Performance and Mesoscopic Simulation of Self-Compacting Concrete Made with Different Lithological Types of Manufactured Sand

Abstract

The development of green building materials and high-performance concrete has promoted the use of manufactured sand (MS) in self-compacting concrete (SCC). To investigate the effect of MS lithology on concrete performance, this study prepared C40-SCC using basalt, limestone, and granite manufactured sand, as well as river sand. Workability and mechanical properties were measured via macro-scale tests. A meso-scale random aggregate model, including mortar, aggregate, and interfacial transition zone (ITZ), was established to simulate uniaxial compression. The macro-test results indicate that workability decreases in the order of river sand, granite, limestone, and basalt, while mechanical strength decreases in the order of granite, limestone, basalt, and river sand. The meso-scale simulation reveals that damage initiates at the ITZ and extends into mortar. The simulated stress–strain curves match the experimental data in the ascending branch, with peak stress errors between 1.1% and 6.9%. The failure modes also align with experimental observations. The consistency between the simulation and experimental results verifies the reliability of the meso-scale model. By combining macro-experiments and meso-simulation, this study compares concrete performance and explains the differences from the perspective of damage evolution. The results indicate that MS lithology affects interfacial properties and damage development, thereby determining macro-mechanical behavior. This research provides a theoretical basis for the appropriate selection of MS in SCC.

1. Introduction

Amid the rapid development of global infrastructure, the demand for sand and gravel aggregates in the construction industry continues to rise. The over-exploitation of natural sand—a finite resource—has led to severe environmental and ecological degradation. In this context, manufactured sand (MS) has emerged as a key sustainable alternative, owing to the abundance of its raw material sources and its lower environmental footprint [1,2]. Meanwhile, self-compacting concrete (SCC), known for its excellent workability, can uniformly fill and achieve dense compaction in complex structural forms without the need for external vibration, thereby enhancing construction efficiency and quality. These advantages align closely with the growing emphasis on green and sustainable construction practices [3].

The physical and chemical properties of MS are primarily determined by the lithology of its parent rock, which represents a fundamental distinction from natural river sand. This inherent difference leads to complex variations in the macroscopic performance of self-compacting concrete incorporating manufactured sand (MS-SCC). Previous studies [4,5] have indicated that MS generally impairs the workability of SCC, while its influence on mechanical properties remains inconsistent. Some research [6,7] suggests that the rough surface texture of MS enhances interfacial bonding, thereby improving concrete strength—a phenomenon that can be characterized as an aggregate interface enhancement effect. In contrast, other scholars [8,9] argue that particle shape and surface texture have a limited effect on strength development. Furthermore, the particle size distribution of MS may exert a more significant influence on mechanical behavior than lithology alone [10]. Additionally, researchers [11] have noted that differences in the micro-morphology and particle gradation of MS are the main reasons for the reduction in workability, reflecting the dominant effect of particle morphology and gradation. These contradictions reveal two competing perspectives on the role of MS: one emphasizes its interface enhancement advantages, while the other focuses on the negative impacts of its morphology and gradation. The interplay between these two effects is difficult to disentangle from the macro-scale alone, hindering a comprehensive understanding of how MS lithology influences SCC performance.

To reveal the meso-scale mechanisms underlying this controversy, numerical simulation approaches that investigate failure mechanisms from the internal material structure have become effective means for deepening understanding. Since Zaitsev [12] first proposed a two-dimensional “aggregate–mortar” two-phase model for concrete, meso-scale simulation techniques have undergone continuous advancement. Cundall [13] introduced the random particle model (RPM), postulating that failure occurs primarily in the softer mortar matrix, with aggregates treated as rigid bodies. Bažant [14] refined the RPM by regarding aggregates as linearly elastic materials and incorporating the interfacial transition zone (ITZ). Subsequent improvements gradually evolved into the discrete element method (DEM). This method offers distinct advantages in simulating the transition of materials from continuum to discontinuum, such as the dynamic fragmentation process of concrete [15]. Recent developments have further extended DEM to incorporate multi-field couplings for more sophisticated analyses of concrete behavior [16]. Mohamed and Hansen [17] proposed a meso-mechanical model (M-H model), applying a random crack approach to describe the tensile failure of concrete. This approach, which explicitly considers the aggregate–mortar mesostructure and the randomness of aggregate distribution, has been further developed and validated in subsequent studies [18,19]. The random aggregate model developed by Wang Zongmin and Liu Guangyan [20,21,22] characterizes concrete as a three-phase composite of aggregate, mortar, and ITZ, which has since become a key research tool. Based on this model, several studies [23,24,25] have demonstrated through simulation that damage initiates at the ITZ due to stress concentration. Jin [26] established a refined two-dimensional meso-scale numerical model by integrating prior approaches, offering a scientific foundation for material optimization. Similarly, Zhang et al. [27] analyzed the uniaxial compressive failure process of concrete using a meso-scale random aggregate model.

