# Fracture Behaviour of Concrete with Reactive Magnesium Oxide as Alternative Binder
## 3 Experimental Results and Discussion  
### Main Mechanical Properties  
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  
Figure 3: Illustration of the application of the splitting load. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Mix**} & \multicolumn{3}{c}{**Compressive Strength**} & \multicolumn{3}{c}{**Tensile Strength**} & \multicolumn{3}{c}{**Modulus of Elasticity**} \\ \cline{2-10}  & **28** & \(\sigma\) & \(\Delta_{\text{RC}}\) & **28** & \(\sigma\) & \(\Delta_{\text{RC}}\) & **28** & \(\sigma\) & \(\Delta_{\text{RC}}\) \\ \hline RC & 51.05 & 1.70 & - & 3.58 & 0.16 & - & 36.57 & 0.02 & - \\ C5\(\cdot\)C & 46.17 & 1.36 & \(-9.6\%\) & 3.20 & 0.24 & \(-10.5\%\) & 42.07 & 0.20 & 15.0\% \\ C10\(\cdot\)C & 40.07 & 1.12 & \(-21.5\%\) & 2.57 & 0.19 & \(-28.2\%\) & 39.03 & 0.59 & 6.7\% \\ C20\(\cdot\)C & 33.60 & 0.70 & \(-34.2\%\) & 2.29 & 0.03 & \(-36.0\%\) & 37.00 & 1.45 & 12.2\% \\ C5\(\cdot\)S0 & 40.83 & 1.27 & \(-20.0\%\) & 3.28 & 0.11 & \(-8.3\%\) & 42.40 & 0.68 & 15.9\% \\ C10\(\cdot\)S0 & 40.74 & 0.01 & \(-20.2\%\) & 2.92 & 0.12 & \(-18.5\%\) & 39.79 & 0.72 & 8.8\% \\ C20\(\cdot\)S0 & 40.11 & 1.04 & \(-21.4\%\) & 2.45 & 0.39 & \(-31.5\%\) & 37.62 & 0.03 & 2.9\% \\ C5\(\cdot\)S1 & 44.59 & 2.40 & \(-12.7\%\) & 2.96 & 0.10 & \(-17.2\%\) & 41.73 & 0.32 & 14.1\% \\ C10\(\cdot\)S1 & 43.70 & 2.40 & \(-14.4\%\) & 2.80 & 0.05 & \(-21.9\%\) & 40.38 & 0.38 & 10.4\% \\ C20\(\cdot\)S1 & 38.32 & 0.54 & \(-24.9\%\) & 2.31 & 0.19 & \(-35.6\%\) & 39.61 & 0.50 & 8.3\% \\ C5\(\cdot\)A & 46.54 & 3.69 & \(-8.8\%\) & 3.50 & 0.20 & \(-2.2\%\) & 41.58 & 0.15 & 13.7\% \\ C10\(\cdot\)A & 43.43 & 0.25 & \(-14.9\%\) & 3.20 & 0.04 & \(-10.5\%\) & 39.37 & 0.00 & 7.7\% \\ C20\(\cdot\)A & 38.46 & 1.56 & \(-24.7\%\) & 2.38 & 0.16 & \(-33.6\%\) & 34.44 & 0.90 & \(-5.8\%\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Main mechanical properties. [PERSON] 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. 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.