## 1 Introduction
Zirconium oxide is a non-reactive ceramic material with a wide range of applications due to its unique combination of physical-chemical and mechanical properties, such as high corrosion resistance, low thermal conductivity and insulating properties, high strength, and excellent fracture toughness [14; 15]. Its properties depend on its crystalline structure and on the phase transformations to which it is subjected, being a great alternative as a reinforcing material for cementitious composites such as mortars and/or concretes [16]. In the literature, it has been reported that ordinary Portland cement (OPC) with partial replacement of zirconium oxide nanoparticles significantly increase the values of split tensile strength, flexural strength, and strength of concrete due to the rapid consumption of Ca(OH)2 and release from cement hydration, which is related to the high reactivity of the zirconia particles. Therefore, the hydration of cement is accelerated; generating large volumes of hydration products, increasing the packing density of the cement particles and reducing the volume of large pores in the cementing paste [17; 18]. It has also been found that the increase in the percent volume fraction of zirconium oxide nanoparticles decreases the initial and final setting time of the concretes, indicating that zirconium oxide nanoparticles have a higher hydration reaction rate than cement, since they are characterized by unique surface effects, smaller particle sizes, and higher surface energy [18; 19]. It has also been reported that the workability of mortar or concrete mixtures decreases with the increasing of zirconium oxide nanoparticle content [20]. With smaller particle sizes and surface area, water demand increases. If the water-to-cement ratio remains constant, and the volume fraction of zirconium oxide nanoparticles increases, the workability of the fresh concrete is diminished. It has been reported that the optimal value of zirconium oxide nanoparticles as partial substitute for cement is 1% by weight of cement [18]. With 1% of zirconium oxide nanoparticles, pore distribution is refined and porosity decreases, compared to cementitious pastes without nanoparticles. The effect of zirconium oxide nanoparticles on the mechanical and thermal properties of cement aluminate-based pastes has also been stated in the literature [21]; for thermal energy-storing materials, it has been determined that the optimum value of zirconium oxide nanoparticle addition is 1% by weight, reaching the best results in the properties. Similarly, thermal conductivity and volumetric heat capacity of pastes enriched with zirconium oxide nanoparticles were improved. Zirconium oxide nanoparticles have also been added in reactive powder concretes (RPC); several works showed that zirconium oxide nanoparticles do not accelerate the hydration process of RPCs, however, they can make the hydration products distribution more uniform, restricting the growth space of CH crystals, obtaining denser microstructures of these concretes. It has also been reported that high contents of zirconium oxide nanoparticles can contribute to better mechanical properties (up to 3% by weight), however, in the absence of a good dispersion, they can agglomerate and weaken the reinforcing effect [22; 23]. Most of the works presented in the literature regarding mortars or concretes have focused on the development and optimization of mechanical properties as a function of curing time, different curing media, and/or percentage of nanoparticle addition. They establish the fine aggregates (natural sand, river sand, or silica sand), cement, and water as part of the composite matrix, where the characteristics in the resistance values and the developed microstructure are not related as a function of the aggregate type or sand. With the addition of zirconium oxide nanoparticles, which is one of the least reported nanoparticles in the literature for similar research, these characteristics may be influenced by the different composites matrices.