The Influence of the Addition of Multi-Scale Zirconia on the Properties of Ultra-High-Performance Concretes

Abstract

This study explores an innovative application of ultra-high-performance concrete (UHPC) by partially substituting cement with nano-zirconia (NZ) and micro-zirconia (MZ). A series of experiments were conducted to explore the influence of zirconia particle size (3860 nm and 320 nm) and varying replacement levels (0%, 0.5%, 1%, and 1.5%) on the workability, mechanical behavior, and microstructural characteristics of UHPC, utilizing the particle packing density model as a basis. Findings reveal that replacing 0.5% of the cement with MZ and NZ results in workability and mechanical performance comparable to the control mix. However, at 1.5 wt% MZ and 1.5 wt% NZ substitution levels, flowability declines by 22.01% and 24.71%, respectively, accompanied by a substantial increase in viscosity. The wet packing density of UHPC exhibits a linear rise with increasing zirconia content, with nano-sized particles exerting a more pronounced effect than their micro-sized counterparts. Specifically, at a 0.5% MZ replacement level, the 28-day compressive and flexural strengths show marginal improvements of 1.82% and 4.48%, respectively. The NZ1MZ0.5 mix achieves the highest 28-day compressive strength increase, reaching 9.45%, with an absolute gain of 11.92 MPa. Analyses using XRD, FTIR, and thermogravimetric analysis (TGA) demonstrate that zirconia incorporation has a negligible influence on the hydration process and does not alter the composition of hydration products in N-UHPC. Although zirconia effectively reduces porosity, excessive amounts (1.5%) increase pore size within the cement matrix, ultimately compromising mechanical properties. Based on these findings, the optimal NZ dosage for UHPC, when used in combination with MZ, is determined to be 0.5%.

1. Introduction

The rapid advancement of the construction sector has intensified interest in ultra-high-performance concrete (UHPC) as a prominent cement-based material. Recognized for its exceptional durability and outstanding mechanical features, the superior performance of UHPC primarily originates from its ultra-dense microstructure [1,2]. Research indicates that concrete achieving a compressive strength (CS) of over 120 MPa within 28 days qualifies as UHPC, an advanced cementitious composite distinguished by its remarkable strength, toughness, and durability. This material is primarily composed of quartz sand, cement, supplementary mineral additives, high-performance water reducers, and a low water-to-binder ratio [3]. UHPC is gradually beginning to be used in structural engineering, including bridge construction and embankment repair [4]. Recently, extensive research has focused on its composition, microstructural characteristics, and manufacturing techniques [5]. Advances in material optimization, fiber reinforcement integration, and refined production methodologies have significantly improved UHPC’s strength, durability, and crack resistance. Despite these enhancements, UHPC remains a quasi-brittle material with intrinsic and surface-level imperfections that can lead to severe deterioration or fatigue accumulation under demanding service conditions [6,7].

In recent years, extensive research has been conducted on incorporating nanomaterials into UHPC to enhance its performance, demonstrating that even small quantities of these materials significantly improve UHPC properties. Due to their ultrafine size and high surface area, nanomaterials can effectively fill microcracks and voids within the cement matrix, leading to a denser structure. Additionally, their unique surface properties and nanoscale interactions promote cement hydration, thereby enhancing the strength and toughness of concrete. With advancements in nanotechnology, numerous studies have explored the integration of different nanomaterials into cementitious systems, including nano-silica (NS) [1], nano-calcium carbonate (NC) [8], nano-titanium dioxide (NT) [9], nano-zirconia (NZ) [10], carbon nanotubes [11], and graphene oxide [12]. Wu et al. [13] investigated the effects of NS and NC on UHPC reinforced with 2% steel fiber, revealing that fiber–matrix bonding and overall mechanical properties improved with these nanomaterials, up to optimal replacement levels of 1% for NS and 3.2% for NC. Similarly, Li et al. [14] incorporated NT into reactive powder concrete (RPC) and observed that its nucleation effect disrupted the crystalline orientation of calcium hydroxide (CH) and restricted CH particle size. Furthermore, the addition of NT enhanced RPC’s electrical conductivity, reducing resistivity by 13.61% [15]. Meanwhile, Chen et al. [16] employed plasma-synthesized carbon nanotubes in UHPC at low dosages, successfully enhancing its mechanical performance. Currently, the integration of nanomaterials into cement-based composites to optimize structural properties and improve binder efficiency remains a prevalent research focus among scholars worldwide.

