Enhancing the matrix-fiber bond in ultra-high-performance fiber-reinforced concrete using a high performance plasticizer. Impact on the flowability, physical and mechanical properties

Enhancing the matrix-fiber bond in ultra-high-performance fiber-reinforced concrete using a high performance plasticizer. Impact on the flowability, physical and mechanical properties J.D. Ruiz Martínez a , J.D. Ríos b , E.M. P´ erez-Soriano c , H. Cifuentes b,* , C. Leiva a,* a Department of Chemical and Environmental Engineering, Escuela T´ ecnica Superior de Ingeniería, Universidad de Sevilla, Seville 41092, Spain b Department of Continuum Mechanics and Structural Analysis, Escuela T´ ecnica Superior de Ingeniería, Universidad de Sevilla, Seville 41092, Spain c Department of Materials Science and Engineering and Transport, Escuela Polit´ ecnica Superior, Universidad de Sevilla, Seville 41011, Spain ARTICLE INFO Keywords: Ultra-high-performance fiber-reinforced concrete High performance plasticizer Thermo-gravimetric analysis Porosity Compressive strength Flexural strength ABSTRACT One of the main problems in the use of ultra-high-performance fiber-reinforced concrete (UHPFRC) with steel fibers is low flowability. The addition of 2,3,4,5,6-Pentahydroxycaproic acid sodium salt (SPS) in small amounts (0.05, 0.1 and 0.15 %wt of the cement content) can significantly increase the flowability and setting time. The effects on porosity, matrix chemical composition and fiber-matrix interaction were examined, as well as its influence on strength. Improving the flowability, pores larger than 50 μ m show a significant decrease, while those smaller than 0.1 μ m show a significant increase. According to the results of thermo-gravimetric analysis, the production of ettringite with finer grains and staggered symbiosis improves the pore structure of the UHPFRC matrix. An improvement of the adhesion of the steel fibers to the matrix was determined by stereomicroscope images, showing evidence of a relationship between SPS dosage and the expanded concrete area surrounding the fiber. The flexural and compressive strength increased by 10 % and 8 %, respectively, when the plasticizer/ cement ratio was 0.15 %. 1. Introduction Within the construction, concrete stands as a profoundly utilized material due to its notable attributes, including its exceptional compressive strength, durability, cost-effectiveness, abundant availability of constituent materials, and its adaptability to various shapes achievable through diverse casting molds [1]. Around the world, concrete is the most widely used building material for residential and commercial structures [2]. Approximately 4.1 billion tons of cement are produced annually worldwide [3]. Ultra-high-performance fiber-reinforced concrete (UHPFRC) represents an innovative cement-based composite material renowned for its exceedingly high strength, exceptional durability, and remarkable toughness. The components utilized in the formulation of UHPFRC predominantly encompass cementitious materials, quartz sand, chemical additives, and water, among others [4–6]. As a result, ultra-high-performance fiber-reinforced concrete (UHPFRC) was developed and is regarded as one of the most promising materials for future sustainable and resilient infrastructure [7,8]. This concrete offers significantly improved mechanical properties, particularly in tensile and flexural strength. Its reinforcement, typically consisting of steel fibers in amounts exceeding 2 % by volume, also provides exceptional energy absorption capacity and ductility [9]. The demand for UHPFRC continues to grow due to its use in large structures. One application of this material is its employ in structures designed for storage or production of thermal energy, such as steam storage tanks in solar thermal power plants or salt storage tanks [10]. Compared to conventional concrete, UHPFRC is known to have higher viscosity and pumping losses. Hence, admixtures, including retarders and additional plasticizers, must be incorporated [11]. The integration of steel fibers is widely acknowledged to potentially decrease the flowability of ultra-high performance fiber-reinforced concrete (UHPFRC), resulting in increased air content in its fresh state, and consequently, heightened porosity in the hardened state [12]. Research has indicated that UHPFRC mixtures incorporating fibers with a smaller aspect ratio are more workable, even at higher fiber dosages, compared to mixtures with fibers possessing a larger aspect ratio. However, it’s noted that fibers with smaller radio tend to lower strength * Corresponding authors. E-mail addresses: [email protected] (H. Cifuentes), [email protected] (C. Leiva). Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat https://doi.org/10.1016/j.conbuildmat.2025.140683 Received 17 April 2024; Received in revised form 10 February 2025; Accepted 2 March 2025 Construction and Building Materials 470 (2025) 140683 Available online 6 March 2025 0950-0618/© 2025 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ). [4,12,13]. Furthermore, improved performance requires a dense microstructure, which in combinations with very little water content cannot be entirely achieved by raising the individual particle’s degrees of reactivity [14]. As a consequence, a high-performance superplasticizer is a crucial element in the preparation of UHPFRC, improving its rheology [15]. Effective physical packing of binder particles in the microstructure is necessary, requiring careful material selection based on synergistic size distributions as well as availability and reactivity [14]. Over the past thirty years, the primary constituents of superplasticizers have been synthetic water-soluble polymers, including modified sugar-free lignosulfonate, sulfonated naphthalene formaldehyde condensate, and sulfonated melamine formaldehyde condensate [16,17]. In the last ten years, several advanced superplasticizers have been developed, all of which are based on the polycarboxylate polymers family as carbohydrate easters which main mechanism is the steric hindrance effect [18, 19]. Nonetheless, solely utilizing these superplasticizers often fails to meet the UHPRFC’s combined fluidity loss standards [10,20]. 2,3,4,5,6-Pentahydroxycaproic acid sodium salt (SPS) is an organic electrolyte known for its high water solubility. SPS, that also acts as a high performance plasticizer [21,22]. While SPS alone has a relatively low water reduction rate, it can make a significant difference improving the water reduction rate or improve the concrete’s flowability and lessen slump loss when paired with other superplasticizer [10,21]. The addition of SPS can delay cement hydration, reduce the adsorption and increase the other superplasticizer concentration in solution, leading to increased fluidity of concrete and reduced slump loss [10]. Several hypotheses outline the setting retarding mechanism of 2,3,4,5,6-Pentahydroxycaproic acid sodium salt. Firstly, it is proposed that SPS impedes the hydration process of tricalcium silicate (C 3 S) by adsorbing onto specific dissolution sites on the surface of the silicate phase. This leads to diminished hydration heat and rate, thereby extending the induction period and delaying the setting [23–25]. Another perspective suggests that the adsorption or complexation of SPS with Ca 2 + hinders the formation of ettringite (AFt) [21,24]. Furthermore, there is a notion that SPS delays AFt formation by impeding the dissolution of calcium sulfate dihydrate (CaSO 4 ⋅2 H 2 O), which is crucial for AFt production [22]. Additionally, a theory revolves around the control of calcium hydroxide (CH) crystals, indicating that SPS suppresses the typical precipitation of CH by restricting the growth of CH nuclei. Nevertheless, the predominant role among these four mechanisms and their interrelation remains unclear at present [10,24]. Subsequent investigations have shed light on the correlation between the retarding effect of superplasticizers and its dosage. At lower levels, superplasticizers prove insufficient to fully counteract the active dissolution site of C 3 S [24], thereby limiting the availability of acid for adsorption onto the tricalcium aluminate (C 3 A) surface. Consequently, the primary mechanism of retardation at lower dosages predominantly involves the inhibition of calcium silicate nucleation and restricting the formation and expansion of hydration products [22]. Conversely, higher dosages result in the complete adsorption of residual superplasticizer onto the C 3 S surface, leading to heightened hydrolysis and concentrations of superplasticizer and Na + . The excess of superplasticizer then hampers the dissolution of CaSO 4 ⋅2 H 2 O and, upon binding to Ca 2+ , is adsorbed onto the C 3 A surface, hindering C 3 A hydration and AFt formation. Consequently, elevated superplasticizers dosages not only delay C 3 S hydration but also impede AFt formation by restraining the dissolution of CaSO 4 ⋅2 H 2 O. Therefore, because excessive SPS addition significantly retards hydration, setting time can be noticeably extended [21,22]. Longer setting times can assist reduce the risk of premature setting in large-scale construction projects or in hot areas, enabling improved workability and more controlled material placement [26]. Longer setting times can also lessen the chance of cracks from quick shrinkage, which will increase the concrete’s lifetime and durability [27, 28]. Saladi et al. found in their work that by adding a blend of super plasticizers to a high-performance cement, the final set occurred between 11 