## 3 Results and Analysis
It is due to the fact that doubling the amount of CNFs added provokes a greater temperature increase in the cement mass, since the dispersion of material with greater conductivity is high, transferring part of this heat to the cement matrix, and as such raising its temperature. Figure 6 shows the evolution of strength in relation to strain, in test pieces without addition and with percentages of 1% and 2% of CNF addition in cement weight after undergoing fire testing. As can be observed, the higher strength levels are reached in test pieces containing 1% of CNFs in cement weight, since in the test pieces containing 2% of addition, strength levels are lower even in concretes without addition. Given the importance of how the strength-strain graph evolves in the behavior of the concrete, it is observed that in the concrete without addition, the graph is more homogeneous, depicting  
Figure 4: Evolution of strength (\(\sigma\)) in relation to strain (\(\epsilon\)) in test pieces without addition, with percentages of 1% and 2% of CNFs. a more ductile material. On the other hand, in the test pieces containing CNF additions, the gradient of the curves is more pronounced and imperfect, demonstrating that its behavior is more homogeneous. The most important piece of information is the strength increase in the test piece incorporating 1% of CNFs, its strength is greater compared to traditional concrete and features less strain, the material becomes tough when exposed to thermal aggression, but after reaching its maximum strength peak, it falls notably, losing ductility and becoming more fragile. Figure 5: Evolution of temperature over time in test pieces without addition, with percentages of 1% and 2% of CNFs in their upper, middle and lower sections. Table 4 shows the most representative mean data values from the strength-strain graphs, highlighting: maximum strength (\(\sigma_{\text{max}}\)), strain due to maximum strength (\(\epsilon_{\text{max}}\)), ultimate strength values (\(\sigma_{\text{u}}\)), ultimate strain values (\(\epsilon_{\text{u}}\)), maximum strain energy density (E\({}_{\text{max}}\)) and ultimate strain energy density (E\({}_{\text{u}}\)). Figure 7 shows the signs and symptoms which occurred in compression testing to breaking point, in test pieces with and without additions and before and after direct exposure to fire. It is observed that the test pieces without addition display a more explosive break, with larger and deeper cracks, displaying a more deteriorated appearance of the material prior to structural collapse, however, the test pieces with the addition of CNFs, before and after being subjected to direct exposure to fire, display a less catastrophic type of breakage. Figure 8 shows a comparison of the morphological characteristics at 300 magnifications of test pieces SA, GF-1% and GF-2% before and after the fire. In the test pieces which were not exposed to fire, excellent uniformity and bonding between the cementitious gel and the aggregate is observed and the formation of ettrinigite and portlandite is detected inside the existing pores. In the test pieces exposed to fire, it can be observed that part of the ettrinigite and the clumps of CaO-SiO\({}_{2}\cdot\)H\({}_{2}\)O (CSH gel) are destroyed and dispersed as a consequence of the heat, giving rise to a smoother textural appearance, where the acicular ettrinigite formations acquire rounded shapes and cracks appear due to heat expansion and contraction.