Efect of nano silicon nitride on the microstructural characteristics and mechanical properties of ultra‑high‑performance steel fber reinforced concrete

Vol.: (0123456789) Materials and Structures (2025) 58:103 https://doi.org/10.1617/s11527-025-02634-9 ORIGINAL ARTICLE Effect ofnano silicon nitride onthemicrostructural characteristics andmechanical properties ofultra‑high‑performance steel fiber reinforced concrete J.D.RuizMartínez· J.D.Ríos· E.M.Pérez‑Soriano· H.Cifuentes· C.Leiva Received: 28 November 2024 / Accepted: 11 March 2025 © The Author(s) 2025 with the matrix. This, in turn, reduced macroporosity around the fibers, enhancing the matrix integrity. TGA indicated a reduction in free calcium hydroxide, corroborating the observed strengthening of the matrix. Furthermore, the addition of 0.75 wt% NSIN yielded the optimal results, with a 17.3% improvement in compressive strength and a 66% increase in crack propagation resistance during the elastic phase. These results highlight the potential of NSIN as a nano-reinforcement to significantly improve the mechanical and microstructural properties of UHPFRC. Keywords Ultra-high-performance concrete· Steel fiber-reinforced concrete· Nano silicon nitride· Fracture· Fiber-matrix interaction 1 Introduction Ultra-high-performance fiber-reinforced concrete (UHPFRC) is considered one of the most promising construction materials for future sustainable and resilient infrastructures due to its high compressive strength (above 120 MPa) and superior flexural tensile strength (15–20 MPa) compared to conventional concrete [1–4]It also has remarkable energy absorption capacity and ductility thanks to its reinforcement, which typically consists of steel fibers in quantities around 2% by volume [3, 5]. However, despite its remarkable performance, UHPFRC faces Abstract This study investigates the incorporation of an innovative nano-reinforcement, nano silicon nitride (NSIN), to enhance the workability and mechanical performance of ultra-high-performance fiber reinforced concrete. The addition of NSIN at dosages of 0.25, 0.5, 0.75, and 1.5 wt% of cement was analyzed to evaluate its impact on the distribution and interaction between steel fibers and the cementitious matrix. Experimental analyses, including thermogravimetric analysis (TGA), transmission electron microscopy (TEM), and scanning electron microscopy (SEM), were conducted to establish a relationship between the microstructural modifications, fiber-matrix interactions, and the resulting mechanical behavior. The findings revealed that NSIN increased workability and extended setting time, enabling improved steel fiber dispersion and interaction J.D.RuizMartínez· C.Leiva(*) Department ofChemical andEnvironmental Engineering, Escuela Técnica Superior de Ingeniería, Universidad de Sevilla, 41092Seville, Spain e-mail: cleiv[email protected] J.D.Ríos· H.Cifuentes(*) Department ofContinuum Mechanics andStructural Analysis, Escuela Técnica Superior de Ingeniería, Universidad de Sevilla, 41092Seville, Spain e-mail: [email protected] E.M.Pérez-Soriano Department ofMaterials Science andEngineering andTransport, Escuela Politécnica Superior, Universidad de Sevilla, 41011Seville, Spain Materials and Structures (2025) 58:103 103 Page 2 of 22 Vol:. (1234567890) challenges such as low workability, erratic matrix fiber interaction, and limited fiber efficacy, which can reduce its potential mechanical properties and durability [4, 6]. Additionally, fiber-matrix interaction is a key factor affecting the strength of the UHPFRC because the stress is transferred through the fiber-matrix interface [7]. Furthermore, Rios etal. [3, 4] concluded that the highest concentration of pores is distributed around the steel fiber. Therefore, improving the interaction between fiber and matrix is crucial to improving the mechanical properties of concrete. On the other hand, because of their large specific surface area, nanomaterials exhibit a variety of special behaviors within the concrete, including the pozzolanic reaction, the nucleation effect and particle filling effect [8–10]. Previous studies have shown that the addition of nanomaterials, such as SiO₂, CaCO₃, Al2O3, TiO2, Fe2O3, graphene oxide and carbon nanotubes can improve the mechanical properties of UHPC by enhancing the hydration process and reducing porosity [11, 12]. Because of their tensile strength and high elastic