*2.2. Nano-Ferric Oxide (Nano-Fe2O3)*

The optimal addition of nano-Fe2O3 in concrete specimens may improve their compressive strength. In addition, the volume electrical resistance of cement mortars with inclusion of nano-Fe2O3 can be altered through the applied load, allowing the measure of compressive stress [73]. It can be used for structural heath monitoring of concrete structures without require additional sensors.

Fang et al. [89] measured the mechanical properties of cement samples with different additions of nano-Fe2O3 (3%, 5% and 10% by cement weight) at ages of 7, 14 and 28 days. Figure 8 shows the SEM image of cement specimens with different additions of nano-Fe2O3. When the nano-Fe2O3 content increases then the surface morphology is denser. For all the measurements, the addition of nano-Fe2O3 in cement mortars increased their compressive strength compared to control mortar. Maximum values of the compressive strength of the cement samples were achieved using 10% of nano-Fe2O3 content. For this nano-Fe2O3 dosage at curing ages of 7, 14 and 28 days, the compressive strength of the cement mortar was increased up to 66.81%, 69.76% and 25.20%, respectively.

**Figure 8.** SEM image of concrete specimens with addition of nano-Fe2O3 (**a**) 0%; (**b**) 3%; (**c**) 5%; (**d**) 10%. Reprinted with permission from [89]. Copyright©2018, Atlantis Press.

Rashad [90] presented a review the effects of nano-Fe2O3, nano-Al2O3, nano-Fe3O4 and nanoclay on some properties of cement composites. These properties were the mechanical strength, hydration heat, water absorption, workability, setting time and durability. For instance, the inclusion of nano-Fe2O3 in the cementitious matrix decreased the water absorption and heat rate values as well as accelerated the peak times. Moreover, the workability of the composite was reduced when the nano-Fe2O3 content was increased. On the other hand, nano-Fe2O3 (0.5%–5% in concretes and 0.5%–10% in mortars) added

into the cementitious matrix improved the compressive strength. Nazari et al. [91] also studied the workability of concrete including nano-Fe2O3. For this case, cement was partially substituted with nano-Fe2O3 (i.e., 0%, 0.5%, 1%, 1.5% and 2% by cement weight) and a water to binder ratio of 0.4 was employed. The workability of concrete is decreased when the nano-Fe2O3 dosage is increased. In addition, Nazari and Riahi [92] developed two models using genetic programming and artificial neural networks to predict the percentage of water absorption and split tensile strength of concrete samples containing nano-Fe2O3.

Khoshakhlagh et al. [93] studied the changes of the concrete properties achieved by adding different percentages (1%–5% by cement weight) of nano-Fe2O3 and superplasticizer. The flexural, compressive and tensile strength, and the water permeability of the concrete specimens were improved with the incorporation of nano-Fe2O3 up to 4% by cement weight. The content of nano-Fe2O3 up to 4wt.% of the concrete specimens increased the coefficient of water absorption. The concrete specimens with nano-Fe2O3 enhanced their hydration heat, workability and the compressive, flexural and tensile strength.
