12620 [32]); siliceous sands (0/2 and 0/4). These products are marked for EN 12620 [32].
The reactive MgO's used in this research were manufactured in three different countries (Australia, Canada, and Spain), and were defined as A, C, and S0, respectively. To analyze the influence of particle size, a grinding treatment was carried out on the MgO from Spain for one hour in a rotary mill with steel balls of diameter 2-4 cm to obtain a particle size distribution comparable to that of cement and named S1. Regarding the MgO's from Australia and Canada, no treatment was performed because their particle size was already similar to that of cement. In summary, the influence of four types of MgO's as partial replacement of cement was analyzed. Table 1 shows the results obtained in the tests to determine the specific surface, purity, reactivity, and skeletal density of the various MgO analyzed. Specific surface affects the reactivity of MgO and is therefore an extremely important factor. In turn, the purity of MgO makes it possible to determine how many other elements there are in the different MgO's received from the manufacturers. This factor also significantly influences the behavior of concrete produced with these materials. From Table 1, it can be concluded that MgO-C, and MgO-A have reactivities and specific surfaces much higher than those of cement and other MgO's. Liska et al. [33] also showed that the reactivity of MgO increases by reducing its particle size and, consequently, increasing its specific surface area. It can also be seen that the product from Australia has the highest content of MgO in its constitution (98.8%). All MgO's have a purity higher than 85%.

Mix Design
The mixes' composition was based on the methodology proposed by Nepomuceno et al. [34], taking into account the following parameters: C30/37 strength class, XC3 exposure class, and S2 slump in accordance with the EN 206 standard [35]. The materials' contents of the mixes, in volume, are provided in Table 2. All the mixes were designed based on the same composition, with different w/c ratios required to keep all mixes within the target slump. Concrete production was done in three phases with a total mixing time of 10 min. The first phase lasted 4 min and the coarse aggregates were mixed with 2/3 of the total mixing water. In the second phase, the fine aggregates were added and mixing lasted two minutes. The third phase consisted of adding the cement with the remaining 1/3 of the water in the mix for four minutes. The replacement of cement with reactive MgO was made by mass at 5%, 10%, and 20%. These percentages were defined according to the results obtained in a previous investigation [22]. There, the mechanical and durability behavior of mortars with reactive MgO was studied, using replacement ratios of 0%, 5%, 10%, 15%, 20%, and 25% by mass. The mixes were named as follows: RC for the reference concrete without any reactive MgO, concrete with partial replacement of reactive MgO from Spain and without grinding treatment was named S0, concrete with reactive MgO and with one-hour grinding treatment was named S1, and concrete with MgO's from Australia and Canada was named A and C, respectively. To designate the MgO incorporation ratio in the mixes, the nomenclature that designates first the amount of substitution and then the origin, was used, e.g., C5:C describes a concrete with 5% MgO from Canada. Table 2 also shows the results of the fresh-state density of concrete. It shows that the density remains constant as the MgO incorporation ratio increases.
By increasing the incorporation ratio of reactive MgO, the consistency of the mixes tended to decrease. Therefore, it was necessary to increase the water/binder ratio in the mixes as reactive MgO increased to maintain the target consistency, as observed in Figure 1a. Figure 1b shows that all mixes are within the S2 consistency range as the content of reactive MgO increases.
10%, 15%, 20%, and 25% by mass. The mixes were named as follows: RC for the reference concrete without any reactive MgO, concrete with partial replacement of reactive MgO from Spain and without grinding treatment was named S0, concrete with reactive MgO and with one-hour grinding treatment was named S1, and concrete with MgO's from Australia and Canada was named A and C, respectively. To designate the MgO incorporation ratio in the mixes, the nomenclature that designates first the amount of substitution and then the origin, was used, e.g., C5:C describes a concrete with 5% MgO from Canada. Table 2 also shows the results of the fresh-state density of concrete. It shows that the density remains constant as the MgO incorporation ratio increases.
By increasing the incorporation ratio of reactive MgO, the consistency of the mixes tended to decrease. Therefore, it was necessary to increase the water/binder ratio in the mixes as reactive MgO increased to maintain the target consistency, as observed in Figure  1a. Figure 1b shows that all mixes are within the S2 consistency range as the content of reactive MgO increases.

Tests
Fracture energy is the quantity of energy required for the propagation of crack through a unitary area. This property is strongly related to the tensile behaviour of the material. Thus, to better analyze the results obtained in the fracture energy test, other mechanical properties of concrete were analyzed (compressive strength, splitting tensile strength, and modulus of elasticity) after 28 days of curing in a wet chamber. The compressive strength of hardened specimens was evaluated according to EN 12390-3 [36]. This test was performed on four specimens of 150 × 150 × 150 mm. The splitting tensile strength test followed the method presented in EN 12390-6 [37]. This test was performed on three cylindrical specimens of 150 mm diameter × 300 mm high. The test procedure used to determine the secant modulus of elasticity is described in LNEC E 397 [38]. This test was performed on two cylindrical specimens of 150 mm diameter × 300 mm high.
The fracture energy (GF) was determined using the wedge splitting test (WST) [39], following the procedure described in NT BUILD 511 [40]. WST was performed on four