modulus, nanomaterials including carbon nanotubes, nanofibers and graphene nanoplatelets (GNPs) have been used to improve the mechanical properties of cementitious composites [13–15]. Moreover, nanoparticles have been included in cement-based materials to enhance their mechanical properties through chemical reactions or physical filling [16]. The chemical composition and large specific surface area of nanomaterials facilitate and accelerate cement hydration nucleation sites, increasing pozzolanic interactions with portlandite (CH) and consequently producing greater quantities of calcium silicate hydrate (C–S–H) gel [17–20]. The proportion of portlandite-Ca(OH)2 in the concrete combines with nanoparticles to produce a denser concrete matrix [21–23]. Regarding UHPFRC improved with nanoparticles, Norhasri et al. [24] found that the addition of 1% nano metakaolin had an optimal effect on compressive strength. Wu et al. [11, 25] enhanced the mechanical properties of UHPC with the addition of nano-SiO2 and nano-CaCO3, but this effect decreased beyond values of 1% and 3.2% respectively, because the reduction of the workability. This was ascribed to a higher volume of entrapped air and capillary voids. By contrast, Camiletti et al. [26] found that the addition of nano-limestone reduced setting time and Ghafari et al. [12]reported that nano-SiO2 reduced the workability of UHPC due to the shape and structure of the nanoparticles. While previous studies have focused on nanoparticles such as SiO₂, CaCO₃, Al2O3, TiO2, Fe2O3, graphene oxide and carbon nanotubes to modify UHPFRC properties [11, 12, 20, 23, 27], this research investigates nano-silicon nitride (NSIN) as a novel nano-reinforcement. NSIN presents a unique combination of properties: its spherical morphology enhances workability, its nano-filling effect refines the matrix structure, and its interaction with hydration products contributes to further strengthening the composite. Nano-Si3N4 (NSIN) has demonstrated exceptional mechanical qualities, high hardness and robust wear resistance in cement-based materials [28]. By using NSIN, Zhang and Wang were able to develop ceramic tool materials with good cutting wear qualities [29]. According to Sun et al. [30], adding NSIN can increase the bending strength and wear resistance of ceramics. Zhu etal. [28] concluded that the surface effect, filling effect and greater surface energy of NSIN are the primary components of its action mechanism, which enhances the mechanical qualities of cement-based materials. Yin et al. [31] used β-Si3N4 whiskers in Cu composites, resulting in enhanced composite hardness and bending strength. However, the use of NSIN in UHPFRC has not been extensively explored, explored in concrete, only in some ceramic compounds [32], presenting a novel opportunity to advance the understanding of nano-reinforced cementitious composites. This study aims to fill this gap by investigating the effects of NSIN on the microstructural and mechanical properties of UHPFRC, with a focus on improving workability, fiber interaction, and crack resistance. This paper presents an experimental analysis of the behavior of UHPFRC composites with varying NSIN concentrations. Four dosages, ranging from 0.25 to 1.5 wt%, were tested. Mercury intrusion porosimetry, thermogravimetric analysis and stereomicroscopy characterized the matrix pore size distribution and composition. Mechanical and fracture parameters were determined using established experimental procedures. The study states a relationship between mechanical properties and characterization data, analyzing how NSIN addition modifies the matrix microstructure, enhancing workability, fiber-matrix Materials and Structures (2025) 58:103 Page 3 of 22 103 Vol.: (0123456789) interaction, and steel fiber orientation, thus impacting the concrete’s fracture and mechanical characteristics. 