## 5 Conclusions  
The microstructural development of cementitious composites with limestone aggregates and the addition of zirconia nanoparticles as a means of reinforcement was evaluated in mortars, in relation to their physical properties and compressive strength.  
It was found that type, shape, and composition of the aggregate contribute to the microstructural development of cement mortars. In mortars with limestone aggregates, the analysis suggests that the failure onset originates within the aggregates and not in the interfacial zone.  
In the zirconia-added mortars, a less porous microstructure was developed with a greater degree of compaction, which is reflected in an increase in the density and the compressive strength.  
The analysis carried out in the work suggests that the reinforcement of the added mortars is due to the high surface activity of the zirconia nanoparticles. Reinforcement in the zirconia-added mortars is derived mainly from three mechanisms: the filler effect, nucleation effect, and the phase transformation effect. The filler effect is exerted by zirconia particles when they are trapped in the empty spaces, or pores, generated by the cement's own hydration process, resulting in the filling of the interstices. Regarding the nucleation effect, zirconia particles serve as preferential nucleation sites for cement hydration products, either within the pore space or at the interface between the cement paste and the aggregates, or between the hydration products, generating a higher (CSH) tobermorite phase content and inhibiting the growth of calcium hydroxide crystals Ca(OH)\({}_{2}\).  
In spite of all of the above, more research is required using specific techniques to analyze the reinforcement mechanism exerted by the zirconia particles on the matrix of the composite. A high void content was not observed in the aggregates, taking into account that the absorption percentage is a proxy of the void content, which was also corroborated by SEM.  
The use of zirconia nanoparticles in cementitious composites with limestone aggregates has produced microstructural development with a direct benefit in the reinforcement of the matrix, which generates an important incentive for a research avenue of a greater scope, leading to better development of the mechanical properties through microstructural control.  
[PERSON][PERSON] conceptualized the whole work, designed the methodology, carried out the SEM tests and reviewed the manuscript. [PERSON] performed the physical-mechanical tests and contributed to the manuscript writing. [PERSON] supervised the experimental work and reviewed the manuscript. [PERSON] carried out the XRD tests, contributed to all sections and reviewed the manuscript. [PERSON] wrote the original draft, supervised the physical-mechanical tests and contributed to all sections. [PERSON] analyzed the physical-mechanical data and contributed to the introduction, methods and discussion sections.  
The authors wish to thank the support of the Chairs Program of the National Council of Science and Technology of Mexico and ECOSUR for the provision of the scanning electronic microscopy equipment.  
The authors declare no conflict of interest.