Tests
Fracture energy is the quantity of energy required for the propagation of crack through a unitary area. This property is strongly related to the tensile behaviour of the material. Thus, to better analyze the results obtained in the fracture energy test, other mechanical properties of concrete were analyzed (compressive strength, splitting tensile strength, and modulus of elasticity) after 28 days of curing in a wet chamber. The compressive strength of hardened specimens was evaluated according to EN 12390-3 [36]. This test was performed on four specimens of 150 × 150 × 150 mm. The splitting tensile strength test followed the method presented in EN 12390-6 [37]. This test was performed on three cylindrical specimens of 150 mm diameter × 300 mm high. The test procedure used to determine the secant modulus of elasticity is described in LNEC E 397 [38]. This test was performed on two cylindrical specimens of 150 mm diameter × 300 mm high.
The fracture energy (G F ) was determined using the wedge splitting test (WST) [39], following the procedure described in NT BUILD 511 [40]. WST was performed on four cubic samples of 150 × 150 × 150 mm tested after 28 days of curing in a wet chamber. G F is determined as the average area under the curve of the splitting force-crack mouth opening displacement (CMOD). This method minimizes the effect of weight on the fracture energy, in contrast to other methods [39][40][41][42]. The geometric configuration of the specimens was generated using special moulds in the casting, the guides and the starting notches were made after 15 days of wet curing using a 1 mm diamond saw as shown in Figure 2a,b. The test procedure applies a vertical force (F v ) at a deformation speed of 0.006 mm/sec that divides the test specimen into two sections. During the test, CMOD was continuously measured with a clip gauge as F v increased. After the test, the depth and width of the split surface (A lig ) were measured. energy, in contrast to other methods [39][40][41][42]. The geometric configuration of the specimens was generated using special moulds in the casting, the guides and the starting notches were made after 15 days of wet curing using a 1 mm diamond saw as shown in Figure 2a,b. The test procedure applies a vertical force (Fv) at a deformation speed of 0.006 mm/sec that divides the test specimen into two sections. During the test, CMOD was continuously measured with a clip gauge as Fv increased. After the test, the depth and width of the split surface (Alig) were measured. The preparation steps to perform WST are illustrated in Figure 3. The splitting force (Fsp) was determined from WST using Equation (3), where α is the angle between the wedge and the vertical load line. The area of the splitting force-COMD curve is the energy dissipated during the fracture, Wf,CMOD, and is normalized with respect to the total split surface (Alig) to complement the fracture. This intermediate specific fracture energy is denoted GF (kN/mm) and is determined from the test result through Equation (4)   The preparation steps to perform WST are illustrated in Figure 3. The splitting force (F sp ) was determined from WST using Equation (3), where α is the angle between the wedge and the vertical load line. The area of the splitting force-COMD curve is the energy dissipated during the fracture, W f,CMOD , and is normalized with respect to the total split surface (A lig ) to complement the fracture. This intermediate specific fracture energy is denoted G F (kN/mm) and is determined from the test result through Equation (4) [39,40,42].
The double K fracture and the double G F fracture criteria are based on asymptotic linear superposition, maintaining the relationship of the linear elastic fracture mechanics valid at all points of the splitting force-CMOD envelope. Therefore, the effective double K I fracture is obtained in terms of equivalent stress intensity factors using the double G F fracture model. Equation (5) relates the two criteria: double K I fracture model and double G F fracture model [43].  The double K fracture and the double GF fracture criteria are based on asymptotic linear superposition, maintaining the relationship of the linear elastic fracture mechanics valid at all points of the splitting force-CMOD envelope. Therefore, the effective double KI fracture is obtained in terms of equivalent stress intensity factors using the double GF fracture model. Equation (5) relates the two criteria: double KI fracture model and double GF fracture model [43].
K G E (5) Table 3 shows the results of the compressive strength, splitting tensile strength, and modulus of elasticity tests, as well as their standard deviations and the percentage of variation with respect to RC.   Table 3 shows the results of the compressive strength, splitting tensile strength, and modulus of elasticity tests, as well as their standard deviations and the percentage of variation with respect to RC. The compressive strength was determined after 28 days of curing in a wet chamber. It is found show that, with the increase of the reactive MgO incorporation ratio, there is a reduction in compressive strength. The effect of the use of MgO can be due to the fact that, firstly, as MgO increases, the amounts of C2S and C3S decrease. Thus, there is a reduction of the formation of C-S-H phases, increasing the formation of Mg(OH) 2 , which is a less resistant hydration product [16,44,45]. The second reason for these findings concerns the expansive nature of MgO. Reactive MgO can cause cracking and thus lead to an increase in porosity and macro-pores, which is a determining factor in strength [12,46]. The concrete mixes most affected by the incorporation of MgO were those made with C, with a reduction of 33.6% for a ratio of 20% of MgO. This negative influence of these MgO's may be because these mixes needed an increase of water/binder ratio. On the contrary, the less affected mixes were those produced with A. In these cases, the use of 20% of these MgO's caused maximum reductions of 24.7%. Regarding the use of 5% MgO, all mixes showed a decrease of between 8.8% and 20%. Similar behavior was found by Choi et al. [17], who observed reductions of between 5% and 13% for curing ages of 7 and 28 days in mixes with incorporation of 5% MgO and 20% FA.