2 Materials andspecimen preparation 2.1 Materials Type I 52.5 R/SR cement compliant with the EN 197–1 standard [33] was used to manufacture the specimens of ultra-high-performance steel fiberreinforced concrete. Silica fume (SF) with a specific surface area of 92 m2/g and Ground-granulated blastfurnace slag (GGBS) were used. Several studies have demonstrated the benefits of using slag in UHPC. For example, [34, 35] found that the combination of slag and silica fume in UHPC resulted in a denser microstructure, improved mechanical properties and durability properties, reducing the CO2 emissions associated to the high amount of cement use. Therefore, the inclusion of slag in our study was based on its proven ability to improve the performance of UHPC, particularly when combined with other materials and proper mix design. For the aggregates, two varieties of quartz sand were added: fine sand (FS) with a maximum particle size of 0.315 mm, and coarse sand (CS) with particles below 0.800 mm. Steel fibers with a length of 13 mm and a diameter of 0.2 mm were used. A polycarboxylate-based superplasticizer (SP) and NSIN with a purity of over 98% were used. The chemical compositions of the binder raw materials were analyzed using X-ray fluorescence spectrometry (XRF), with detailed results presented in Table1. The NSIN powder was analyzed using a ZEISS LIBRA 120 transmission electron microscope (TEM) to determine the morphology of its nanoparticles. Figure1 depicts the results obtained via TEM, which confirmed the predominantly spherical morphology of the particles, with an approximate diameter of 50 nm. Additionally, particle agglomeration was observed, underscoring the need to use specific dispersion techniques to achieve a uniform distribution and prevent agglomeration. Although NSIN could be more easily incorporated into cement-based materials with lower energy consumption through conventional mixing processes [28], in this research we opted for dispersion via ultrasonication to obtain optimal dispersion [36]. The microstructure of the UHPFRC matrix and the distribution of nano silicon nitride (NSIN) particles were analyzed using a Hitachi S5200 scanning electron microscope (SEM) equipped with EnergyDispersive Spectrometry (EDS). SEM with EDS was Table 1 XRF chemical composition of the binder raw materials Cement GGBS SF Loss on ignition (LOI) 19.42 17.39 18.97 Al2O36.59 9.83 0.20 BaO 0.06 – – CaO 45.61 35.12 0.30 Cl2O30.07 – – CuO 0.04 – – Fe2O32.85 0.31 0.06 K2O 1.09 2.01 0.42 MgO 1.00 6.63 0.35 MnO20.05 0.11 – Na2O 0.29 0.21 0.12 P2O50.13 – – SiO218.29 27.81 79.58 SO34.02 – – SrO 0.05 0.08 – TiO20.41 0.48 – ZnO 0.02 – – Fig. 1 TEM image of NSIN particles Materials and Structures (2025) 58:103 103 Page 4 of 22 Vol:. (1234567890) chosen in this study because it allowed us to obtain high-resolution images of the matrix microstructure while simultaneously performing elemental mapping to confirm the presence and distribution of NSIN particles. This approach provided a comprehensive understanding of the interaction between the NSIN particles and the cementitious matrix. 2.2 Mix proportions and specimen preparation Five different mixes of UHPFRC that differed only in NSIN content were manufactured. Table2 shows the designation of the five mixes and the proportion of constituents of each. The first mix NSIN0, contained no NSIN and was the reference mix. For the other mixes, NSIN was added in the following proportions: 0.25 wt%, 0.50 wt%, 0.75 wt%, and 1.50 wt%, respectively. The manufacturing process of these types of concretes plays a crucial role in the final properties of the matrix, which is why it is detailed below. Initially, NSIN was added to the mixing water along with the superplasticizer, and moderate sonication was applied to the mix. Specifically, an ULTRASONS 3000513 device with a power of 150 W was used for 30 min to improve the dispersion of NSIN particles in the water. In addition, the solid materials—cement, silica fume, blast furnace slag, fine and coarse aggregates—were mixed for 5min in a vertical mixer to achieve a homogeneous solid mix. Subsequently, all constituents, both solid and liquid, were combined in the mixer and mixed for an additional 20 min until the desired workability was achieved. Next, steel fibers were added and mixed for five more minutes. For each of the five mixes, three 40 × 40 × 160 mm3 prisms were cast. After 24 h, the samples were demolded and subjected to a further curing period of 25 days submerged in water at room temperature. Next, the samples were left to dry for 48 h. 3 Experimental program 3.1 Thermogravimetric analysis Specimens have been stored in sealed bags to avoid carbonation after curing time, then samples weighing 100–150 mg was collected from the surface of the samples for the TG SDTA measurements. A thermogravimetric analysis (TG-SDTA MettlerToledo 851) was conducted from room temperature to 900 °C. A heating rate of 10 °C/min was chosen, using N2 as the purging gas, consistent with standard practice [37, 38]. 