Ultra-High-Performance Concrete Prepared with Manufactured Sand: Effects of Stone Powder Content on Fresh-State Fluidity and Mechanical Properties

Abstract

This study investigates the preparation and performance of ultra-high-performance concrete (UHPC) incorporating manufactured sand as a full replacement for quartz sand. The mix design was optimized by integrating the compressible packing model (CPM) with an orthogonal experimental design. The influence of stone powder content in manufactured sand—0, 5, 10, and 15% by mass of fine aggregate—on fresh-state fluidity and 7d-mechanical properties was systematically evaluated. Hydration products and microstructural features were analyzed using X-ray diffraction (XRD), scanning electron microscope (SEM), and mercury intrusion porosimetry (MIP). Results show that the manufactured sand-based UHPC achieved a fresh-state fluidity of 185 mm and a 7-day compressive strength of 152.4 MPa. Both fluidity and compressive strength exhibited a unimodal trend with increasing stone powder content, reaching maxima at 10%. Microstructural analysis revealed intimate interfacial bonding between unhydrated particles and calcium silicate hydrate (C–S–H) gel; notably, the UHPC matrix with 10% stone powder displayed the densest microstructure. MIP results further demonstrated that an optimal stone powder content effectively reduced total porosity, with the lowest overall porosity and the highest volume fractions of harmless (≤20 nm) and less harmful (20–100 nm) pores observed at 10%. These microstructural refinements collectively underpin the superior mechanical performance of manufactured sand-based UHPC.

1. Introduction

Ultra-high-performance concrete (UHPC) exhibits exceptional mechanical properties and durability [1,2], enabling structural weight reduction, enhanced load-carrying capacity of components, and extended service life. Consequently, UHPC has been increasingly deployed in critical engineering applications, including long-span bridges, nuclear containment structures, offshore wind turbine foundations, and seismic-resistant prefabricated systems, owing to its exceptional compressive strength (>120 MPa), high fracture energy, and superior resistance to chloride ingress, carbonation, and freeze–thaw cycling [1,2,3,4,5,6].

However, conventional UHPC formulations rely predominantly on quartz sand as the fine aggregate [7,8]. In China, natural quartz sand reserves are not only geographically constrained and economically depleted but also subject to increasingly stringent ecological regulation. Rising demand has exacerbated supply instability [9], thereby limiting the broader engineering deployment of UHPC.

Concurrently, large-scale earthwork excavation associated with mega-projects, such as inter-basin water transfer systems, high-speed rail networks, and national expressway corridors, produces vast quantities of excavated rock that can be processed and utilized as aggregate for concrete production. Among these by-products, mechanically processed fine aggregates in specified particle size distributions below 4.75 mm [10] are commonly classified as manufactured sand (MS). Manufactured sand features angular particle shapes, a rough surface texture, and mechanical strength comparable to that of the parent rock, while also offering economic advantages [11,12].

A growing number of researchers have adopted manufactured sand as fine aggregate in the formulation of UHPC. Xiong Hongrui et al. [13] substituted high-quality manufactured sand for quartz sand to produce Reactive Powder Concrete with enhanced mechanical properties (compressive strength > 100 MPa). Shen et al. [12] optimized the particle gradation and developed manufactured sand-based UHPC, achieving a compressive strength of 120 MPa. Kay et al. [14] employed manufactured sand with particle sizes below 1.2 mm sourced from various rock types to produce manufactured sand-based UHPC with a compressive strength of 200 MPa. Wang Quanchao et al. [15] developed manufactured sand-based UHPC exhibiting a slump flow of 680 mm, a compressive strength of 135.6 MPa, and a flexural strength of 25.8 MPa through the application of well-optimized gradation techniques. Liu Chen [16] reported that the particle morphology of manufactured sand exerts a dual effect on the performance of UHPC. Specifically, when granite-derived manufactured sand replaced 60% of natural sand, the resulting UHPC exhibited superior fresh-state workability and retained compressive and tensile strengths comparable to those of the natural sand reference mixture. Yang et al. [17] partially replaced river sand with manufactured sand in the preparation of UHPC. The study demonstrated that although the incorporation of manufactured sand decreases UHPC fluidity, it substantially improves compressive strength and increases autogenous shrinkage. Bian Chen et al. [18] found that the workability of manufactured sand-based UHPC and river sand-based UHPC decreased by 17.3% and 15.8%, respectively, while the compressive strength declined by 7.0% and 9.7%, correspondingly. Chu Hongyan et al. [19] replaced river sand with manufactured sand in UHPC, achieving compressive strength of 169.9 MPa (+5.2%), flexural strength of 18.8 MPa (+15.3%), and elastic modulus of 47.8 GPa (+5.8%)—all exceeding those of the river sand reference mixture.

The aforementioned research findings suggest that manufactured sand can function as a feasible alternative to quartz sand in the preparation of UHPC. However, the production process of manufactured sand typically results in a significant amount of stone powder (particle size below 0.075 mm), with content levels generally ranging from 5% to 20% [20,21]. Owing to its high specific surface area, stone powder significantly increases the water demand of concrete mixtures, thereby reducing fresh-state workability, impairing particle packing density and microstructural homogeneity, and ultimately hindering strength development [17,22]. Nevertheless, systematic investigations into the influence of stone powder on UHPC performance remain scarce in the current literature.