and 24 h [29]. Different cement varieties showcased distinct optimal dosages, highlighting concerns regarding superplasticizers -cement compatibility. However, the mechanism underlying superplasticizers’ enhancement of cement compressive strength remains elusive, since the amount of superplasticizer, as well as its aspect ratio, has varied in previous investigations. Ma et al. [24] propose that high performance plasticizer alters the calcium silicate hydrate surface energy (C-S-H) through adsorption, thereby augmenting C-S-H cohesion and bolstering compressive strength. Ren et al. [25] reported that small amounts of SPS contribute to increase the concrete setting and strength development [30] However, excessive superplasticizer content may increase porosity, affect long-term durability, and raise production costs [31]. Although no adverse effects were observed within the dosage range analyzed in this study, SPS content optimization should take these factors into account. Furthermore, the synergistic effect of multiple admixtures is not yet fully understood, as their combination can alter viscosity, thixotropy, and the structural performance of the material [32]. This paper delves into the investigation of the effect of SPS on UHPFRC. The exploration focuses on three different dosages of SPS, as established in previous research, aiming to comprehend their influence on the physical and mechanical properties of this advanced material. Various evaluation methods have been employed to analyze and characterize the properties of SPS-modified UHPFRC, with the objective of delving into the alterations induced by this admixture on the structure and performance of concrete. This multifaceted approach aims to provide a comprehensive and detailed understanding of the effects that SPS can exert on UHPFRC, thus contributing to the knowledge in the field of high-strength construction materials. 2. Materials and methods 2.1. Materials and mix preparation Three different materials were utilized as binders in this study. Type I cement of 52.5 R/SR, according to EN 197–1 [33], was produced by Portland Valderrivas. The SF S-92-D silica fume was provided by the SIKA company, while the ground-granulated blast-furnace slag (GGBS) was obtained from the Arcelor-Mittal company. The SF S-92-D silica fume was provided by the SIKA company, while the ground-granulated blast-furnace slag (GGBS) was procured from Arcelor-Mittal. Regarding the aggregates used, two types of quartz sand were utilized: the finest sand (FS) with a maximum particle size of 0.315 mm, and the coarse sand (CS) with a particle size below 0.800 mm [34]. Steel fibers measuring 13 mm in length and 0.2 mm in diameter, sourced from Beckaert, were also employed. The superplasticizer (20HE) was supplied by SIKA, while SPS with a purity of 99.8 % was obtained from Scharlab. The chemical compositions of the binder materials were analyzed using X-ray fluorescence spectrometry and are detailed in Table 1. Four different formulations of UHPFRC were produced, each differing in the quantity of SPS incorporated. The matrix dosage remained consistent across all mixtures (refer to Table 2), whereas varying proportions of SPS additions were added (0 %, 0.05 %, 0.10 %, and 0.15 % of the cement content). These four formulations are denoted as SPS-0 (representing the concrete reference), SPS-0.05, SPS-0.10, and SPS-0.15, respectively. First, the solid materials (cement, silica fume, blast furnace slag, fine and coarse aggregate, and SPS) were added to the vertical mixer and mixed for four minutes. Next, the superplasticizer was added to the water and mixed for one minute to achieve a homogeneous liquid. Finally, all constituents (solids and liquids) were mixed for 20 minutes to achieve the desired consistency of the mixture. Afterward, the steel fibers were added and mixed for an additional five minutes. In this study, the extended mixing time was not a result of the addition of SPS. Instead, it was necessary to ensure the mixture attained the optimal consistency and workability required for UHPC. This process was carefully J.D. Ruiz Martínez et al. Construction and Building Materials 470 (2025) 140683 2 controlled to maintain the integrity and performance of the material. Each mixture was then utilized to cast three prisms measuring 40 mm ×40 mm ×160 mm. After 48 hours, the specimens were removed from their molds and submerged in water at room temperature for 25 days to cure. 2.2. Experimental methods 2.2.1. Thermo-gravimetric analysis A thermo-gravimetric study (TG-SDTA Mettler-Toledo 851) was conducted from room temperature to 600◦C. To this end, samples of 100–150 mg for the TG-SDTA measurements were taken from the surface of the test panels. A heating rate of 10 ◦C/min was chosen, using air as the purging gas. 2.2.2. Abrams miniature slump test A modified miniature slump test was devised to assess the workability of fresh concrete, following the guidelines outlined in EN 12350–2:2020 [35]. The setup for the mini-slump test, depicted in Fig. 1, employed a scaled-down geometric Abrams cone with specific dimensions: 50 mm diameter at the top, 100 mm diameter at the base, and 150 mm height. The test was conducted on a flat surface composed of the same material. After testing in accordance with the standard procedure, measurements were taken using a standard gauge with one-millimeter precision, and the results were averaged across four readings. 