3.2 Workability A downscaled slump test was devised to evaluate the workability of fresh concrete, following the procedures outlined in the EN 12350–2:2020 [39] standard. Each test was performed on a flat aluminum sheet using a steel mini-slump cone. The setup for the mini-slump test in this study involved a modified Abrams cone geometry with dimensions of 50 mm at the top, 100 mm at the bottom and 150 mm in height. The fresh concrete was promptly introduced into the mini-slump cone and gradually lifted to minimize inertial impacts. Following the usual protocol, measurements were made using a standard gauge and averaged across four readings with a precision of one millimeter. The self-compacting ability of the concrete was determined using the average value of the flow extension according to the specifications and guidelines for self-compacting concrete [40] 3.3 Initial and final setting time The initial and final setting times of the UHPFRC mixtures were determined in accordance with the EN 196–3 standard [41], which specifies the Table 2 Mix proportions and designation Constituent (kg/m3) Mix Cement SF GGBS FS CS Water SP Steel fibers NSIN NSIN0 540 210 310 470 470 205 42 196 0 NSIN0.25 540 210 310 470 470 205 42 196 1.35 NSIN0.50 540 210 310 470 470 205 42 196 2.70 NSIN0.75 540 210 310 470 470 205 42 196 4.05 NSIN1.50 540 210 310 470 470 205 42 196 8.10 Materials and Structures (2025) 58:103 Page 5 of 22 103 Vol.: (0123456789) procedure for measuring the setting times of cement pastes using the Vicat needle method. Additionally, the EN 480–2:2007 [42] standard, which outlines the test methods for determining the setting time of concrete admixtures, was also followed to ensure consistency and accuracy. The initial setting time was defined as the time when the Vicat needle penetrated the cement paste to a depth of 6 ± 3mm, while the final setting time was identified as the time when the needle could no longer penetrate more than 0.5 mm into the paste. These measurements were conducted at a controlled temperature of 20 °C to simulate real-world conditions. 3.4 Steel fiber—matrix interaction A SMZ25 Nikon stereomicroscope (New York, USA) was used for this study. It was connected to a computer equipped with NIS-Elements BR software for image capture. The study involved the careful manufacturing of small specific specimens measuring 20 × 40 × 40 mm3, maintaining the integrity of the unmolded surface. This precaution ensured the identification of matrix-fiber interactions, allowing for the visual detection of any discernible impacts of NSIN on the UHPFRC. Stereomicroscopic imaging enables for a wide field of view, exact microscopic features in 3D, and quantitative measures [43–45]. The results of this methodology are consistent with prior research [46–49], demonstrating the method’s usefulness to investigate fiber-matrix bonding in UHPRC. Alongside image magnification, the equipment generated a 3D model of the image and a Z-axis graph, offering micrometric precision to illustrate the average profile of the mix’s surface. 3.5 Porosity Porosity was assessed with a Micromeritics Autopore IV (Norcross, USA) mercury intrusion porosimeter. Pore size ranged from 0.007 to 150 μm. To prepare the samples, 5 mm pellets were formed and then subjected to drying in an oven at 105 °C until a constant mass was obtained. 3.6 Mechanical properties 3.6.1 Compressive strength For each type of mix, three 80 × 40 × 40 mm3 specimens were used to assess compressive strength in accordance with the EN 12390–3 standard [50]. A 3000 kN load capacity servo-hydraulic testing machine was used for this test. 3.6.2 Flexural strength All the concretes designed in this study underwent three-point bending tests. The tests were conducted according to the EN 14651 standard [51]. The dimensions of the specimens prepared for these tests were 40 × 40 × 160 mm3, similar to previous studies [11, 52]. All specimens were notched at the center to a depth equivalent to 1/6 of their width. The use of notched specimens with a notch depth of 1/6 of the specimen width is widely accepted in the study of UHPFRC [53, 54]. This method effectively captures the material’s response to cracking and fiber bridging, providing valuable insights into the flexural behavior of UHPFRC. By following this