## 4 Results and Discussion
### Factors Influencing the Behavior of the Products
#### 4.2.4 The Effect of Nano MgO/Fly Ash on the Cement Porosity  
As shown in Figure 11, we observed different pore distribution in the cement paste directly depending on the content of nano MgO/fly ash. Pore distribution and total porosity are factors that affect the final strength, permeability, and volume changes in the hardened cement paste. The pore size distribution and porosity of cement paste is affected by the nano MgO, as can be seen in Figure 11. The total porosity of the cement decreased as the curing age increased. The porosity of the produced cement containing 8% of nano MgO oxide was higher than that of cement containing 4% of nano MgO oxide, which may be due to the excessive content of nano MgO oxide, which led to harmful expansion and increased the harmful pore size of mortar. This observation may be due to the presence of MgO as well as the process of MgO hydration; there are three main stages that may occur at the same time but can be dominant at different times. In the first two stages, reactive nano MgO particles dissolve and leave pores in the whole system, and then minor Mg(OH)\({}_{2}\) crystals grow freely to fill these pores without generating crystal growth pressure. The third stage is dominated by confined crystallization, and expansion is mainly caused by microcracks induced by the crystal growth pressure in the whole system. Hydration of MgO makes the pore structure partially dense or increases the tortuosity of pores, which will improve durability; furthermore, if high expansion occurs, high porosity will reduce  
Figure 10: Effect of nano MgO content on expansion performance of cement paste. durability. The strength of hardened cement is considered to be its most important property. Each of the basic components of cement presents a specific behavior where, by varying their relative percentages, the mechanical properties of the cement can be modified. It should also be noted that as the total porosity of the cement decreased as the curing age increased, the cement became denser, and the strength became higher. This is exactly what is shown in Figure 12, where the division into three groups with clear boundaries of specific ratios of nano MgO/fly ash is clearly shown. The clear separation of the three groups, from the beginning of the study, with the factors affecting the cement is obviously shown in Figure 12, which is directly related to the total of the results given so far. For example, the differentiation of the two groups in terms of porosity and its distribution (Figure 11) is in complete agreement with the groups of Figure 12, and the distribution of pores is affected by the ratio of water to cement and the degree of hydration of the cement. Large pores (>50 nm) affect the compressive strength, while smaller ones have a greater effect on drying shrinkage and creep. Figure 11: Pore size distribution of mortars containing MEA: (**a**) 4% MEA; (**b**) 8% MEA. Figure 12: The compressive strength of cement paste samples with MgO. 4.2.5. The Effect of Nano MgO on the Mechanical Performance of the Produced Cement  
Moreover, the physical interpretation of the diagrams of Figures 9 and 10 is harmonious with the final behavior and strength of the hardened cement directly dependent on the mineral raw materials, where they participate in the composition of the cement (Table 1). In general, the cement strength seems to show increasing trends in the tests of 7, 28, and 90 days for contents of nano MgO up to 4% of the volume of cement, probably because the microstructure improved after nano MgO incorporation. With the extension of the curing age, the compressive strength of all samples gradually increased, most likely because fly ash underwent a pozzolanic reaction, which improved the interface transition zone of the cement [37].