## 3 Results and Discussion
Adding an air-entraining agent makes the tail smaller. After the partial replacement of Portland cement in concrete R+F+A with zeolitic tuff (10% by mass), the fracture behavior of such concrete under load is similar to concrete R+F. However, this concrete will have a better resistance to freezing and thawing cycles and, as a result, a durability that conforms to a sustainable development strategy [13]. The area under the complete load-deflection curve (Figure 5) represents the total fracture energy \(\mathrm{G_{F}}\). Figure 6 shows the fracture-energy magnitudes of concretes. The obtained results indicate that the fracture-energy difference between concrete R0 and R is negligible. The fracture energy of concrete incorporating different types of fibers has the highest value, reaching 441 N/m. The addition of an air-entraining agent causes a reduction of fracture energy by 32.9%. The fracture energy of concrete denoted R+Z+F+A increases by 35.1% compared to the reference concrete R. This underlines the positive effect of the optimization of technological factors when a concrete mix is designed to provide appropriate fracture parameters and freeze-thaw resistance. Figure 5: Typical load–deflection curves for studied concretes. Figure 6: Fracture energy of concretes. A similar trend can be observed for the characteristic length of studied concretes. Figure 7 reveals that when PP fibers are added to the concrete mix, the characteristic length of concrete R+F increases by 55.0% compared to R. Entrained air bubbles in R+F+A series are flaws in the cement matrix, increasing concrete brittle behavior. This results in a 19.9% lower characteristic length for such a composite. However, the complex addition of zeolitic tuff and air-entraining agent to concrete incorporating different types of fibers improves the characteristic length, which reaches 176 mm and is the highest among tested concretes. The use of a scanning electron microscope equipped with EDS explains the improvement of the mechanical properties and fracture parameters of concrete incorporating zeolitic tuff and air-entraining agent. Despite its crystalline structure, the zeolitic tuff, which is a hydrated alumina-silicate by nature, shows proper pozzolanic activity. The microstructure of the above-mentioned concrete is characterized by an increased homogeneity (Figure 8b) and the formation of tight submicroscopic clusters of hydrosilicates and calcium hydrosulfouluminates (Figure 8d). Such concrete also has a structure with an evenly distributed, dispersed air bubble system (Figure 8a). Using a surface active agent as an air-entraining admixture, which consists in most cases of amphiphilic molecules, induces the adsorption of fine cement particles at the air/paste interface, and a particle-armed bubble shell is formed (Figure 8c). The shell's composition depends on the type of cement and used mineral additions. As can be seen from Figure 8e, due to the use of zeolitic tuff, the low-base calcium hydrosilicates in the non-clinker part of the cement matrix of concrete form a tight and robust bubble shell, which in combination with the modified microstructure of concrete results in the enhancement of the mechanical and fracture parameters of concrete. [PERSON] et al. [26] confirm the formation of the shell and indicate that the bubble shell improves strength and stability, reducing coalescence. Figure 7: Characteristic length of concretes. Figure 8: (**a**–c) Microstructure and (**d**,**e**) EDS spectra of concrete cement matrix. That is why when a load is applied to the air-entrained FRC containing zeolitic tuff, stresses distribute uniformly around the evenly scattered armored bubble shell. This results in an improvement in the compressive strength and fracture parameters of the designed concrete.