Zirconia (ZrO₂) is widely recognized for its exceptional physical and chemical properties, as well as its biocompatibility, making it a valuable material in ceramic toughening, high-performance refractory applications, and biomedical fields. Currently, NZ is extensively used in concrete technology due to its remarkable strength, toughness, and wear resistance. While extensive research has explored the integration of various reactive nanomaterials, such as NS, NC, and nano-Al₂O₃ [16], into UHPC, studies on multi-scale zirconia applications remain limited. Inspired by the reinforcement mechanisms of NZ in ceramics, some researchers have incorporated it into cementitious composites to examine its strengthening effects. Soleymani [17] investigated the influence of zirconia at replacement levels of 0.5%, 1.0%, 1.5%, and 2.0% in ordinary Portland cement (PC), reporting flexural strength enhancements of 13.6%, 25.0%, 20.5%, and 6.8%, respectively, after 28 days of curing. However, beyond a certain threshold, zirconia increased porosity, weakening its ability to refine the pore structure of cement paste. Similarly, Shekari et al. [18] observed a 20.2% rise in 28-day compressive strength when 1.5% NZ was added to cement paste. Nazari and Riahi [19,20] examined the impact of lime water on the setting time and compressive strength of NZ-reinforced cement-based composites. Han et al. [21] conducted experiments and demonstrated that UHPC incorporating NZ exhibited 36.6%, 16.3%, and 34.0% improvements in flexural, compressive, and splitting tensile strengths, respectively, compared to conventional UHPC. Most existing research has concentrated on the effects of NZ in ordinary PC, with significantly fewer studies assessing its role in UHPC performance enhancement at multiple scales. Prior investigations have primarily examined the impact of nano-scale zirconia on the reg mechanical properties of UHPC, while there is no comprehensive investigation focusing on the combined influence of different zirconia particle sizes. This knowledge gap underscores the necessity for further exploration into the influence of multi-scale NZ on the workability and mechanical behavior of UHPC.

In conclusion, zirconia is a high-strength, tough, and wear-resistant material that has demonstrated its suitability in conventional cement-based materials. However, its potential as an effective admixture to decrease the production costs and environmental impact of UHPC remains an area for further investigation. To explore the broader use of zirconia in UHPC and significantly improve its overall performance, this study examines the feasibility of incorporating NZ and micro-zirconia (MZ) as partial replacements for traditional PC in UHPC production. By adopting an enhanced Modified Anisotropic Axiomatic (MAA) model for Z-UHPC mix design, various particle sizes of zirconia were used to replace PC at different replacement rates (0%, 0.5%, 1%, 1.5%). A comprehensive evaluation approach, covering fresh properties, hardened properties, hydration behavior, and microstructural analysis, was applied to analyze the impacts of zirconia incorporation on the performance of UHPC. The present research intends to enhance the performance and environmental impact of UHPC, offering valuable insights into zirconia’s role in UHPC applications.

2. Materials and Methods

2.1. Raw Materials

The study utilized P·O 52.5 cement (PC), which had a specific surface area of 506 m²/kg. Silica fume (SF) was characterized by a specific surface area of 18,230 m²/kg and a silica content of over 90%. Fine aggregate consisted of 40–70 mesh quartz powder (QP), which had a specific gravity of 2.36 and a specific surface area of 4523 cm²/mg. The mineral powder (MP) used was of Grade S95, with a density of 2832 kg/m³ and a specific surface area of 397 m²/kg. The polycarboxylate-based superplasticizer (SP) employed had a water reduction rate of at least 30%. Copper-coated steel fibers, with a diameter of 0.2 mm and a length of 13 mm, were used. Tap water served as the mixing medium, and polycarboxylate superplasticizer (PS) was included to modify the workability of UHPC. The chemical compositions and macroscopic features of the raw materials are presented in

Table 1. Chemical composition of PC, SF, MP, and QP.

Properties	PC	SF	MP	QP
Physical
Specific gravity	2.98	2.06	2.61	2.39
Specific area (cm2/gm)	506	18,230	2832	-
Color	Grey	Light Grey	White	White
Chemical composition (%)
SiO2	32.79	98.12	35.17	96.58
Al2O3	6.22	0.97	15.92	-
Fe2O3	2.99	0.36	0.71	-
CaO	52.29	0.14	32.58	0.35
MgO	1.28	0.10	11.60	0.14
SO3	2.06	0.07	1.00	-
K2O	0.81	0.06	0.26	0.62
Na2O	0.85	0.04	0.24	0.16

and Figure 1, respectively.

The nano-enhancers used in this study were zirconia (ZrO₂) and NZ, both sourced from Suzhou Yuante New Materials Technology Co., Suzhou, China. They were supplied in white powder form, with average particle sizes of 3860 nm for zirconia and 320 nm for NZ. The microscopic structures of these materials are illustrated in Figure 1. Zirconia displays high crystallinity, a complete crystal structure, and a dense lattice. In contrast, NZ has smaller crystals and more lattice distortions, though it maintains good structural dispersion without signs of agglomeration.