2.2.3. Initial and final setting time The penetrating resistance approach described in the EN 196–3:2017 was used to determine the UHPFRC’s initial and final setting timeframes [36]. This method describes the procedure for gauging the initial and final setting times of concrete. The initial setting time is pinpointed as the moment a needle penetrates the cement paste 6 ±3 mm. The final setting time is identified when the needle can no longer penetrate the cement paste by more than 0.5 mm. 2.2.4. Heat of hydration The heat of hydration of UHPC was measured a differential scanning calorimeter (TA DSC 2920 Instrument), To prepare the samples and analyze the influence of the binder, steel fibers were removed, and the other components base percentage and the mixing duration were manteined, because only 2 g were put in to the calorimeter. The temperature in the calorimeter was always maintained at 20 ◦C during the test and the heat flow was recorded for 40 h. 2.2.5. Stereomicroscope The stereomicroscopic allows imaging a large field of view, provides precise microscopic details in 3D, and it allows quantitative measurements [37–40]. The observations obtained from this approach align with previous studies [41–44], further validating the applicability of this method to examine the fiber-matrix bonding in UHPRC. A Nikon model SMZ25 microscope (New York, U.S.A.), commonly referred to as a stereomicroscope, was used for this study. The instrument is connected to a computer equipped with NIS Elements BR software, through which the images were captured. For the examination, small specimens measuring 20 mm×40 mm x 40 mm were fabricated for the purpose of using this microscope. In addition to the magnification of the images, the utilized equipment generated a 3D model of the image and a Z-axis graph. This graph delineates, with micrometric precision, the profile of the mixture surface, enhancing the depth of analysis. 2.2.6. Porosity A porosimetry study was carried out using a Micromeritics Autopore IV mercury intrusion porosimeter (Norcross, U.S.A.). The analysis covered a pore size range spanning from 0.007 to 150 μ m. To prepare the samples, they were shaped into 5 mm pellets, and subsequently dried in an oven at 105◦C. Key parameters included a surface tension of 480 mN/m, a contact angle of 140◦, and a maximum applied pressure of 413 MPa. These factors were critical in ensuring accurate and reliable Table 1 XRF chemical composition of binder materials and mix composition of reference concrete (kg/m 3 ). Cement GGBS SF CaO 45.61 35.12 0.30 SiO 2 18.29 27.81 79.58 Al 2 O 3 6.59 9.83 0.20 SO 3 4.02 - - Fe 2 O 3 2.85 0.31 0.06 K 2 O1.09 2.01 0.42 MgO 1.00 6.63 0.35 TiO 2 0.41 0.48 - Na 2 O0.29 0.21 0.12 P 2 O 5 0.13 - - Cl 2 O 3 0.07 - - BaO 0.06 - - SrO 0.05 0.08 - MnO 2 0.05 0.11 - CuO 0.04 - - ZnO 0.02 - - Table 2 Mix proportions and designation. Constituent (kg/m 3 ) Mix Cement SF GGBS FS CS Water SP Steel fibers SPS SPS−0 540 210 310 470 470 205 42 196 0 SPS−0.05 540 210 310 470 470 205 42 196 2.7 SPS−0.10 540 210 310 470 470 205 42 196 5.4 SPS−0.15 540 210 310 470 470 205 42 196 8.1 Fig. 1. Abrams miniature slump test for fresh concrete. J.D. Ruiz Martínez et al. Construction and Building Materials 470 (2025) 140683 3 porosity measurements 2.2.7. Mechanical properties 2.2.7.1. Compressive strength. The evaluation of compressive strength was conducted on three cubic samples of 40 mm of side, for each type of concrete, following the guidelines of the EN 12390–3 standard [45]. These assessments took place using a servo-hydraulic testing machine with a load capacity of 3000 kN. Each mix was represented by six samples measuring 80 mm ×40 mm ×40 mm. 2.2.7.2. Flexural tensile strength. Three-point bending tests were performed for each type of manufactured concrete, utilizing prismatic specimens with dimensions of 40 mm×40 mm×160 mm. The residual strength is calculated using Eq. (1), and the tests and outcomes were carried out in accordance with the EN 14651 standard [46]. fRj =3PjL 2bh2 sp (1) where fRj and Pj, which correlate to CMODj, respectively, stand for the applied load and the residual flexural tensile