Recent advances in three-dimensional (3D) meso-scale modeling have enabled more realistic simulations of aggregate spatial distribution and crack propagation [28,29,30]. However, these studies—whether two-dimensional (2D) or 3D—have focused primarily on conventional concrete systems. Systematic investigations into the meso-scale behavior of SCC incorporating different manufactured sand lithologies remain scarce.

Given that the present study focuses on a comparative analysis of lithological effects under identical modeling conditions, a 2D meso-scale approach was adopted. This choice is supported by the fact that 2D models have been widely validated for comparative studies of material heterogeneity and damage mechanisms [31], while offering significant computational efficiency for parametric analyses.

In summary, current research is characterized by two key gaps. First, findings concerning the macroscopic properties of MS-SCC prepared with different lithologies remain inconsistent and have not been systematically compared. Second, the mechanistic connection at the meso-scale between the lithological characteristics of MS and the resulting variations in SCC performance has yet to be adequately established. To address these issues, three types of MS—basalt, limestone, and granite—were selected, along with natural river sand as a control group. These lithologies were chosen based on both their geological significance and local availability in the Shaanxi region of China, where they are commonly used for MS production. In the experimental phase, the influence of these sands on the workability and macro-mechanical properties of the four groups of SCC was first investigated. Subsequently, based on experimentally obtained material parameters, a two-dimensional random aggregate model incorporating mortar, aggregate, and the ITZ was developed to simulate the uniaxial compression process at the meso-scale. Through this integrated experimental–simulation approach, the study aims not only to compare macroscopic performance but also to elucidate, from the perspective of mesoscopic damage evolution, the mechanism by which different lithological MS types influence the mechanical behavior of SCC. The findings are expected to lay a theoretical foundation for the rational selection of MS and the performance optimization of SCC.

2. Materials and Methods

2.1. Raw Materials

The cement used was Conch P·O 42.5 ordinary Portland cement; its chemical composition is presented in

Table 1. Main Chemical Compositions of P·O 42.5 Portland Cement (%).

SiO2	Al2O3	Fe2O3	CaO	MgO	SO3	R2O	Loss on Ignition
22.15	5.63	3.79	63.84	1.62	2.74	0.61	1.74

. Grade I fly ash, supplied by Gongyi Borun Refractory Materials Co., Ltd. (Gongyi, China), was employed, and its key properties are listed in

Table 2. Basic Properties of Grade I Fly Ash.

Apparent Density (g/cm3)	Bulk Density (g/cm3)	Residue on Sieve (%)	Loss on Ignition (%)	Water Content (%)	Al2O3 (%)	SiO2 (%)	Fe2O3 (%)	CaO (%)
2.55	1.12	16	2.8	0.85	24.2	45.1	5.6	2.1

. Silica fume, selected as a high-activity microsilica powder, was sourced from Zhengzhou Henghuo Filter Materials Co., Ltd. (Zhengzhou, China). A polycarboxylate-based high-performance water reducer (superplasticizer) produced by Qinfen Building Materials (Xi’an, China) was used, with a water reduction rate of 28%.

The fine aggregates comprised natural river sand from the Xi’an region of Shaanxi Province and MS processed from basalt, limestone, and granite parent rocks sourced from the Hanzhong region (Figure 1). All sands were classified as Zone II medium sand. The fineness moduli were 2.6 for river sand and 2.74, 2.99, and 2.86 for basalt, limestone, and granite MS, respectively. Coarse aggregate consisted of 5–20 mm crushed limestone. The particle size distributions of all aggregates are presented in Figure 2, and the relevant basic properties of the manufactured sands are provided in

Table 3. Basic Properties of Manufactured Sand.

Item	Basalt	Limestone	Granite
Stone Powder Content (%)	9.5	3.3	6.7
MB Value	1.3	0.7	0.5

.

2.2. Phase Analysis of MS by XRD

The three types of lithological MS were ground to pass through a 320-mesh sieve (approximately 47 µm) and subsequently oven-dried to remove moisture prior to analysis. Each prepared sample was mounted in a holder to ensure a flat surface and minimize preferred orientation. Phase identification was conducted using a D8 Advance powder X-ray diffractometer. Data collection was carried out under standard instrumental conditions, and the resulting diffraction patterns were recorded. The mineral compositions of the three MS types are presented in Figure 3, while the major chemical components of their parent rocks are summarized in

Table 4. Main Chemical Compositions and Contents of MS Parent Rocks (%).