Given these considerations, this study adopts granite-derived manufactured sand—rather than quartz sand—as the fine aggregate in UHPC and systematically investigates its mix design, preparation, and performance. The work adopts a rigorously structured, three-phase technical framework—(1) theoretical design, (2) performance validation, and (3) mechanistic elucidation—to systematically investigate how stone powder modulates the properties of manufactured sand-based UHPC. First, guided by the dense particle packing theory, specifically the Compressible Packing Model (CPM) [23], the particle size distribution of manufactured sand and the composition of the UHPC binder system (Portland cement, silica fume, and fly ash) were jointly optimized via orthogonal experimental design to achieve synergistic densification. Second, the quantitative effects of stone powder content (0%, 5%, 10%, and 15% by mass of fine aggregate) on fresh-state fluidity and hardened-state mechanical properties (compressive strength, splitting tensile strength, and flexural strength) were systematically evaluated. Third, X-ray diffraction (XRD), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP) were integrated to elucidate the hydration kinetics and microstructural evolution of manufactured sand UHPC, thereby revealing how stone powder governs macroscopic performance through hydration products and quantifiable modifications to pore structure (e.g., total porosity, critical pore diameter).

2. Materials and Methods

2.1. Material

In this study, ordinary Portland cement (OPC) complying with GB 175–2023 [24], Grade 52.5, was used as the primary binder; its key physical properties are presented in

Table 1. Physical properties of cement.

Density (kg/m3)	Specific Surface Area (m2/kg)	Setting Time (min)		Compressive Strength (MPa)		Flexural Strength (MPa)
		Initial	Final	3 d	28 d	3 d	28 d
3060	372	151	204	30.7	60.4	6.3	8.7

. Supplementary cementitious materials comprised silica fume (SF) and fly ash (FA). The SF exhibited a density of 2220 kg/m³, a 7-day activity index of 105%, and a specific surface area of 23,453 m²/kg. The FA had a density of 2440 kg/m³ and a specific surface area of 545 m²/kg. Their respective chemical compositions and particle size distributions are reported in

Table 2. Chemical composition of cement, silica fume, and fly ash (wt%).

	SiO2	Al2O3	Fe2O3	CaO	K2O	SO3	Loss on Ignition (%)
C	25.06	7.62	3.36	57.73	0.76	4.59	1.36
SF	98.22	/	0.05	0.63	0.4	0.44	1.48
FA	57.78	31.76	3.07	3.0	1.83	/	2.62

and illustrated in Figure 1. To ensure adequate workability, a polycarboxylate-based superplasticizer (SP) with a water-reduction capacity of 35% was employed. The fine aggregate consisted of quartz sand (particle size range: 0.15–1.18 mm), supplied by Henan Mishang Environmental Technology Co., Ltd. (Zhengzhou, China) It features a SiO₂ content of 98%, a fineness modulus of 2.6, and an apparent density of 2630 kg/m³.

The manufactured sand used in this study was obtained by crushing granite at a construction site, followed by sieving to obtain particles within the size range of 0.15–1.18 mm. These particles were used as fine aggregate for the preparation of ultra-high-performance concrete (UHPC). The apparent density of the manufactured sand is 2650 kg/m³, with a fineness modulus of 2.83. The chemical composition of the manufactured sand is presented in

Table 3. Chemical composition of manufactured sand (wt%).

Chemical Composition	SiO2	Al2O3	CaO	Fe2O3	K2O	SO3
Content (%)	60.05	17.50	12.32	5.36	3.13	0.45

. Stone powder is defined as the fraction of particles finer than 0.075 mm in manufactured sand, isolated via dry sieve analysis of the parent granite-derived manufactured sand. Manufactured sands with controlled stone powder contents (0, 5%, 10%, and 15% by mass of fine aggregate) were prepared by blending a stone powder fraction with the base manufactured sand (0.15–1.18 mm), both derived from the same granite parent material. Additionally, copper-plated micro-fine steel fibers were employed as reinforcement materials, and their performance characteristics are summarized in

Table 4. Steel fiber performance characteristics.

Diameter (mm)	Length (mm)	Tensile Strength (MPa)	Elastic Modulus (GPa)	Density (kg/m3)
0.2	13	2850	200	7850

.

2.2. Design for the Gradation of Manufactured Sand and Cementitious Material Mix Proportion Based on the Compressible Packing Model (CPM)

2.2.1. A Concise Overview of Compressible Packing Model (CPM)

The fundamental principle of UHPC formulation lies in the optimization of particle grading, which is based on the theory of dense compaction. This approach is intended to enhance the overall density of the UHPC mixture, thereby facilitating the attainment of ultra-high mechanical properties and superior durability [25,26,27]. To evaluate the packing density of concrete particles, researchers have developed various mathematical models aimed at guiding the improvement of material performance. These models include the Furnas model [28], the Aim and Goff model [29,30], the modified Toufar model [31], and the Dewar model [32].