strength. In particular, the matching CMOD values of 0.5 mm, 1.5 mm, 2.5 mm, and 3.5 mm were used to determine fR1, fR2, fR3, and fR4. The variables b, hsp, and L represent the beam’s width, notch height, and span length, respectively, in Eq. (1). The tests were conducted using hydraulic equipment with a maximum load capacity of 50 kN, employing (crack mouth opening displacement) CMOD displacement control. Both the support and loading rollers were equipped with anti-torsion mechanisms. In addition to measuring the 5 mm CMOD, deflection at the center of the specimen was recorded using a 10 mm vertical transducer (Fig. 2). Following the failure of all specimens, a visual inspection of the fracture zone confirmed that the fractures resulted from bending. The determination of the Young’s modulus was carried out based on the results of the load-CMOD curves, following the guidelines provided by Jenq and Shah [47]. To do this, the initial slope of the load-CMOD curve was used, and, knowing the dimensions of the specimens used in the three-point bending tests, the Young’s modulus was calculated for each test. 3. Experimental results 3.1. Thermo-gravimetric analysis Several peaks were observed in the TG curves depicted in Fig. 3, ranging from 50◦C to 600◦C. The initial endothermic peak is at 50◦C attributed to humidity evaporation, followed by dehydration of C-S-H and ettringite at 90◦C. The third endothermic peak at 148◦C was caused by the dehydration of hydrated calcium aluminates, followed by the fourth peak at 435◦C due to the decomposition of calcium hydroxide. Although the adsorption of SPS or the complex between SPS and calcium ions (Ca 2+ ) inhibits the formation of ettringite during the setting time, there is an small increase in ettringite at 28 days. This is evident in the leftward shift of the second peak at 0. Although no major changes were observed in the cementitious matrix, it is observed that the addition of 15 % SPS generated more ettringite with finer grains and staggered symbiotics in interfaces and pores, due to the dehydration of the ettringite. 3.2. Abrams miniature slump test The results of the Abrams slump tests for each mix are depicted in Figs. 4 and 5. Notably, these figures reveal a consistent trend: as SPS content increases, the spread values also increase. For instance, at an SPS/cement ratio of 0.15, there is a substantial 55 % increase compared to the UHPFRC without high performance plasticizer. Furthermore, it is worth highlighting that all mixes including SPS have been increased in workability, while maintaining consistent viscosity and uniform texture throughout the process, devoid of the need for vibration. On the contrary, as far as the concrete slump is concerned, a consistent positive effect trend is followed, mirroring the slump flow relationship, which invariably determines the fluidizing effect of SPS on concrete. Additionally, it is also noticeable that none of the pastes exhibited significant deformation or segregation during the mini-slump test, as illustrated in Fig. 5. Previous study [24] demonstrated that the fluidity of SPS-doped cement mortar was greater than that of mortar without SPS, with the maximum effective dose being 0.03 %, beyond this dosage, the workability of the cement mortar did not improve further. In UHPFRC, a similar effect can be observed; however, the addition of two other binders alongside cement allows higher dosages to continue enhancing workability. 3.3. Initial and final setting time In Fig. 6, the progression of penetration resistance within the UHPFRC matrix is depicted using the Vicat needle test apparatus for all SPS mixes. Notably, it is evident that the higher the SPS dosage, the longer the delay in the build-up of matrix penetration resistance, thereby extending both initial and setting times. For the determination of setting times, Eq. (2) was applied through regression analysis as described by: log(PR) = a+b⋅log(t)(2) where log (PR) represents the penetration distance in millimeters, t signifies the elapsed time in minutes, and a and b denote the regression coefficients. The resulting regression coefficients, coefficient of determination (R 2 ), and the initial and final setting times are presented in Table 3. As shown in Fig. 6, there are differences of 10.5 hours between the initial setting time of SPS-0 and SPS-0.05. However, these disparities escalate to 16.7 and 25.7 hours for SPS-0.10 and SPS 0.015 respectively, compared to SPS-0. The differences become more pronounced at the final setting time, since the dosages with higher SPS in their composition took 18.8 (SPS-0.10) and 27.1 (SPS-015) hours longer than SPS-0 to set, Fig. 2. Setting up and instrumenting three-point bending tests. J.D. Ruiz Martínez et al. Construction and Building Materials 470 (2025) 140683 