**1. Introduction**

Construction engineering requires materials that enhance the mechanical properties of the cement-based composites for modern concrete infrastructure. For instance, the compressive, tensile and flexural strength of concrete structures need to be improved. For this, nanomaterials can be mixed with cementitious matrices to obtain concrete with high mechanical strength [1–10]. Nanotechnology can facilitate the development of nanomaterials incorporated into cement-based materials to increase their mechanical strength [11–25], decreasing their environment impact [26]. The CO2 emissions generated during the production of ordinary Portland cement can represent approximately between

5% and 7% of the world man-made emissions of this gas [27,28]. A main challenge of the cement industry is the reduction of the CO2 emissions. One alternative solution is the construction of concrete structures with higher mechanical strength and higher durability, which will decrease their maintenance requirements or need for early replacement. Thus, the concrete structures can have thinner sections, which will require less quantity of cement-based composites for their construction.

The cement-based materials can be mixed with nanomaterials such as nano-silica (nano-SiO2), nano-alumina (nano-Al2O3), nano-ferric oxide (nano-Fe2O3), nano-titanium oxide (nano-TiO2), carbon nanotubes (CNTs), graphene and graphene oxide. In recent years, several researchers [29–49] have studied the incorporation of nanomaterials into cement-based materials. The mixture of cementitious composites and nanomaterials can increase the mechanical strength of the resulting concrete structures. Thus, the life cycle of these structures can be extended or they can require smaller amounts of steel reinforcing bars. A common nanomaterial employed in cement-based composites is nano-silica. This material accelerates the cement hydration due to the generation of calcium-silicate-hydrate (C–S–H) and dissolution of tricalcium silicates (C3S) [50]. In addition, this acceleration of cement hydration is caused by the nano-silica acting as a seed for nucleation of C–S–H [50]. Nano-silica can improve the durability, workability and mechanical properties of cement-based materials [51–59]. On other hand, nano-Al2O3 particles can increase the compressive strength of cement-based materials [41,60–63]. Al2O3 nanofibers with a dosage of 0.25% by cement weight may enhance the compressive strength of cement-based materials by up to 30% [50]. Another nanomaterial that can be added to cementitious matrices is nano-Fe2O3. Optimal values of this nanomaterial improve the compressive strength of concrete specimens [64,65]. The cement added with TiO2 nanoparticles can be used to build a photocatalytic concrete with self-cleaning and air-purification characteristics [66]. This concrete type can allow effective photocatalytic decomposition of pollutants, including volatile organic compounds, carbon monoxide, chlorophenols and aldehydes generated from automobiles and industrial emissions [66–68]. Also, graphene family nanomaterials can be incorporated into cement composites to enhance their mechanical strength and durability, as well as provide self-sensing abilities [69–72]. Other novel properties of cement-based materials containing nanomaterials are their low electrical resistivity and self-sensing capabilities [73]. For instance, cement-based composites with CNTs have strain-sensing abilities, which may allow the measurement of their electrical resistance under applied loads [74]. It represents an advantage to obtain strain-sensing concrete structure systems for structural health monitoring [75,76].

This review includes recent studies about the effects on the mechanical strength, durability and workability of cement-based composites due to the incorporation of nanomaterials such as nano-SiO2, nano-Fe2O3, nano-TiO2, nano-Al2O3, CNTs, graphene and graphene oxide. In addition, these studies include nanomaterials that provide self-cleaning and self-sensing abilities to concrete structures. Also, the main challenges of using nanomaterials in cement-based materials are discussed.

#### **2. Nanomaterials in Cement-Based Materials**