2.2. Mixture Design and Sample Preparation

The Z-UHPC mix design was based on the Chinese standard GB/T 31387-2015 using an internal mixing method [22]. In this design, cement, silica fume, and slag were partially substituted with nanomaterials in amounts ranging from 0.5% to 1.5% of the total binder mass (which includes the cement, silica fume, and slag). The study also examined the effects of using only zirconia or NZ as supplementary cementitious materials (SCMs) on UHPC performance for comparison. The dosage of polycarboxylate superplasticizer was set at 2.2% of the binder mass, while copper-coated micro-steel fibers were incorporated at 2% of the total volume. The water-to-binder ratio was maintained at 0.18. The compositions of various mixtures are listed in

Table 2. Compositions of various mixtures.

Sample	Mix Proportion/(kg·m−3)
	ZrO2	Nano-ZrO2	PC	SF	MP	QZ	Water	Steel Fiber
Control	0	0	880	240	160	798	216	156
M0.5	6.4	0	873.6	240	160	798	216	156
M1	12.8	0	868.2	240	160	798	216	156
M1.5	19.2	0	860.8	240	160	798	216	156
N0.5	0	6.4	873.6	240	160	798	216	156
N1	0	12.8	868.2	240	160	798	216	156
N1.5	0	19.2	860.8	240	160	798	216	156
N0.5M0.5	6.4	6.4	868.2	240	160	798	216	156
N0.5M1	12.8	6.4	860.8	240	160	798	216	156
N1M0.5	6.4	12.8	860.8	240	160	798	216	156

.

The process began by thoroughly mixing the measured water with the superplasticizer. Next, the appropriate amount of ZrO₂ was added to the solution and stirred for two minutes, followed by ultrasonic dispersion for 10 min to achieve a stable and evenly distributed ZrO₂ suspension. The pre-weighed powders (cement, silica fume, slag) and quartz sand were blended for 2 min, after which the prepared ZrO₂ suspension was gradually incorporated into the mixture and mixed for an additional 7 min. Steel fibers were then introduced gradually at the specified ratios and stirred for another 3 min. The fresh Z-UHPC mixture was poured into molds and vibrated for 30 s on a vibrating table to eliminate air bubbles. The molds were sealed with plastic film and maintained at room temperature for 24 h before demolding. The specimens were then cured at 20 ± 2 °C with 98% ± 2% relative humidity.

2.3. Methods

2.3.1. Mechanical Properties

The mechanical properties were evaluated following the GB/T 17671-2021 on specimens that were cured for 1, 3, 7, and 28 days [23]. Flexural strength was assessed using specimens measuring 40 mm × 40 mm × 160 mm, tested under a three-point bending configuration with a 100 mm span and a loading rate of 50 N/s. Compressive strength was tested on cubic specimens of 40 mm × 40 mm× 40 mm at a loading rate of 2.4 kN/s. For each test group, three specimens were analyzed, and the average value was used as the final result.

2.3.2. Fresh Behavior

Fresh behavior was assessed through three experiments: workability, wet packing density, and mini-V-funnel time. The flowability of Z-UHPC samples was tested following the EN1015-3 flow table test. Fresh UHPC was poured into a steel mold with a top diameter of 70 mm, a bottom diameter of 100 mm, and a height of 60 mm. After the vertical lifting of the mold, the UHPC was allowed to flow until it stabilized. Measurements were conducted twice, and the average value was recorded. The mini-V-funnel test, following Meng’s research methodology [24], was carried out to explore the influence of NZ addition on UHPC viscosity. Fresh paste was quickly poured into the V-funnel, and the surface was leveled with a scraper along the upper edge of the funnel. After allowing the paste to rest for 1 min, the discharge gate at the bottom was opened, and a stopwatch was used to measure the time from opening until all paste had flowed out. The time was recorded to the nearest 0.1 s.

The wet packing density was evaluated using the method proposed by Kwan [25] to assess the actual filling state of UHPC with various zirconia contents. Fresh mixed UHPC paste was poured into three 250 mL containers, degassed using a vibrator, and scraped off until the mixture was fully compacted. The wet bulk density was then obtained through the following expression, with the average of three parallel samples recorded as the final value.

$$\varphi ={\displaystyle \frac{\mathrm{M}/\mathrm{V}}{{\rho }_{\mathrm{W}}{\mathrm{R}}_{\mathrm{W}}+{\rho }_{\mathrm{s}}{\mathrm{R}}_{\mathrm{S}}+{\displaystyle \sum {\rho }_{\mathrm{X}}{\mathrm{R}}_{\mathrm{X}}}}}$$

(1)

where M is the mixture mass and V represents the container volume. The variable x corresponds to different cementing materials. ρw, ρs, and ρx denote the apparent densities of water, quartz sand, and cementing materials, respectively. Similarly, R_w, R_s, and R_x represent the volume ratios of water, quartz sand, and cementing materials to the total solid materials, respectively.