4 thus accumulating an increase of 204.3 % and 295 % respectively in the final setting time. Additionally, the inclusion of SPS resulted in a proportionate increase in the disparity between the initial and final setting timeframes. The time window expanded by 7.6 % for the dosage with the lowest SPS addition, while reaching a 61 % increase for the composition with the highest SPS. These results indicate that SPS primarily functions have been to delay both the initial and final setting times, as well as to widen the time gap between them. Consequently, these changes will reflect in the physical and mechanical properties of the compositions. While this characteristic may limit the use of SPS in certain timesensitive applications, it can provide significant benefits in others, and we believe these potential applications warrant further exploration. The prolonged setting times in concrete mixtures can offer several advantages in specific applications. For example, in large-scale construction projects or in hot climates, longer setting times can help mitigate the risk of premature setting, allowing for better workability and more controlled placement of the material. Furthermore, extended setting times can reduce the risk of cracks due to rapid shrinkage [48–50]. ultimately contributing to the durability and longevity of the concrete. 3.4. Effect of SPS on UHPC heat of hydration Fig. 7 illustrates how the hydration heat flow varies with varying SPS doses. Exothermic wetting and an early-stage reaction are responsible for the first peak in Fig. 7, resulting in a gelatinous coating containing ettringite rods [24]. Subsequently, there is a time interval known as the induction period, where the heat flow remains almost constant (btween the end of the first peak and the beginning of the second). The second Fig. 3. Thermo-gravimetric analysis of SPS-0 and SPS-0.15 at 28 days. Fig. 4. Mini slump flow of UHPFRC with different SPS contents. J.D. Ruiz Martínez et al. Construction and Building Materials 470 (2025) 140683 5 peak corresponds to the period where the primary products, C–S–H and CH, are produced, and is known as the acceleration period [24]. The hydration reaction process is closely related to the cement setting process. The diference between the initial and final setting times correspond to the start and finish of the acceleration period (the time between the start of the second peak and the maximum point of that second peak) in the hydration process. [51]. The addition of SPS considerably changed the hydration kinetics. The induction period increased dramatically with the increase in SPS dosage, suggesting that SPS delayed the hydration of CH and C-S-H, producing that the acceleration period began with a delay, which is consistent with the difference in the initial setting time. The study’s experimental findings mostly concur with those of SPS in conventional cement [24]. Fig. 5. Mini slump flow results for each mix: a) SPS-0, b) SPS-0.05, c) SPS-0.10, and d) SPS-0.15. Fig. 6. Initial and final setting time. J.D. Ruiz Martínez et al. Construction and Building Materials 470 (2025) 140683 6 3.5. Stereomicroscope Fig. 8 displays images depicting the bond between one steel fiber and the cementitious matrix, initially posing challenges in discerning any noticeable impact of SPS. Nonetheless, Fig. 9 portrays the profile formed in the area just preceding the appearance of the steel fiber, showcased at the center of Fig. 9, revealing compelling evidence that suggests a correlation between the dosage of SPS and the enlarged area of concrete surrounding the fiber. The results depicted in Fig. 9 prompt the hypothesis that the fluidization of the cementitious mix could potentially enhance the adhesion of the steel fibers. Furthermore, it is inferred that in UHPFRC containing approximately 2 % steel fibers, the fluidization of the cementitious matrix facilitates a more uniform formation, thereby diminishing notable pores and potentially augmenting the contact of the matrix and fibers and the friction during a pull-out test [52]. 3.6. Porosity Porosity plays a fundamental role in evaluating the compressive strength of concrete. The presence of pores in the concrete matrix reduces the solid surface area available for stress transmission, potentially compromising the material’s load-bearing capacity. It is crucial to consider not only total porosity but also the size, distribution, and interconnectivity of pores, as these factors significantly influence the mechanical behavior of concrete [9]. To provide a comprehensive analysis, pores were classified into four categories in Fig. 10: gel pores (<10 nm), large mesopores (10–50 nm), macropores (0.05–10 μ m), and entrained air (>10 μ m) [53]. In particular, pores with a size in the range between 0.01 