standardized approach, we ensure that our results are comparable to those of other studies and that our findings can be reliably applied to the design and evaluation of UHPFRC in practical applications. During the test, measuring instruments included a 10-mm vertical transducer to record the deflection at the center of the specimen and a 5 mm clip gauge to measure the CMOD. The experiments were conducted using a servo-hydraulic machine capable of withstanding a maximum load of 3000 kN, using CMOD displacement control. Anti-torsion devices were installed on the loading and support rollers. The tests and results were conducted according the EN 14651 standard [51], where the residual strength is calculated using Eq. (1): where fRj and Pj represent the residual flexural tensile strength and the applied load, respectively, corresponding to CMODj. Specifically, fR1, fR2, fR3 and fR4 were calculated using the matching CMOD values of (1) f Rj= 3P j L 2bh2 sp Materials and Structures (2025) 58:103 103 Page 6 of 22 Vol:. (1234567890) 0.5 mm, 1.5 mm, 2.5 mm and 3.5 mm, respectively. In Eq. (1), b is the width of the beam, hsp is the height of the beam excluding the notch height, and L is the span length (Fig.2). 4 Results anddiscussion 4.1 Thermogravimetric analysis The TGA curves, presented in Fig.3, show the variation of mass loss (%) and heat flow (mW) from room Fig. 2 SEM image and EDS of the UHPFRC matrix: a identification of C-S–H gel and NSIN particles; b elemental composition mapping of NSIN particles Fig. 3 Thermogravimetric analysis of NSIN0, NSIN0.75 and NSIN1.5 at 28 days Materials and Structures (2025) 58:103 Page 7 of 22 103 Vol.: (0123456789) temperature to 900 °C. Several peaks can be observed in heat flow. In the first stage (20–250 ºC), an endothermic peak observed (50 ºC) corresponds to the evaporation of moisture and water in capillary pores [55, 56], followed by the dehydration of C–S–H gel at 150 °C. Figure3 shows that a higher NSIN dose accelerated the cement hydration reaction through the nucleation effect (as seen in Fig. 2), resulting in a lower mass loss of 3.83% in NSN0 compared NSN1.5., while NSIN0.75 remains at parity. In the second stage (250–500 ºC), an endothermic peak at 450 °C was due to the decomposition of calcium hydroxide [56]. The addition of NSIN slowed down the decomposition of Ca(OH)2, with mass loss decreasing to 2.56% in NSIN1.5 and 2.05% in NSIN0.75. In the third stage (500–800 ºC), a peak in the thermogravimetric data at 580 °C revealed a mass loss of 2.94% in NSIN0, while NSIN compositions reached 2.34% in NSIN0.75 and 2.56% in NSIN1.50. This indicates that NSIN helped prevent the decomposition of calcium carbonate [57, 58]. In the last stage (800–900 ºC), weight loss occurred due to the loss of OH − residuals. Unlike all other peaks, this last reaction in the compositions containing NSIN was exothermic [58]. The thermogravimetric analysis (TGA) revealed that the addition of nano silicon nitride (NSIN) significantly reduces the content of free calcium hydroxide (CH) and promotes the formation of additional calcium silicate hydrate (C–S–H) gel, leading to a denser and more homogeneous matrix. This reduction in CH content is consistent with the observed improvements in compressive strength and crack resistance, as a denser matrix provides better stress transfer and reduces the formation of macroporosity [55]. Additionally, the TGA results showed that NSIN helps stabilize the calcium carbonate (CaCO₃) content, reducing its decomposition at high temperatures. This stabilization is likely due to the filler effect of NSIN, which reduces the porosity and improves the thermal stability of the matrix [58]. These findings are supported by previous studies [59], which also observed similar improvements in thermal stability and mechanical performance with the addition of nanomaterials in cement-based materials. 