2.3.3. Microscopic Characteristics Determination

The Z-UHPC cement mortar samples, after curing for 28 days, were partially dried and ground into 200-mesh powder for XRD analysis. The diffraction patterns were used to study the crystalline phases in the material. The microstructure morphology of the 28-day cured Z-UHPC materials was examined using an SEM (VEGA3 TESCAN, Country). Before SEM observation, the fracture surface samples were soaked in anhydrous ethanol and then dried in a vacuum drying oven. The pore structure at 28 days was analyzed using the Brunauer–Emmett–Teller (BET) method, with pretreatment at 100 °C and testing conducted at 77 K. It is worth noting that BET analysis provides insights into the refinement of the pore structure due to nano-zirconia doping. Figure 2 illustrates the research flowchart.

3. Results and Discussions

3.1. Fresh Behavior

3.1.1. Workability

Figure 3 shows the mechanical properties of Z-UHPC. The addition of zirconia to UHPC resulted in reduced flowability and increased mini-funnel passing time, leading to decreased workability. Specifically, the flowability of the control sample was 259 mm, while adding 0.5 wt% Zr and 0.5 wt% NZ reduced the flowability to 242 mm and 232 mm, respectively, a decrease of just 6.3% to 2.1%. This suggests that low doses of zirconia had a negligible impact on flowability, though nano-zirconia had a more noticeable effect. However, when the dosage was increased to 1.5 wt% Zr and 1.5 wt% NZ, the flowability dramatically dropped to 202 mm and 195 mm, reflecting reductions of 22.01% and 24.71%, respectively. This significant decrease may originate from the increased specific surface area introduced by the nanomaterials [26]. Due to the higher internal surface area of NZ and its strong Van der Waals forces, it adsorbs more free water, which, in turn, reduces the flowability of fresh UHPC [27]. Moreover, the high reactivity of nano-zirconia creates nucleation sites for cement hydration products, accelerating hydration and further consuming free water, thus significantly lowering the flowability of the Z-UHPC mixture [28]. Also, the zirconia particles are small and hard, which may affect the dispersion of the concrete, and the smaller particles increase inter-particle friction during mixing, which further limits flow. However, optimizing the zirconia particle size usually provides the best overall performance and improves durability and corrosion resistance. Yu et al. [5] showed that when the proportion of zirconia substituting for cement exceeds a certain limit, the flowability decreases significantly. Therefore, by optimizing the zirconia particle size and substitution ratio, flowability and performance enhancement can be effectively balanced.

Compared to the control sample’s funnel time of 58 s, the funnel times for zirconia contents of 0.5%, 1%, and 1.5% were 65 s, 77 s, and 92 s, respectively, showing an increase of 11% to 35%. For NZ contents of 0.5%, 1%, and 1.5%, the funnel times were 65 s, 95 s, and 105 s, respectively, which increased by 28% to 53%. This suggests that adding zirconia to UHPC increases its viscosity. As the NZ content increases, the zirconia particles are absorbed onto the cement particle surfaces through intermolecular forces, reducing spatial hindrance and electrostatic repulsion. This process enhances the flocculation structure in the mixture, thereby increasing the viscosity of the UHPC. Yu et al. [5] explained that higher nanoparticle content significantly increases the viscosity of fresh mixtures, as the entrapped voids cannot escape, increasing the void content within the cementitious matrix and the ions of zirconia reacted chemically with the calcium and aluminum ions in the Z-UHPC mixture as well as the silicate ions in the silicate to form a stable chemical bonding, which reduced the friction between the particles, and this reaction not only promoted the homogeneous dispersion of particles but also improved the compactness of the Z-UHPC mixture. In addition, when the content of zirconia nanoparticles was increased, the bonding point of the material could be significantly increased due to its extremely small particle size and large specific surface area, which further enhanced the interaction force between the viscosity molecules.