and 10 μ m have a detrimental effect by weakening the concrete matrix, creating stress concentration points, and facilitating crack propagation. In contrast, gel pores, with a size smaller than 0.01 μ m, do not have a significant impact on compressive strength. In fact, a small quantity of gel pores can be beneficial by improving the workability of concrete and reducing shrinkage. Regarding the results of each type (Table 4), it was observed that as the percentage of SPS increases, the total porosity decreases except for the 0.15 % dosage, which is slighly increase respect to 0.1 %. In the different ranges, the trend is similar to that of total porosity. These findings align with observations made with a stereomicroscope (see subsection 3.5), where it was assumed that with higher SPS dosage, the number of large pores would be reduced. This phenomenon suggests an expected increase in compressive strength. However, it is important to consider other factors that may influence porosity concentration, such as an excess of superplasticizers, which could lead to an increase in pore size and total porosity in pastes, mortars, and concretes [24]. 3.7. Mechanical properties 3.7.1. Compressive strength In this section, the average values of compressive strength obtained from four repetitions are presented. As depicted in Fig. 11, the average compressive strength of the concrete increased with the rise in SPS concentration. For the SPS-0.05 concentration, a 0.4 % increase of compressive strength was observed compared to plain concrete, which proved to be negligible. However, in the case of the SPS-0.10 concentration, there was a 4.5 % increase, while for SPS-0.15, it was 9.5 % higher compared to plain concrete. These findings are directly linked to porosity. In the case of SPS-0.05, the increase in compressive strength was imperceptible because, in the range of pores larger than 1 µm, it was very similar to that of SPS-0, as can be seen in Fig. 10. It is in the case of SPS-0.10 and SPS-0.15 where a more significant reduction in porosity is observed in the range of pores larger than 1 µm, especially in the band of pores larger than 90 µm. This highlights the consistency and correlation between compressive strength results and the internal porosity of the matrix. 3.7.2. Flexural tensile strength Fig. 12 presents the Load-CMOD displacement curves for all tested specimens (SPS-0, SPS-0.05, SPS-0.10, and SPS-0.15), obtained from the three-point bending tests. These curves provide a detailed representation of the flexural behavior, particularly the peak load capacity, postTable 3 Coefficients of Eq. (2) and setting times at 25ºC. Mix A b R 2 Initial Set [h] Final set [h] SPS−0.0 −54.11 124.92 0.9531 9.30 13.20 SPS−0.05 −138.70 418.01 0.9836 15.80 20.10 SPS−0.10 −165.20 554.18 0.9643 23.15 28.15 SPS−0.15 −205.10 739.66 0.9877 31.00 36.65 Fig. 7. UHPC heat flow of hydration with different dosages of SPS. J.D. Ruiz Martínez et al. Construction and Building Materials 470 (2025) 140683 7 Fig. 8. Images with their respective 3D modeling for each mix: a-b) SPS-0, c-d) SPS-0.05, e-f) SPS-0.10, and g-h) SPS-0.15. J.D. Ruiz Martínez et al. Construction and Building Materials 470 (2025) 140683 8 peak softening response, and energy absorption. As observed, the incorporation of SPS leads to an increase in peak load and enhanced post-peak resistance, with higher SPS dosages exhibiting a more gradual decline in load, indicative of improved fracture toughness [54]. The CMOD curves allow for a more precise evaluation of the crack propagation and ductility of the material. Fig. 13 displays the values of the proportional limit (LOP) relative to the EN 14651:2007 +A1:2008 standard [46], serving as a representation of the flexural strength or initiation of flexural cracking, as well as the residual flexural tensile strength of all mixes admixed with SPS and their respective standard deviation. In all SPS concentrations, an increase in the initiation of flexural cracking is observed (black squares in Fig. 13), by 10 % for SPS-0.05, 31 % for SPS-0.10, and 38 % for SPS-0.15. This improvement in the initiation of flexural cracking is attributed to the reduction of macropores deduced from the porosimetry results (Fig. 10) and because there is a higher local friction between matrix and fibers when SPS is added Fig. 9. Profile related to EDF images. Fig. 10. Pore size distribution for each concrete with different SPS concentration. Table 4 Total porosity (mL/g) and pore range distribution (mL/g). Range SPS-0 SPS-0.05 SPS-0.1 SPS-0.15 >10 μ m 0.158 0.205 0.126 0.125 10–0.05 μ m 0.345 0.337 0.334 0.306 0.01–0.05 μ m 0.100 0.091 0.105 0.159 <0.01 μ m 0.045 0.006 0.014 0.022 Total 0.635 0.555 0.566 0.593 J.D. Ruiz Martínez et al. Construction and Building Materials 470 (2025) 140683 9