4.2 Workability The outcomes of the slump tests for each mix are depicted in Fig.4. As illustrated, it is evident that the slump values did not follow a linear relationship with the different NSIN dosages used. Instead, a curve was formed that initially showed a larger slump radius with increasing NSIN, achieving a higher workability (14.4% greater than NSIN0) with the inclusion of NSIN at 0.75%. This may be due to a ball-bearing effect of the spherical granulate shape of NSIN Fig. 4 Mini-slump flow measurements were conducted in UHPFRC samples containing varying NSIN contents Materials and Structures (2025) 58:103 103 Page 8 of 22 Vol:. (1234567890) (Fig.1) within the concrete [48]. Nevertheless, at the highest NSIN dosage (1.5%), an opposite trend was observed, with the slump value decreasing to a level like that of the mix without NSIN. This suggests that oversaturation of NSIN hinders further improvement in workability. Additionally, all mixes demonstrated selfcompacting properties. This indicates the concrete’s ability to flow and fill formwork under its selfweight, even in the presence of dense reinforcement, without the need for vibration, maintaining uniform consistency throughout [49]. 4.3 Initial and final setting time As depicted Table3, variations in setting times were observed across different NSIN dosages. Specifically, there was a 2.58-h difference in initial setting time between NSIN0 and NSIN1.5, suggesting a proportional variation depending on the NSIN dosage. The disparities were more marked in the final setting time, with compositions containing more NSIN setting 2.65 h (NSIN0.75) and 5.3 h (NSIN1.5) later than NSIN0, showing a significant increase of 20% and 40.1% respectively. The results show that the addition of NSIN significantly reduced both the initial and final setting times. For example, the initial setting time decreased from 8.10 h for the reference mix (NSIN0) to 5.52 h for the mix with 1.5% NSIN. Similarly, the final setting time decreased from 12.80 h for NSIN0 to 7.50 h for NSIN1.5. This reduction in setting time is attributed to the nucleation effect of NSIN, which accelerates the hydration process by providing additional sites for the formation of hydration products. These findings are consistent with previous studies [60], which also observed a reduction in setting times with the addition of nanomaterials in cement-based materials. Moreover, the gap between initial and final setting times contracted with higher NSIN concentrations, decreasing by 14.89% for the lowest NSIN addition and by 42.12% for the highest NSIN content. These findings underscore the role of NSIN in decreasing both initial and final setting times, thus narrowing the time interval between them. These temporal shifts suggest consequential alterations in the physical and mechanical characteristics of the compositions. 4.4 Steel fiber—matrix interaction Figure5 shows images of the interaction between a steel fiber and the cementitious matrix. Initially, it was challenging to discern the influence of NSIN. Table 3 Setting times at 20 ºC Mix Initial setting time (h) Final setting time (h) NSIN0 8.10 12.80 NSIN0.25 7.50 11.50 NSIN0.50 7.05 11.10 NSIN0.75 6.55 10.15 NSIN1.5 5.52 7.50 Fig. 5 Images with their respective 3D modeling for each mix of NSIN: a NSIN0, b NSIN0.25, c NSIN0.50, d NSIN0.75, and e NSIN1.5 Materials and Structures (2025) 58:103 Page 9 of 22 103 Vol.: (0123456789) Nonetheless, the profile formed just before the emergence of the steel fiber, depicted at the center of Fig.5, provides compelling evidence of a correlation between NSIN dosage and the expanded concrete area surrounding the fiber. This expanded area indicates enhanced adhesion and integration of the steel fiber within the matrix, likely attributable to the varying NSIN dosages. Results depicted in Fig.6 support the hypothesis that the addition of NSIN, due to its particle-filling and interface effects [8, 9], potentially increased the surface area of the fiber embedded in the matrix. Furthermore, it was inferred that, in UHPFRC with approximately 2% steel fibers, the fluidization of the cementitious matrix helped to achieve a more uniform distribution of steel fibers, thereby mitigating significant pores and potentially enhancing the mechanical properties of the specimens. It is important to note the clear and direct relationship observed in Figs. 4 and 6. Both tests showed that, as the NSIN content increased, workability also increased, resulting in a greater interaction surface between the matrix and the fiber. By contrast, an opposite trend was observed in the composition with the highest NSIN content (NSIN1.5), which may be explained by an oversaturation of NSIN, suggesting an optimal point below the 1.5% addition. Despite this, the interaction surface remained larger than that of the reference sample. The stereomicroscope images (Fig. 6) revealed that