3.1.2. Bulk Density

Figure 4 illustrates the bulk density of UHPC mixtures with varying zirconia contents. As the zirconia replacement ratio rose, the fine zirconia particles filled the voids in the UHPC matrix, and the packing density increased. The 28-day bulk densities of M0, UHPC-M0.5, UHPC-M1.0, and UHPC-M1.5 were 2410, 2425, 2430, and 2446 kg/m³, respectively. Compared to the control UHPC, the densities of UHPC-M0.5, UHPC-M1.0, and UHPC-M1.5 increased by 0.62%, 0.83%, and 1.49%, respectively. The 28-day bulk densities for UHPC mixtures with NZ were 2435, 2458, and 2432 kg/m³, with UHPC-N0.5, UHPC-N1.0, and UHPC-N1.5 showing density increases of 1.04%, 1.99%, and 0.91%, respectively, compared to the control UHPC. This demonstrates that the presence of NZ improved the bulk density more substantially. However, excess NZ (1.5%) is prone to agglomeration due to high surface energy, forming micro-sized agglomerates, leading to an increase in localized porosity coupled with a significant decrease in mobility, which results in a decrease in bulk density. The addition of zirconia in UHPC mixtures adsorbs excess water molecules, providing water for key cement components, such as dicalcium silicate and tricalcium silicate, thus enhancing hydration kinetics and releasing free calcium hydroxide [20,21]. Initially, zirconia particles react with inert components in the mixture; however, over time, they also interact with cement hydration products like calcium hydroxide (C-H) and calcium silicate hydrate (C-S-H). These C-S-H particles effectively fill internal open pores, leading to the removal of water from the pores. As a result, the bulk density of Z-UHPC rises while the total pore volume reduces, indirectly influencing the mechanical and microstructural characteristics of the internal structure of the UHPC composite materials. In addition, it can be seen that the MZ and NZ combinations as a whole (N0.5M0.5, N0.5M1, N1M0.5) have more pronounced bulk density enhancement for UHPC. This suggests that the synergistic blending of MZ and NZ achieves a more significant bulk density enhancement than single blending through multiscale pore filling, enhanced dispersion stability, and optimized rheological properties.

3.2. Mechanical Properties

Compressive Strength

Figure 5 presents the compressive strength of Z-UHPC with varying contents of NZ and MZ at 3, 7, and 28 days. At all ages, the compressive strength of UHPC increased first and then dropped with increasing zirconia content, with all zirconia-containing UHPC specimens exhibiting higher strength compared to the control group. At 3 days, compared to control, the M0.5, M1, and M1.5 mixtures showed strength increases of 1.82%, 6.79%, and 1.16%, respectively. For NZ mixtures, N0.5, N1, and N1.5 exhibited increases of 9.08%, 15.61%, and 12.67%, respectively. The combined nano- and micro-zirconia mixtures (N0.5M0.5, N0.5M1, and N1M0.5) achieved the highest strength improvements of 14.47%, 8.21%, and 19.27%, respectively. As time progressed, the strength of the specimens became more predictable, with all mixtures from M0.5 to N1M0.5 showing significant strength increases ranging from 1.58% to 9.45% at 28 days. At this stage, among the zirconia-containing mixtures, the UHPC-N1M0.5 specimens exhibited the highest compressive strength, while the UHPC-M1.5 mixtures displayed the lowest. This is mainly due to the better dispersion of zirconia nanoparticles; in Z-UHPC mixtures, the nanoparticles can be more uniformly distributed in the matrix, which reduces the aggregation phenomenon between particles. This good dispersion helps to improve the densification of the material, thus enhancing its mechanical properties.

When the MZ replacement reached 1.5% PC, although there was a slight decrease in strength, it exceeded that of the control UHPC samples. This observation originates from two main effects: first, the filling effect, where the very fine grain size of MZ effectively fills voids in the UHPC matrix; second, the pozzolanic effect, where MZ reacts with CH, resulting in a denser microstructure in the fiber–matrix interfacial transition zone [1,10,15]. Over time, the material effects, particularly those of zirconia, became more pronounced as the nano-sheets reacted with cement hydration products, increasing compressive strength with higher zirconia content at 28 days. This enhancement may originate from the hydration process, the two-dimensional geometric shape, and the high contact surface area of zirconia within the microstructure of UHPC [29]. In this study, the N1M0.5 group achieved the maximum increase in compressive strength, with a 9.45% improvement at 28 days, corresponding to an absolute increase of 11.92 MPa. This demonstrates superior strengthening effects; it indicates that nano-zirconia can accelerate the hydration process and promote the generation of calcium-silica hydration products in the cement matrix, mainly because the nanoparticles have higher surface energy, which can effectively catalyze the hydration reaction and enhance the hydration kinetics of the cement. The increase in the hydration products can help to fill the pore space and improve the denseness of the material. While microzirconia particles are larger, with lower surface energy, the hydration reaction is slower. Therefore, the optimal zirconia dosage for enhancing the compressive strength of UHPC is 1% NZ combined with 0.5% MZ.