De Larrard F. integrated principles of statistical mechanics with experimental data to develop the compressible packing model (CPM), which is derived from both the linear packing model and the solid suspension model. This model considers the interactions among particles of varying sizes and the influence of packing configuration on compaction density. The core formula is given in Equation (1) [23,33,34,35,36]:

$$\begin{gathered} {\gamma }_i={\beta }_i/\left\{1-{\sum }_{j=1}^{i-1}\left[1-{\beta }_i+b_{ij}{\beta }_i\left(1-1/{\beta }_j\right)\right]y_j-{\sum }_{j=i+1}^n\left(1-a_{ij}{\beta }_i/{\beta }_j\right)y_j\right\} \\ {a}_{ij}=\sqrt{1-{\left(1-d_j/d_i\right)}^{1.02}} \left(j=i+1,\dots ,n\right) \\ {b}_{ij}=1-{\left(1-d_i/d_j\right)}^{1.50} \left(j=1,\dots ,i-1\right) \\ K={\sum }_{i=1}^n{K}_i={\sum }_{i=1}^n{\displaystyle \frac{\raisebox{1ex}{$y_i$}\!\left/ \!\raisebox{-1ex}{${\beta }_i$}\right.}{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$\mathrm{\varnothing }$}\right.-\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{${\gamma }_i$}\right.}} \end{gathered}$$

(1)

Revision: In this model, ${\mathsf{\gamma }}_{\mathrm{i}}$ represents the virtual packing density of the system when class i is prevailing; while ${\mathsf{\beta }}_{\mathrm{i}}$ denotes the residual packing density of class i under individual compaction; the parameter ${\mathrm{y}}_{\mathrm{i}}$ signifies the volumetric proportion of each size fraction relative to the total solid volume; the loosening and wall effect coefficients are expressed by ${\mathrm{a}}_{\mathrm{i}\mathrm{j}}$ and ${\mathrm{b}}_{\mathrm{i}\mathrm{j}}$, respectively; diameters of particle classes i and j (based on sieve sizes) are denoted as ${\mathrm{d}}_{\mathrm{i}}$ and ${\mathrm{d}}_{\mathrm{j}}$; furthermore, $\mathrm{K}$ stands for the compaction index, and $\mathrm{\varnothing }$ defines the actual packing density of the particulate assembly.

2.2.2. Design Steps Based on Compressible Packing Model

The model is constructed based on a monodisperse particle system comprising $\mathrm{i}\left(\mathrm{i}=1\mathrm{t}\mathrm{o}\mathrm{n}\right)$ continuous particle sizes classes. The characteristic particle size, denoted as ${\mathrm{d}}_{\mathrm{i}}$ and ${\mathrm{d}}_{\mathrm{j}}$, along with its corresponding volume fraction ${\mathrm{y}}_{\mathrm{i}}$, is obtained through a laser particle size analyzer. Based on the measured packing density, the virtual packing of the i-class compacted alone ${\mathsf{\beta }}_{\mathrm{i}}$ for each particle size is calculated using Equation (1) [37]. Different compaction indices, $\mathrm{K}$, corresponded to distinct packing processes, and the value of $\mathrm{K}$ was selected in accordance with reference [38].

The gradation design of manufactured sand and the mix proportion design of cementitious materials are conducted as follows [34]: First, a MATLAB (2024)-based computational program is developed according to Equation (1). The characteristic particle sizes (d_i and d_j) of each size fraction, along with its volume fraction (y_i) and the virtual packing density (β_i) of the i-th fraction compacted in isolation, are input into the program. The theoretical packing density (γ_i) of each fraction is then computed. Second, incorporating the experimentally determined compaction index K, the program calculates the actual packing density (∅) [34,38]. Finally, through iterative optimization of the manufactured sand gradation and the cementitious material composition ratio, the program identifies the combination that maximizes the overall packing density, thereby determining the optimal manufactured sand gradation and cementitious material proportion for achieving the densest possible microstructure.

2.3. Test Methods

2.3.1. Measurement of Fluidity of Fresh UHPC Mixture

The preparation of UHPC mixtures, including those with manufactured or quartz sand, followed a standardized procedure of mixing, casting, and vibrating. The dry components, comprising the binders (cement, SF, and FA) and fine aggregates, were first homogenized for 60 s. This was followed by the addition of the superplasticizer and 66% of the mixing water, with a subsequent 2 min blending period. In the final stage, the steel fibers and the residual water were incorporated, and the mixing continued for another 3 min, bringing the total duration to approximately 6 min. The fresh-state flowability was then determined adhering to the Chinese Standard GB/T 2419-2005 [39].

2.3.2. Assessment of Compressive, Tensile, Flexural, and Shear Strength of UHPC

To assess the mechanical performance (compressive, tensile, flexural, and shear strengths), various specimens were fabricated for each UHPC batch. These included six 100 mm cubes for compression testing and six dog-bone-shaped samples (total length 368 mm; tensile Section 100 mm × 50 mm × 50 mm), which were strengthened with carbon fiber fabric as per [40]. Additionally, seven prisms were cast, consisting of three units with a 100 mm × 100 mm × 400 mm geometry and four others measuring 100 mm × 100 mm × 300 mm.