NSIN improved the fiber-matrix interaction, reducing gaps between the fibers and the matrix. This improvement is attributed to there is a higher local friction between matrix and fibers when NSIN due to the lower porosity around the fiber (Fig. 5), and therefore, the greater force of the pull-out of the steel fibers from the matrix is necessary. The addition of NSIN significantly enhanced the hydration process and refined the pore structure of the UHPFRC matrix, as evidenced by the stereomicroscope and porosimetry results (Figs.6 and 7). This improvement in fiber-matrix interaction is attributed to the nucleation effect of NSIN, which provides additional sites for the formation of hydration products, accelerating the early-age hydration process, as shown in TGA (Fig.3). These findings are consistent with previous studies [11], which also observed that nanomaterials can enhance the hydration process and refine the pore structure of cement-based materials. Furthermore, the improved fiber-matrix interaction observed in this study aligns with previous research on the importance of fiber interaction in UHPFRC. Huang et al. [61] demonstrated that better fibermatrix interaction correlates with increased flexural Fig. 6 Stereomicroscope images detailing the average fiber-matrix interaction related to 3D images Materials and Structures (2025) 58:103 103 Page 16 of 22 Vol:. (1234567890) advanced damage stages in the matrix. Furthermore, the addition of NSIN, which improved fiber-matrix adhesion (see Fig.6), improved residual strengths in the later stages fR3 and fR4. Figure 14 illustrates the relationship between residual strengths and NSIN content. Each set of bars in the diagram shows the percentage of improvement of each residual strength compared to its equivalent value in the NSIN0 diagram (light blue). A significant improvement in all residual strengths was evident when the matrix was supplemented with NSIN. The 0.5 wt% content showed the most notable increases in residual strengths. The most significant improvement was recorded for fR3 and fR4 in NSIN0.5, with increases of 103% and 99%, respectively. This behavior was attributed Fig. 13 Relationships between the residual strengths and the fibermatrix interfacial surface Fig. 14 Relationships between residual strengths and NSIN content Materials and Structures (2025) 58:103 Page 17 of 22 103 Vol.: (0123456789) to the combination of a high LOP (Fig.11.a) and an increased fiber-matrix interfacial area (Fig. 6). It is important to highlight the crucial role of reinforcing the matrix with NSIN in the post-cracking behavior of steel fiber-reinforced concretes. 4.6.5 Influence ofLOP andfiber‑matrix interfacial surface onenergy absorption capacity This research also explored how the LOP and the fiber-matrix interfacial surface influenced energy absorption capacity in UHPFRC reinforced with NSIN. Figure 15 presents the results of the linear regression analysis between the first residual strength at a CMOD of 0.5 mm and the subsequent residual strengths at larger notch openings of 1.5, 2.5 and 3.5 mm CMOD. It was observed that fR2 and fR3 exhibited a very high linear correlation with fR1, with R2 values of 0.95 and 0.99, respectively. However, this correlation was weak for fR4, with an R2 value of 0.56. The high correlations between fR2 and fR3 on one side and fR1 on the other were due to the fact that, for small CMOD openings where matrix damage mainly manifested as microcracking, residual strength was more dependent on the matrix’s properties (i.e., porosity, fiber distribution and orientation). For fR4 (3.5 mm CMOD), the resistance mechanism was quite different. It was characterized by macrocracks in the matrix where the fibers were more activated, primarily resisting tensile stresses through bridging effects. This difference in the resisting mechanism explains the loss of correlation with the earlier stages. The ability to absorb energy can be quantified by the rate of decrease, which corresponds to the slope of the linear regression lines. For fR2, the rate of decrease relative to fR1 was 86%, for fR3 it was 77%, and for fR4 it was 53%. This indicates that the energy absorption capacity decreased moderately for 1.5 and 2.5 mm openings but dropped by about half for 3.5 mm openings, reflecting the extent of internal damage in the matrix. A final concept worth highlighting is that the highest residual