3.3. Flexural Strength

Figure 6 presents the flexural strength of UHPC at different ages for various mix ratios. The effect of zirconia on early-age flexural strength was not significant, likely due to its compressive strength characteristics. The trends observed for flexural and compressive strengths of UHPC with zirconia at different ages were similar; both first rose and then dropped with increasing zirconia content while consistently remaining higher than the control group. This observation demonstrates that zirconia can enhance the flexural performance of UHPC. For MZ content ranging from 0.5% to 1.5%, the 28-day flexural strength varied between 21.61 MPa and 22.57 MPa, with slight differences between the M0.5 and M1.5 specimens. NZ exhibited slightly higher flexural strength compared to MZ, the main reason is that nano-zirconia has a larger specific surface area, which means that more surface area can react with the hydration products in the cement matrix to form a more stable chemical bond, whereas micro-zirconia has a smaller surface area and a limited reaction surface. Notably, the N1M0.5 specimens exhibited the highest flexural strength at 28 days, with an 11.98% improvement over the control specimens. Zirconia-containing specimens demonstrated significant improvements in flexural strength at both 7 and 28 days compared to the control group.

Across all tests at various curing ages, UHPC specimens consistently failed through vertical cracking, exhibiting identical failure mechanisms with no visible differences in the fracture surface appearance. The overall results suggest that zirconia’s impact on compressive strength enhancement in UHPC was more pronounced than its effect on flexural strength. This aligns with the findings of Han et al. [21], who indicated that while zirconia participates minimally in the hydration of UHPC, its primary role is in filling voids within the matrix. This filling effect results in a denser UHPC structure, which limits the space and size for CH crystal growth, thereby significantly enhancing the mechanical properties of UHPC.

3.4. Mechanism Analysis

3.4.1. SEM Microstructure

To investigate the influence of NZ on the microstructure of UHPC, SEM observations were conducted on Z-UHPC specimens. Figure 7 presents SEM images of the control, M1, N1, N1.5, and M1N0.5 specimens after 28 days of curing. The surface of the control specimen showed numerous loose accumulation layers, with many pores and extended cracks, leading to inadequate density and uniformity at the microscopic level. Internally, a significant presence of ettringite (AFt) and CH crystals was observed. The CH crystals, which have low strength and poor stability, reduced the bonding strength at the microstructural interface, making them the weakest link and prone to extensive cracking in the control group. After incorporating 1% NZ particles, nanoparticles were effectively dispersed, and C-S-H gel encapsulated the zirconia, improving the bonding strength between the zirconia and hydration products. This enhanced the density of the hydration products and significantly reduced early internal defects. Figure 7d reveals that with higher zirconia content, excessive aggregation and agglomeration within the UHPC matrix occurred, which made dispersion challenging. The large amount of zirconia adsorbed due to Van der Waals forces, leading to agglomeration [30]. The low water–binder ratio in UHPC limited the dispersion of large quantities of zirconia in the small water volumes, resulting in loose agglomerates that did not bond with the cement matrix. These agglomerates created multiple interfaces in the same region, acting as defect sites and promoting crack formation within the UHPC. To address this problem, a dispersant (e.g., polymer or surfactant) can be coated on the surface of zirconia in advance to make it easier to disperse in the substrate material by changing the hydrophilic or lipophilic nature of the zirconia surface.

EDS technology was used to analyze the chemical composition of UHPC specimens containing zirconia after 28 days of curing. The distribution of elements, such as C, O, Mg, Al, Si, Na, K, Ca, Fe, and Zr, in the UHPC matrix interface was examined. The primary components of the UHPC were found to be Ca, Si, and O, with trace amounts of Fe, Mg, and K. Cement clinker phases, such as C₃S, C₂S, tricalcium aluminate (C3A), and calcium aluminoferrite (C₄AF), along with hydration products, such as C-S-H, Ca(OH)₂, and C₃AS₃H₃₂ (ettringite), were identified as the main components of the hardened matrix. Elements such as Fe, Mg, and K, which primarily originate from the C₃A and C₄AF phases, can potentially modify the structure of the cement matrix. While NZ particles participate poorly in cement hydration reactions [2], their nucleation effect can accelerate these reactions. Additionally, NZ particles help fill the pores in the cement mortar, leading to a reduction in the growth space for CH crystals. This ultimately results in a more refined microstructure, enhancing the overall performance of the UHPC.