After the UHPC mixture was uniformly mixed, it was poured into pre-prepared molds that had been coated with mold release oil to minimize interfacial friction between the molds and the mixture. The mixture was then compacted using a vibration table. The cast specimens were covered with plastic sheets prior to demolding to prevent moisture loss due to evaporation. The specimens were demolded 24 h after casting and subsequently transferred to a steam curing chamber. They were then heated at a rate of 10–15 °C/min to a temperature of 90 °C and maintained at this temperature for 72 h. Following this thermal curing phase, the specimens were gradually cooled to room temperature and then cured at a temperature of 20 ± 2 °C and a relative humidity of 95% (standard curing) until reaching the 7-day testing age.

Mechanical characterization of the UHPC, specifically compressive and tensile strengths, was performed following the T/CBMF 37-2018 guidelines [41]. For compression, a YAW-Y2000F1W machine (Jinan Shijin Group Co., Ltd., Jinan, China) was utilized with a constant stress increase of 1.3 MPa/s. Tensile properties were determined using a WAW-Y1000C universal tester (Jinan Shijin Group Co., Ltd.) at a displacement-controlled rate of 0.1 mm/min. To ensure precise data collection, carbon fiber fabric was used to reinforce the variable cross-sections of the tensile specimens, while strain development was tracked via dual-sided strain gauges [42], as illustrated in Figure 2a.

Flexural strength testing was carried out in compliance with the Chinese standard GB/T 31387-2015, Reactive Powder Concrete [43], using prismatic specimens measuring 100 mm × 100 mm × 400 mm. The tests were conducted using a WAW-Y1000C universal testing machine at a loading rate of 0.1 MPa/s, as illustrated in Figure 2b.

In accordance with the Chinese standard CECS 13-2009 [44], shear strength was evaluated using 100 mm × 100 mm × 300 mm prismatic specimens, with four replicates for each experimental group. The testing was performed on a YAW-Y2000F1W compression system, applying a constant stress rate of 0.08 Mpa/s. To facilitate the testing setup, each sample’s lateral surface was partitioned into three equal segments, and 2.5 mm wide, 10 mm deep grooves were machined at the base for angle iron reinforcement. As illustrated in Figure 3, three displacement transducers (LVDTs) were fixed via magnetic bases to track the deformation within the central shear region.

2.3.3. Microstructure Examination

For microstructural characterization, samples were extracted from the core of 100 mm cubic specimens at the specified ages. To terminate the hydration process, these fragments were submerged in anhydrous ethanol for a minimum of 24 h, followed by vacuum drying at 40 °C for another 24 h. For XRD testing, the material was pulverized into a fine powder (particle size < 10 μm, Rigaku Corporation, Tokyo, Japan). In contrast, specimens for SEM and MIP analysis were carefully trimmed into small blocks of approximately 3 mm × 3 mm × 2 mm. Mineralogical phases were identified using a Bruker AXS D8-Advance diffractometer (scanning from 5° to 80° 2θ at a 0.02° step). Morphological observations were conducted via a JSM-7500F SEM (JEOL, Tokyo, Japan) at 20 kV. Additionally, pore structure features (4 nm to 400 μm) were quantified using an AutoPore IV-9500 mercury intrusion porosimeter (Micromeritics Instrument Corporation, Norcross, GA, USA).

3. Results and Discussion

3.1. Optimal Mix Proportion Design for UHPC Incorporating Manufactured Sand

3.1.1. Gradation Design of Manufactured Sand and Optimization of Cementitious Material Composition

The manufactured sand listed in Table 3 was subjected to sieve analysis in accordance with GB/T 14684-2022 “Sand for construction” [10]. Particles within the size ranges of 1.18~0.60 mm, 0.60~0.30 mm, and 0.30~0.15 mm were selected and blended in varying volume proportions. Subsequently, the packing density of the resulting sand mixtures at each blending ratio was calculated based on the CPM using the method outlined in Section 2.2.2, with results presented in Figure 4.

As shown in Figure 4, the packing density of blended manufactured sand exhibits a unimodal trend—increasing initially and then decreasing—with rising volume fraction of finer particles. For binary particle size combinations (1.18–0.60 mm and 0.60–0.30 mm; Figure 4a), the packing density peaks at 0.5954 when the volume fraction of the 0.60–0.30 mm fraction reaches 43%, corresponding to a volumetric ratio of 0.57:0.43. In ternary blends comprising 1.18–0.60 mm, 0.60–0.30 mm, and 0.30–0.15 mm fractions (Figure 4b), the maximum packing density of 0.6298 is achieved at a 30% volume fraction of the finest fraction (0.30–0.15 mm), with an optimized volumetric composition of 0.40:0.30:0.30. Given the apparent density difference among the three types of manufactured sand with particle sizes of 1.18–0.60 mm, 0.60–0.30 mm and 0.30–0.15 mm is less than 1.2%, the mass proportions of the three in the blended manufactured sand can also be taken as 0.40:0.30:0.30.

Similarly, based on the CPM method (see Section 2.2), the packing densities of cementitious material systems at various composition ratios can be determined. The results are presented in Figure 5. Specifically, Figure 5a illustrates the variation trend of packing density in the “cement + silica fume” binary system as a function of silica fume content, while Figure 5b depicts the corresponding trend for the “cement + silica fume + fly ash” ternary system with respect to fly ash content.