strength fR1 was obtained for the NSIN0.50 material. However, the highest LOP was achieved with NSIN0.75 (Fig.13). This is because a less porous matrix is initially stronger but also more brittle. Thus, although it had a higher LOP, once this first cracking (LOP) was exceeded, fR1 was reached more quickly, as demonstrated by the results. Figure16 illustrates the energy absorption capacity determined from three-point bending tests for UHPFRC modified with different NSIN contents. Energy absorption capacity increased with NSIN content up Fig. 15 Relationships between residual strengths fR2, fR3, fR4 and fR1 Materials and Structures (2025) 58:103 103 Page 18 of 22 Vol:. (1234567890) to 0.75 wt%. However, it decreased with a higher content of 1.50 wt%. This trend was qualitatively similar to that observed in compressive strength (Fig.9.a) and in the LOP (Fig.11.a). The energy absorption and CMOD curves showed a nonlinear relationship for all UHPFRCs, suggesting that the rate of increase in energy absorption decreased as CMOD increased. Results suggested that the addition of NSIN to UHPFRC significantly improved its mechanical and fracture behavior. This improvement was due to two fundamental reasons: the reduction of macropores in the matrix (Fig. 7), leading to higher compressive strength and LOP; and the increase in the fiber-matrix interfacial surface area, which improved fiber adhesion. In the initial stages of loading, internal stresses generated microcracks in the matrix whose propagation was hindered by fibers acting as barriers. As microcracks coalesced and the cracks widened (increasing CMOD), the fibers bridged the gap between the crack faces. The addition of NSIN increased the fiber-matrix interfacial surface, improving fiber adhesion. Nevertheless, the efficiency of fiber bridging decreased as CMOD increased. Fibers can slip, stretch, or even break, reducing the material’s ability to absorb additional energy effectively. 5 Conclusions This study investigated the effects of nano silicon nitride (NSIN) on the microstructural and mechanical properties of ultra-high-performance fiber-reinforced concrete (UHPFRC). The most significant findings can be summarized as follows: 1. The addition of NSIN significantly enhanced the workability and extended the setting time of UHPFRC, facilitating fiber-matrix interaction. This led to an increase of the matrix around the fibers according to Stereomicroscope analysis. 2. NSIN acted as both a pozzolanic activator and a filler agent, reducing the presence of capillary pores and macropores. This dual role contributed to a 17.3% increase in compressive strength and a remarkable 66% improvement in crack propagation resistance during the elastic phase, particularly at an optimal dosage of 0.75 wt% NSIN. Fig. 16 Energy absorption capacity and CMOD curves generated from experimental three-point bending tests Materials and Structures (2025) 58:103 Page 19 of 22 103 Vol.: (0123456789) 3. The unique relationship between compressive strength and flexural performance was a key finding of this study. Specifically, we observed a linear increase of 4.37 MPa in the Limit of Proportionality (LOP) for every 10 MPa increase in compressive strength. This rate of improvement is significantly higher than those reported in previous studies involving other nanomaterials. 4. The improvement in interfacial adhesion was particularly evident in the post-cracking behavior, where residual flexural strengths (fr values) showed significant increases, especially for larger crack openings (e.g., 103% and 99% improvements in fr3 and fr4, respectively). 5. The energy absorption capacity of UHPFRC was also enhanced by NSIN, with the optimal dosage (0.75 wt%) showing the highest performance. This improvement was attributed to the combined effects of reduced macroporosity and enhanced fiber-matrix interaction. The addition of NSIN offers a promising approach to improving the mechanical and microstructural properties of UHPFRC, particularly in applications requiring high flexural strength and crack resistance. Funding Funding for open access publishing: Universidad de Sevilla/CBUA. This research was funded by the Spanish Ministry of Science and Innovation under projects number PID2019 - 110928RB-C33 and PID2023 - 147971OB-C32. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. 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