Main Mechanical Properties
The results of the tensile strength test performed after 28 days show that, for the mixes with incorporation ratio of 5% MgO, the reduction was between 2% and 17%. The C5-S0 and C5-A mixes are the ones that had the best behavior with the use of 5% MgO. This can be possibly explained because these mixes have a w/b ratio equal or slightly higher than the w/b ratio of the RC mix. The mixes with MgO S1 had the worst results, with reductions of 17.2%, 21.9%, and 35.6%, for ratios of 5%, 10%, and 20%, respectively. This behavior is because the grinding of MgO from Spain decreased the surface area of the particle, not completely hydrating the MgO particles. Mavroulidou et al. [18] found reductions in tensile strength when 10% reactive MgO was added to the fuel ash and metakaolin binary mixes.
There is an increase in modulus of elasticity when MgO's C, SO and S1 are used at almost all ratios of substitution. These increases are up to 16%, presenting better results for MgO S1 mixes, with values between 8% and 14%. Mixes with MgO A had an improvement at 5% and 10% MgO ratios, with values of 14% and 8%, respectively, and a reduction of 6% for 20% MgO. This increase can be explained by the expansive nature of MgO, decreasing the shrinkage and the micro-cracks and increasing the bulk density of concrete [28,47,48]. Figure 4a shows the relationship between compressive strength and splitting tensile strength in cylindrical specimens for all mixes at 28 days. All mixes were found to be well within the confidence intervals for compressive strength of conventional concrete.   Figure 4b shows the theoretical relationship between the compressive strength and the modulus of elasticity of concrete, according to Equation (6) proposed by EC2 [49], where α = 0.9 for coarse limestone aggregates (the case of this paper). It can be observed that all mixes have values of modulus of elasticity much higher than those proposed by EC2. Thus, it is concluded that the proposal established by EC2 [49] is quite conservative.

Fracture Energy
The results of the fracture energy after 28 days of curing are shown in Figure 5a-d and Table 4, as well as their standard deviation and percent variation relative to RC for each substitution ratio. As can be seen there, the fracture energy decreases with the use of reactive MgO. This can be explained because GF is directly related to the interaction between cement matrix and aggregates. Therefore, the greater the replacement of cement  Figure 4b shows the theoretical relationship between the compressive strength and the modulus of elasticity of concrete, according to Equation (6) proposed by EC2 [49], where α = 0.9 for coarse limestone aggregates (the case of this paper). It can be observed that all mixes have values of modulus of elasticity much higher than those proposed by EC2. Thus, it is concluded that the proposal established by EC2 [49] is quite conservative.

Fracture Energy
The results of the fracture energy after 28 days of curing are shown in Figure 5a-d and Table 4, as well as their standard deviation and percent variation relative to RC for each substitution ratio. As can be seen there, the fracture energy decreases with the use of reactive MgO. This can be explained because G F is directly related to the interaction between cement matrix and aggregates. Therefore, the greater the replacement of cement with MgO, the lower the amount of CSH produced and the greater the amount of Mg(OH) 2 produced, reducing the properties of the cementitious paste (as inferred from the results of Table 3) and, consequently, the G F of the concrete mixes [9,10,50,51]. The mixes with MgO from Canada and Australia are the most affected, with a reduction of approximately 50%, relative to RC, for ratio of 20%. In addition to being related to the hydration products formed by the MgO, these mixes have a higher w/b ratio and K F decreases with the increase of this ratio, as observed by Wittmann and Sadhana [50].     Figure 5a-d, when the amount of MgO increases, the splitting force-CMOD curve tends to be flatter. Furthermore, CMOD increases with the incorporation of MgO. This increase occurs because the modulus of elasticity of concrete increases with the use of the expansive agent (MgO), improving the quality of concrete [31,48], as was observed in the analysis of results of the modulus of elasticity.
The peak splitting force will also increase for the mixes with MgO S0 and S1, in relation to the RC mix. A similar behavior was observed by Guo et al. [31], when they studied the effect of expansive agents, including MgO, in concrete with FA. The authors observed that toughness exceeded 1.1 times that of the reference concrete between 28 and 60 days using this type of agents.
As G F and K I are correlated parameters. In Figure 6a it is found that there is a real logarithmic correlation between these parameters with an R 2 of 0.96. On the other hand, G F and the compressive strength of each type of concrete mixes are also correlated with a strong quadratic fit (R 2 between 0.93 and 0.99), as shown in Figure 6b. This occurs because one of the most relevant parameters that determine the fracture energy of normal strength concrete is the w/b ratio [52], as is also the case of compressive strength. However, some other properties can also affect the value of G F , such as the relative strength between aggregates and cement paste [52].

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 GF, 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. KI, being related to GF 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.

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 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 I , being related to G 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.