As shown in Figure 5, the packing density of the cementitious material mixture first increases and then decreases with increasing supplementary cementitious material content. In the binary system (cement + silica fume), the maximum packing density of 0.5611 is achieved at a silica fume volume fraction of 30%, corresponding to a cement-to-silica fume volume ratio of 0.70:0.30. In the ternary system (cement + silica fume + fly ash), the maximum packing density of 0.5626 is attained at a fly ash volume fraction of 20%, with a volumetric composition of 0.56:0.24:0.20. Given the densities of cement, silica fume, and fly ash—3060 kg/m³, 2220 kg/m³, and 2440 kg/m³, respectively (Section 2.1)—the mass proportions of the ternary cementitious material system are 0.63:0.19:0.18.

3.1.2. Orthogonal Design of Mix Proportions for Manufactured Sand-Based UHPC

Based on the experimentally determined mass ratio of the blended manufactured sand and the mass ratio of the ternary cementitious material system (Section 3.1.1), three key factors—water-to-binder mass ratio (W/B), sand-to-binder mass ratio (S/B), and steel fiber volume fraction (Fiber fraction)—were selected as control factors for the orthogonal experimental design of manufactured sand-based ultra-high-performance concrete (UHPC) mixtures. The corresponding factor levels are listed in

Table 5. Factor levels for orthogonal experimental design.

Factor	Water-to-Binder Mass Ratio (H)	Sand-to-Binder Mass Ratio (K)	Steel Fiber Volume Fraction/% (L)
Level
1	0.14	0.8	1.5
2	0.16	1.0	2.0
3	0.18	1.2	2.5

. All mixtures were proportioned using the absolute volume method, and their compositions are detailed in

Table 6. Orthogonal design of mix proportions for manufactured sand-based UHPC (kg/m³).

No.	MS	C	SF	FA	W	Steel Fiber	SP
H1K1L1	976	764	238	218	171	118	24
H1K2L3	1104	692	215	197	155	196	22
H1K3L2	1227	641	199	183	143	157	21
H2K1L3	943	738	230	210	189	196	24
H2K2L2	1085	680	212	194	174	157	22
H2K3L1	1208	631	196	180	161	118	20
H3K1L2	925	725	225	206	208	157	23
H3K2L1	1067	669	208	190	192	118	21
H3K3L3	1172	612	190	174	176	196	20

. UHPC specimens were prepared according to Table 6, and their fresh-state fluidity and 7d-compressive strength were evaluated. The results are presented in Figure 6.

Based on the orthogonal test results, the water-binder ratio, sand-binder ratio, and steel fiber volume fraction were initially optimized at 0.16, 0.8, and 2.0%, respectively. However, Figure 6 reveals that increasing the steel fiber volume fraction from 2.0% to 2.5% induces negligible deterioration in UHPC fluidity while concurrently enhancing compressive strength. Furthermore, consistent with findings reported in references [45,46,47,48,49], elevating the steel fiber volume fraction to 2.5% yields substantial improvements in key mechanical properties—particularly tensile strength—with only a marginal reduction in fluidity. Given that both workability and mechanical performance are critical design objectives, and considering the favorable trade-off achieved at 2.5%, this value was adopted as the final optimized steel fiber volume fraction. Accordingly, the optimized UHPC reference mixture proportions are summarized in

Table 7. Reference mix proportion for manufactured sand-based UHPC (kg/m³).

Cementitious Materials 1			W	Steel Fiber	SP	MS 2
C	SF	FA
738	230	210	189	196	24	943

¹ The mass proportions of cement, silica fume, and fly ash in the cementitious materials are 63:19:18. ² The mass proportions of the manufactured sand fractions, namely, 1.18–0.60 mm, 0.60–0.30 mm, and 0.30–0.15 mm, are 40:30:30.

.

3.2. Effect of Stone Powder Content on Fluidity and Mechanical Properties of UHPC Containing Manufactured Sand

Based on the reference mixture proportions in Table 7, an experimental study was conducted to investigate the effect of stone powder content in manufactured sand—varied at 0%, 5%, 10%, and 15% by mass of fine aggregate—on the fresh-state flowability and 7-day mechanical properties of ultra-high-performance concrete (UHPC), including compressive strength, tensile strength, flexural strength, and shear strength. To rigorously isolate and systematically quantify the independent effect of stone powder content on both fluidity and mechanical performance, the water dosage, water-binder ratio, polycarboxylate superplasticizer dosage, and all other constituent proportions were held constant across all mixtures. The corresponding mixture proportions are listed in

Table 8. Mix proportions of manufactured sand-based UHPC with varying stone powder contents (kg/m³).

No.	C	SF	FA	W	Steel Fiber	SP	QS	MS *	MS Stone Powder
									Mass	Content *
MS-0	738	230	210	189	196	24	/	943	0	0
MS-5							0	895.85	47.15	5%
MS-10							0	848.70	94.30	10%
MS-15							0	801.55	141.45	15%
QS-0							936	/	/	/

* The stone powder content is defined as the mass fraction of stone powder in the manufactured sand–stone powder mixture, expressed as a percentage.