3.4.2. XRD

Figure 8 presents the XRD results for different mixtures. The crystalline phases of the mixtures remained unchanged after the addition of zirconia, and the incorporation of nano-zirconia did not form new phases. However, with the increasing dosage of zirconia, the characteristic diffraction peaks in the range of 21°~27° in the XRD pattern were enhanced, suggesting that a high dosage of zirconia may change the structure of the hydration products (e.g., C-S-H phases) of the calcium silicates in the hydration process of the cement and slightly alter its crystallinity. This observation aligns with experiments [31,32,33]. The zirconia-based UHPC samples primarily contained ettringite, C-S-H, and CH as hydration products. Most samples exhibited characteristics typical of inert fillers. The XRD analysis revealed peaks for ettringite, calcite, quartz, aluminate, and silicate at 21°, 27.5°, 28.9°, 32.5°, 39.5°, 40°, 52.5°, and 55.8°, corresponding to the expected crystalline phases in the UHPC composites.

The control group exhibited high-intensity CH peaks, reflecting the significant CH production during the hydration reaction. The addition of ZrO₂ particles reduced the size and orientation of the CH crystals, leading to decreased CH peak intensities [20]. The N1M0.5 mixture displayed the lowest CH peak, which aligns with the substantial strength improvement observed in the UHPC containing NZ after 28 days of curing. Additionally, the XRD results for N1, M1, and M1.5 mixtures showed comparatively higher C-S-H peak intensities. This suggests that the inclusion of NZ particles enhances cement hydration reactions, consuming CH crystals, reducing their size and orientation, and, thus, creating more space for other hydration products to form. Consequently, more C-S-H gel is produced, contributing to a denser UHPC microstructure. The XRD findings indicate that the optimal dosage in this study was 1.5% for the N1M0.5 mixture, which exhibited the lowest CH peak intensity, the highest C-S-H peak intensity, and the most significant improvements in microstructure.

3.4.3. BET Pore Structure Analysis

Figure 9 illustrates the pore size distribution of UHPC specimens for various zirconia contents at 28 days, obtained using the BET method. The primary pore diameter distribution for all five samples falls between 5 and 30 nm, with relatively small overall differences and similar distribution trends. Notably, the N1M0.5 mixture, containing 1.5% zirconia, exhibits a significantly lower pore volume compared to the control UHPC, which helps explain the observed mechanical property improvements. For gel pores and interlayer pores (diameter < 10 nm), a significant increase was noted in specimens with 1.5% zirconia, while a slight decrease was observed in specimens with MZ. The pore structure of UHPC samples with 0.5% zirconia was similar to that of the control group. This similarity may be due to the appropriate zirconia content improving UHPC’s packing density [18]. As zirconia content increased, total nitrogen adsorption rose significantly, and pore size distribution notably improved. Particularly, cumulative pore volume exhibited a negative correlation with zirconia content. At 28 days of curing, these values were 0.048 cm³/g (control), 0.033 cm³/g (M1), 0.027 cm³/g (N1), 0.041 cm³/g (N1.5), and 0.024 cm³/g (M1N0.5). This effect is attributed to variations in hydration reactions, pore structure, and internal microstructure, with particular enhancement in the formation and development of smaller pores [34].

Cement paste pores can be divided into four distinct types based on size: gel pores (1–10 nm), small capillary pores (10–50 nm), medium capillary pores (50–100 nm), and large submicron pores (100–1000 nm). As the content of nano- and micro-sized zircon increased, the pore size distribution became more refined, with a larger proportion of smaller pores. This improvement resulted in a more uniform microstructure characterized by pores in a narrower size range and fewer large pores. At the same time, nano-zirconia can further fill the tiny pores in the material through its very small particle size, improve the distribution and density of the pores, so as to make the pore structure of the material more refined, and this refined pore structure helps to improve the mechanical properties of the material, effectively reduce the propagation of micro-cracks in the cement matrix, and enhance its densification and load-bearing capacity. The control group’s pore structure was mainly characterized by larger capillary pores, particularly in the 10–50 nm range. However, with increased zirconia content and optimized particle size, substantial gel formation occurred, filling and replacing the initially formed larger pores. This led to a refinement of medium and large pores, while more gel and small capillary pores formed [6,32]. Consequently, the pore structure became denser, and the pore size distribution became more uniform and refined.

3.4.4. FTIR Analysis

To further investigate the influence of zirconia replacement on hydration products, FTIR analysis was performed on five different mixture samples to identify their phase composition and determine the presence of specific functional groups at certain wave numbers. Figure 8 presents the FTIR spectra of UHPC mixtures containing zirconia after 28 days of curing. The main IR absorption bands include those for H₂O and CH, observed between 620 and 820 cm⁻¹, along with the symmetric and asymmetric stretching Si-O bands in the range of 950 to 1200 cm⁻¹. Additionally, the bending frequency of H₂O was observed at 1630 cm⁻¹. The early stages of C-S-H formation in UHPC were marked by C-S-H bond stretching and asymmetric stretching of Si-O bonds at frequencies around 962, 970, 1978, and 975 cm⁻¹, corresponding to the formation of C₃S and C₂S phases during hydration. The highest silica content was detected across all UHPC mixtures, with a prominent double stretching peak at 1092 cm⁻¹.

