## 4 Discussion  
Zirconia particles in tetragonal and monoclinic phases with submicronic particle sizes with crystallite sizes of 15.18 and 17.79 nm, respectively, were synthesized by coprecipitation of zirconia nanoparticles doped at 7.5 mol% with YO\({}_{1.5}\).  
Cement mortars with addition of these synthesized particles presented a microstructural development with a greater compaction degree than the reference mortars, smaller volume of pore space, and higher compressive strength, which allows inferring a good surface activity due to the high surface area of the obtained particles.  
The efficiency of ZrO\({}_{2}\) nanoparticles, in terms of the increase in resistance, was around 19%, possibly due to nanoparticle surface effects, which have a faster reaction rate than that of cement and may influence compressive strength at both ages. In addition, limestone aggregates have irregular shapes, rough texture, and they react or produce greater bonding between the aggregate particles and cement paste; there is an increment in the adhesion strength that can be correlated to a greater resistance [3]. Additionally, in the literature [16], it has been reported that phase transformation effect occurs when zirconia in tetragonal phase is subjected to mechanical stresses and undergoes a martensitic transformation to the monoclinic phase, which involves an increase in volume of approximately 4%, which should contribute to the inhibition of the growth of cracks and the increase of compressive strength [24]. However, more studies will have to be carried out to identify and isolate the mentioned reinforcement mechanism in the matrix of the cementitious composite. According to the literature, since ZrO\({}_{2}\) nanoparticles are characterized by their particle sizes with greater surface area, they increase the packing density of the cement particles, reducing the volume of permeable pores in the mixtures [18].  
According to SEM analysis, the reference mortars made with limestone aggregates presented a microporosity ranging from 10 nm to 1 \(\upmu\)m, which depends mainly on the hydration degree, and they also presented a macroporosity consisting of entrained air in the form of spherical bubbles and sizes between 20 and 200 \(\upmu\)m, besides the aggregate's porosity, that can greatly influence the properties of the mortar [36]. According to the literature [37], pores smaller than 1 \(\upmu\)m contribute to the transfer of capillary water while, in finer pores, the water is bound to the material and these pores do not contribute to water transfer. Pores greater than 100 \(\upmu\)m can contribute to water permeability by gravity or to the entry of water driven by the wind.  
Figure 9: SEM micrographs of the fracture surface of ZrO\({}_{2}\) nanoparticle-added mortars cured at 28 days.  
The exposure of mortars on exposed surfaces in coastal areas is subject to high environmental loads, such as a high degree of salinity, high relative humidity, and high temperatures for most of the year. The soluble salts in mortars, supplied from external sources, may present a serious risk for their durability. Salts may come from the raw material or be supplied from external sources, such as the salt-laden wind and salt from the soil and groundwater that is transported through the walls or from wastes. Therefore, a low percentage of permeable pores in the mortars is required.  
It is suggested that the type, form, and composition of the aggregate (dolomitic limestone) contribute, to a certain extent, to the microstructural development of mortars and also to the physical-mechanical behavior. In the reference mortars, these characteristics increase the adhesion with the cement paste and decrease the porosity in the interface zone, generating a greater compaction volume, which implies that faults may be originated in the aggregates. In the zirconia-added mortars, SEM images showed that aggregates tend to cleave along the planes, producing flat surfaces.