. Manufactured sands with controlled stone powder contents (0, 5%, 10%, and 15% by mass) were prepared by blending a stone powder fraction with the base manufactured sand (0.15–1.18 mm). Furthermore, a control mixture designated as QS-0 was prepared by replacing the manufactured sand in the MS-0 mixture with an equivalent volume of quartz sand for comparative analysis.

3.2.1. Effect on Fluidity of UHPC

Figure 7 illustrates the effect of stone powder content in manufactured sand on the fluidity of ultra-high-performance concrete (UHPC). As shown in the figure, the fluidity of manufactured sand UHPC initially increases and subsequently decreases with increasing stone powder content, reaching a maximum at 10% stone powder content. The control group QS-0 (quartz sand-based UHPC, 0% stone powder content) exhibits a fluidity of 190 mm. For mixtures with 0%, 5%, 10%, and 15% stone powder content—designated as MS-0, MS-5, MS-10, and MS-15, respectively—the measured fluidity values are 185 mm, 190 mm, 194 mm, and 189 mm. Notably, the fluidity of Mixture MS-10 is 4.9% higher than that of MS-0 and 2.1% higher than that of QS-0. Mixtures MS-5 and MS-15 show improvements of 2.7% and 2.2%, respectively, compared to MS-0, with fluidity values close to that of the QS-0 reference.

As the stone powder content increases, the proportion of fine particles in manufactured sand rises, whereas that of coarse particles declines correspondingly. These fine particles act as micro-fillers, enhancing particle packing density within the UHPC matrix and thereby improving its rheological behavior—specifically, reducing plastic viscosity and yield stress, which collectively contribute to a significant improvement in the fluidity of the fresh paste [30]. However, excessive stone powder can adsorb free water, increasing the paste’s water requirement and consequently impairing fluidity [17]. Furthermore, the fluidity of manufactured sand-based UHPC without stone powder (MS-0) is slightly lower than that of quartz sand-based UHPC (designated as QS-0), primarily due to the rough surface texture and pronounced angularity of manufactured sand particles, which result in higher internal friction. Therefore, an appropriate stone powder content can optimize the particle gradation, enhance packing density, reduce requirement demand, and consequently improve the fluidity of UHPC.

3.2.2. Effect on Mechanical Properties of UHPC

Figure 8 presents the effect of stone powder content on the mechanical properties of manufactured sand-based UHPC. As illustrated in the figure, with increasing stone powder content, all measured strength indices—including compressive strength, axial tensile strength, flexural strength, and shear strength—exhibit an initial increase followed by a subsequent decrease, reaching peak values at a stone powder content of 10%. Furthermore, the strength values of the stone powder-free mixture (MS-0) are slightly higher than those of the quartz sand-based reference UHPC (QS-0).

It can be noticed from Figure 8a that the compressive strength of quartz sand UHPC (QS-0) is measured at 147.3 MPa. For manufactured sand UHPC mixtures with stone powder contents of 0%, 5%, 10%, and 15% (denoted as MS-0, MS-5, MS-10, and MS-15, respectively), the corresponding compressive strengths are 152.4 MPa, 156.9 MPa, 160.3 MPa, and 156.2 MPa in sequence. Specifically, compared with MS-0 and QS-0, the compressive strength of MS-10 exhibits 5.2% and 8.8% increases respectively. In comparison with MS-0, the compressive strengths of MS-5 and MS-15 have increased by 3.0% and 2.5% respectively; when compared with QS-0, they have improved by 6.5% and 6.0% respectively. Furthermore, the compressive strength of MS-0 exceeds that of QS-0 by 3.5%. The trends in tensile strength, flexural strength, and shear strength as a function of stone powder content are broadly consistent with those observed for compressive strength.

The strength development of MS-UHPC follows a trend of initial growth followed by a decline as stone powder dosages increase, a phenomenon largely governed by the synergistic impact of the filling effect, nucleation sites, and chemical reactivity of the stone powder [50]. Specifically, the fine stone powder particles occupy the interstitial spaces among the larger manufactured sand grains. This micro-filling action refines the overall aggregate grading, which in turn enhances the packing density of the entire UHPC matrix [15]. Simultaneously, fine stone powder particles possess numerous surface-active sites that act as effective nucleation centers for cement hydration products, promoting the hydration process and accelerating the formation of hydrates to a certain extent [50,51]. Simultaneously, the rough surface texture and angular morphology of manufactured sand and stone powder enhance the interfacial bond strength between the cementitious paste and fine aggregates [52]. These characteristics also contribute to improved dispersion and mechanical anchoring of steel fibers within the matrix [53]. Consequently, when an appropriate amount of stone powder is incorporated, the compressive, tensile, flexural, and shear strengths of UHPC are all enhanced. However, excessive stone powder content may disrupt the optical aggregate gradation of the UHPC system, thereby reducing overall packing density. Furthermore, the water absorption capacity of stone powder can lead to moisture retention in the interfacial transition zone, which inhibits further cement hydration, weakens interfacial bond strength, and diminishes the aggregate skeleton’s load-bearing contribution, ultimately impairing mechanical performance [11,18,19]. Additionally, manufactured sand-based UHPC without stone powder (MS-0) exhibits slightly higher strength compared to quartz sand-based UHPC (QS-0), primarily attributed to the rough surface texture of manufactured sand particles, which enhances interfacial adhesion with cementitious materials and promotes mechanical interlocking with steel fibers [52].