A strong vibration observed at 1008 cm⁻¹ may originate from the irregular stretching of T-O-Si, where T represents tetrahedral Si or Al, indicating the presence of the C-S-H, Aft, and AFm phases. As the zirconia replacement ratio increases, the N1M0.5 samples displayed a trend of absorption peak wave numbers shifting toward higher frequencies in the 2930–3230 cm⁻¹ range, likely due to the increased zirconia content [35,36]. However, no significant changes were observed in other absorption peaks, suggesting that the addition of zirconia does not impact the hydration process or the resulting products in Z-UHPC. These findings from FTIR analysis align with the results obtained from XRD testing.

3.5. Innovation and Limitations

This article represents a significant improvement in creating sustainable, non-proprietary UHPC by replacing traditional supplementary cementitious materials (SCMs) with NZ and MZ. This strategy not only reduces PC’s environmental impact but also addresses its high cost and the diminishing availability of FA in UHPC. The findings show that NZ and MZ can effectively enhance the mechanical properties of UHPC mixtures, with their combined use particularly boosting long-term compressive strength under different curing conditions.

However, the study has some limitations. Although Z-UHPC exhibited strong compressive strength, the inclusion of NZ and MZ in the mixtures decreased flowability and increased viscosity. This suggests that zirconia’s impact on the mixture’s rheology might not be as favorable as that of other mineral admixtures, such as cement, fly ash, or slag. To maintain the flowability of NZ and MZ-modified mixtures, a higher dosage of superplasticizers is required, which could pose practical challenges for large-scale applications. Additionally, while NZ and MZ improved strength development at 7 days, their long-term development patterns remain uncertain, raising questions about their necessity for sustainable construction practices. Moreover, the zirconia production process is characterized by high energy consumption, high production costs and a short lifespan, while zirconia is relatively difficult to recycle, which may make it more difficult to dispose of when it is discarded. Despite these challenges, the research demonstrates the potential of NZ and MZ as cost-effective, eco-friendly SCMs in UHPC formulations, providing a foundation for future optimization and research into their broader applicability in construction.

4. Conclusions

The present research examined the feasibility of using zirconia with particle sizes ranging from 0.3 to 3.8 μm as a partial replacement for traditional PC in the preparation of Z-UHPC with environmentally friendly potentials, offering a feasible approach to enhance the comprehensive performance of UHPC. The effects of varying zirconia percentages on key UHPC properties, such as flowability, compressive strength, flexural strength, and packing density, were thoroughly investigated through a series of tests. Advanced material characterization techniques, including SEM, XRD, BET, and FTIR, were employed to analyze the microstructural characteristics and mechanisms of strength enhancement in UHPC. Based on the findings, the primary achievements are as follows:

(1) Z-UHPC flowability decreased as zirconia content increased. At 0.5% replacement, the flowability was almost unaffected; however, at 1.5% replacement, significant flow reductions of 22.01% and 24.71% were observed, alongside viscosity increases of 36.91% and 43.21%. UHPC’s wet packing density increased linearly with zirconia replacement rates from 0 to 1.5%, with nano-sized particles showing more pronounced effects than micron-sized ones.

(2) UHPC’s mechanical properties initially increased with zirconia content, followed by a decrease. The 0.5% MZ replacement had a minimal influence on the 28-day mechanical and durability properties of Z-UHPC, with compressive and flexural strength increases of 1.82% and 4.48%, respectively. The N1M0.5 group showed the highest 28-day compressive strength increase of 9.45% (an absolute increase of 11.92 MPa) and a flexural strength improvement of 11.98%, demonstrating better enhancement effects.

(3) XRD, FTIR, and thermogravimetric analyses revealed that zirconia addition slightly affected the hydration process of N-UHPC, without altering the types of hydration products. BET results indicated that zirconia reduced N-UHPC’s porosity; however, at 1.5% content, the cement matrix pore size increased, leading to a decrease in mechanical properties. These BET findings were consistent with SEM analysis, which showed a gradual reduction in UHPC microstructure density with increasing zirconia content.

In summary, the use of micro–nano-zirconia materials holds potential as a technical solution for significantly improving the toughness of the UHPC matrix, particularly in enhancing early strength. However, the mixed effects of micro–nanomaterials on various toughness parameters warrant further investigation to fully understand their impact.