3.3. Influence of Stone Powder Content on the Microstructure of Manufactured Sand-Based UHPC

3.3.1. XRD Analysis

Figure 9 presents the XRD patterns of hydrated samples from the selected compositions of UHPC, namely, MS-0, MS-10 (manufactured sand-based UHPC containing 0% and 10% stone powder, respectively), and QS-0 (quartz sand-based UHPC).

As illustrated in Figure 9, the main crystalline phases identified in the hydrated UHPC samples across different mix proportions include SiO₂, unhydrated C₃S and C₂S minerals, as well as hydration products such as ettringite (AFt) and calcium hydroxide (Ca(OH)₂), with the characteristic diffraction peaks of SiO₂ exhibiting relatively high intensity. The quartz crystals (SiO₂) originate from manufactured sand (Samples MS-0, and MS-10) or quartz sand (Sample QS-0), which contain 60% and 98% SiO₂, respectively (see Section 2.1). This finding further confirms that manufactured sand and quartz sand particles can function as inert fillers in UHPC matrix, contributing to the densification of the particle packing [54]. Moreover, the XRD peak intensities of C₃S and C₂S in the MS-10 sample were lower than those in QS-0 and MS-0, supporting the hypothesis that the nucleation effect of stone powder can partially accelerate the hydration of cementitious materials [50,51]. Additionally, the diffraction peaks corresponding to Ca(OH)₂ and AFt crystals were weak in all three hydrated samples. The reduced Ca(OH)₂ content is primarily attributed to its extensive consumption during secondary hydration reactions involving high volumes of reactive mineral admixtures (e.g., silica fume, fly ash), particularly under steam curing conditions [55,56]. Conversely, the low AFt content may be due to restricted crystal growth caused by the system’s low porosity or possible thermal decomposition during the steam curing process [57,58,59].

3.3.2. SEM Analysis

Figure 10, Figure 11 and Figure 12 display SEM images of hydrated samples from manufactured sand-based ultra-high-performance concrete (UHPC) mixtures MS-0 and MS-10, containing 0% and 10% stone powder, respectively, as well as quartz sand-based UHPC (QS-0).

Figure 10, Figure 11 and Figure 12 demonstrate that both manufactured sand-based UHPC and quartz sand-based UHPC exhibit highly dense microstructures. The hardened cement paste in both systems consists primarily of abundant hydration products, unhydrated cement particles, and a minimal volume of pores. These hydration products, predominantly calcium silicate hydrate (C-S-H) gel, tightly envelop supplementary cementitious materials such as fly ash and aggregate particles (Figure 10a, Figure 11a and Figure 12a), thereby enhancing the microstructural integrity of the cementitious matrix. In the case of the manufactured sand-based UHPC sample with 0% stone powder content (MS-0), Figure 10 illustrates that certain pores are filled with gel-like hydration products (Figure 10a) or calcium hydroxide (Ca(OH)₂) crystals (Figure 10c), which is consistent with the XRD analysis results previously discussed for MS-0 (Figure 9). For the quartz sand-based UHPC sample (QS-0), although quartz particles are well-encapsulated by hydration products, distinct interfacial transition zones (ITZs) are observed between the particles and the surrounding cement paste due to the layered morphology of the quartz grains (Figure 12a), potentially compromising the overall structural density. Figure 11b reveals that in the manufactured sand-based UHPC sample containing 10% stone powder (MS-10), a limited number of fine hydration product nuclei have formed on the surface of the stone powder, confirming the nucleation effect associated with stone powder. This phenomenon contributes to the promotion of hydration reactions within the UHPC system, increasing the total amount of hydration products and further improving the microstructural characteristics [50,51].

In summary, it can be concluded that the filling and nucleation effects induced by an appropriate amount of stone powder in manufactured sand not only enhance the compaction density of the UHPC system [31], but also promote the hydration process to a certain extent. Consequently, this results in improved compactness of UHPC specimens and a reduction in porosity. This conclusion will be further supported by the mercury intrusion porosimetry analysis presented in Section 3.3.3.

3.3.3. MIP Analysis

The mercury intrusion porosimetry (MIP) test results for manufactured sand-based UHPC with varying stone powder contents (MS-0, MS-10, and MS-15), as well as for the control group QS-0 (quartz sand-based UHPC), are presented in Figure 13 and

Table 9. Porosity index of each UHPC specimen.

No.	Total Invasion Volume (mL/g)	Most Probable Pore Diameter (nm)	Porosity (%)	Pore Diameter Distribution (%)
				5~20 nm (Harmless Pore)	20~100 nm (Slightly Harmful Pore)	100~200 nm (Harmful Pore)	>200 nm (Highly Harmful Pore)
QS-0	0.017	5.5	3.77	37.6	14.2	5.0	43.2
MS-0	0.012	5.5	2.72	39.7	14.8	4.6	40.9
MS-10	0.009	6.0	2.06	45.1	21.4	3.1	30.4
MS-15	0.010	6.0	2.31	37.0	24.3	3.9	34.8

.

As illustrated by the data in Figure 13 and Table 9, the porosity of manufactured sand-based UHPC (MS-UHPC) follows a parabolic trend with increasing stone powder dosage, first declining and then rising. A minimum porosity of 2.06% was achieved at the 10% stone powder level. Notably, the peak pore frequency (most probable diameter) for all four groups remained below 6 nm. Based on the pore classification proposed in Reference [58]—comprising harmless (<20 nm), slightly harmful (20–100 nm), harmful (100–200 nm), and highly harmful pores (>200 nm)—the MS-10 group demonstrated the most optimized structure. Specifically, at 10% stone powder content, the cumulative volume of harmful and highly harmful pores reached their lowest points (30.4% and 3.1%), while the combined fraction of harmless and slightly harmful pores peaked at 66.5%. This trend can be attributed to two mechanisms: first, the filling effect of an appropriate amount of stone powder enhances the packing density of the UHPC matrix; second, the nucleation effect of stone powder particles promotes cement hydration, thereby increasing the formation of hydration products [50,51], which contributes to enhanced microstructural densification and improved mechanical properties. However, excessive stone powder may compromise the overall compactness of the system, resulting in reduced mechanical strength—a finding consistent with the compressive strength results presented in Section 3.2.2.

Furthermore, a comparative analysis of MIP results between quartz sand-based UHPC (QS-0) and manufactured sand-based UHPC reveals that the porosity of the latter with 0% stone powder content (MS-0) is 2.72%, lower than that of QS-0, which is 3.77%. Additionally, the volume fractions of highly harmful and harmful pores in all three manufactured sand-based UHPC mixtures (MS-0, MS-10, and MS-15) are lower than those in the quartz sand-based counterpart, with corresponding values of 43.2% and 5.0%, respectively. This difference is likely attributable to the predominantly flaky morphology of quartz sand particles, which may result in less favorable particle packing and higher interstitial porosity.

4. Conclusions

This study utilizes granite-derived manufactured sand as a complete quartz sand replacement in the formulation of ultra-high-performance concrete (UHPC). By integrating experimental evaluation of fresh-state workability and 7d-mechanical properties with quantitative microstructural characterization, the study systematically elucidates the influence mechanism of stone powder content in manufactured sand on UHPC performance. The principal findings are summarized as follows:

(1)

When the manufactured sand is graded into three size fractions—1.18–0.60 mm, 0.60–0.30 mm, and 0.30–0.15 mm—with a mass distribution of 40:30:30, and the cementitious system comprises Portland cement, silica fume, and fly ash in mass proportions of 63:19:18, ultra-high-performance concrete (UHPC) incorporating manufactured sand as the sole fine aggregate achieves excellent fresh-state workability and enhanced mechanical performance through an optimized mix design. The finalized composition is: Portland cement, 738 kg/m³; silica fume, 230 kg/m³; fly ash, 210 kg/m³; granite-derived manufactured sand, 943 kg/m³; straight steel fibers, 196 kg/m³; polycarboxylate-based superplasticizer, 24 kg/m³; and a water-to-binder ratio of 0.16. Under this formulation, the UHPC exhibits a fresh-state workability of 185 mm and attains a 7-day compressive strength of 152.4 MPa.

(2)

The stone powder content in manufactured sand exerts a pronounced influence on both the fresh-state workability and mechanical performance of UHPC, encompassing compressive, tensile, flexural, and shear strengths. As the stone powder content increases from 0% to 5%, 10%, and 15% by mass of fine aggregate, UHPC workability first rises then declines, peaking at 194 mm with 10% stone powder—exceeding that of the quartz sand reference mixture (prepared with quartz sand as the sole fine aggregate) by 2.1% under identical mix proportions. Concurrently, all four mechanical properties improve progressively with rising stone powder content and attain their maxima at 10%. Specifically, compressive strength reaches 160.3 MPa (+8.8%), tensile strength 10.43 MPa (+45.5%), flexural strength 24.18 MPa (+8.9%), and shear strength 40.57 MPa (+6.5%), relative to the quartz sand reference values of 147.3 MPa, 7.17 MPa, 22.19 MPa, and 38.09 MPa, respectively.

(3)

Microstructural characterization of manufactured sand-based ultra-high-performance concrete (MS-UHPC) reveals that an optimal stone powder content (10 wt.%) accelerates the hydration of the cementitious paste, thereby promoting the formation of calcium silicate hydrate (C-S-H) and other hydration products. Concurrently, it improves particle packing density, yielding a more homogeneous and densified microstructure in the hardened matrix. SEM and XRD results show that at a 10% stone powder content, the hydration of clinker phases, particularly C₃S and C₂S, is markedly accelerated; unhydrated particles are uniformly enveloped by a continuous C-S-H gel network, resulting in a compact and low-defect cementitious matrix. MIP further demonstrates that this stone powder dosage minimizes total porosity to 2.06%, while suppressing highly harmful pores (≥200 nm) and harmful pores (100–200 nm) to 30.4% and 3.1% of the total pore volume, respectively. In contrast, the combined volume fraction of slightly harmful (20–50 nm) and harmless (<20 nm) pores reaches its maximum of 66.5%. Collectively, these microstructural refinements—improved particle packing, and optimized pore structure—directly underpin the superior mechanical performance of MS-UHPC.