Parent Rock Type	SiO2	Al2O3	Fe2O3	CaO	MgO	Na2O	K2O	Others
Basalt	48.7	14.4	12.7	8.6	3.1	2.5	1.2	8.8
Limestone	16.3	1.0	1.1	59.0	4.2	0.9	0.4	17.1
Granite	69.4	12.1	2.7	1.9	2.2	1.8	0.4	9.5

.

As shown in Figure 3, the basalt MS consists primarily of quartz and albite, reflecting its mafic volcanic origin. The limestone MS is composed predominantly of calcite, with its characteristic diffraction peaks clearly evident in the corresponding pattern. Granite MS consists mainly of quartz and feldspar, both of which exhibit distinct peaks in the XRD pattern.

X-ray fluorescence (XRF) analysis was performed on the parent rocks of the different MS types, and the results are summarized in Table 4. The basalt MS is characterized by an SiO₂ content of 48.7%. In the limestone MS, calcium (expressed as CaO) accounts for up to 59% of the composition. The granite MS is predominantly composed of SiO₂ and Al₂O₃, which together account for over 80% of its chemical composition.

2.3. Mix Proportion Design

The mix proportion for C40-SCC was designed following the method described in Ref. [32]; the results are presented in

Table 5. Mix Proportion of C40 Self-Compacting Concrete (kg·m⁻³).

Fine Aggregate	Cement	Silica Fume	Fly Ash	River Sand	MS	Crushed Stone	Admixture	Water
Basalt MS	403.5	21.2	106.2	0	826.8	784	5.3	196.5
Limestone MS	403.5	21.2	106.2	0	826.8	784	5.3	196.5
Granite MS	403.5	21.2	106.2	0	826.8	784	5.3	196.5
River Sand	403.5	21.2	106.2	746.0	0	784	5.3	196.5

. Moreover, given that basalt MS has a higher specific surface area, greater porosity, and significantly higher water absorption compared to other lithological types of MS [33], all four fine aggregates were pre-soaked prior to mixing to achieve a saturated surface-dry condition. A flowchart illustrating the specimen preparation procedure is presented in Figure 4.

2.4. Test Methods

To evaluate the workability of the four SCC mixtures, tests for slump flow, T₅₀₀ time, J-ring flow, and V-funnel flow time were conducted in accordance with standards [34,35]. For each mixture, six 100 mm cubes were cast to determine the cube compressive strength and splitting tensile strength at 7, 14, 28, and 56 days. In addition, three 100 mm × 100 mm × 300 mm prismatic specimens were prepared per mixture. After 28 days of standard curing, these prisms were used to measure the axial compressive strength and to perform stress–strain tests. All specimens were demolded 24 h after casting and then cured in a standard curing room at a temperature of 20 ± 2 °C and relative humidity of no less than 95% until the designated test age.

3. Results

3.1. Workability

All four SCC mixtures satisfied the workability requirements specified in the standards [34,35]. The testing procedure is illustrated in Figure 5, and the measured data are summarized in

Table 6. Workability indicators of four types of SCC.

Lithological Types	Slump Flow (mm)	T500 Flow Time (s)	V-Funnel Time (s)	J-Ring Flow Difference (mm)
Basalt	580	8	20	43
Limestone	665	6	16	28
Granite	701	5	12	22
River Sand	718	4	11	17

.

As shown in Table 6, the slump flow of each mixture fell within the specified limits. Flowability, ranked from highest to lowest, was as follows: river sand, granite, limestone, and basalt. The V-funnel flow times ranged from 11 to 20 s, all meeting the acceptable criteria. In terms of passing ability evaluated by the J-ring test, the mixtures with granite and river sand satisfied the second-grade requirement (blocking step height difference of 0–25 mm), while those with limestone and basalt met the first-grade requirement (difference of 25–50 mm).

3.2. Mechanical Properties

3.2.1. Cubic Compressive Strength of SCC

The cube compressive strength of SCC specimens prepared with the three lithological types of MS and with natural river sand was measured at different ages. Testing was conducted using a WHY-2000 computer-controlled compression testing machine, as shown in Figure 6. The typical failure modes observed during testing are illustrated in Figure 7.

The development of cube compressive strength of the SCC specimens over different curing ages is shown in Figure 8. With increasing curing time, the compressive strength of all four SCC mixtures continued to increase. At 7 days, the strengths reached approximately 79.87% for basalt MS-SCC, 80.99% for limestone MS-SCC, 74.28% for granite MS-SCC, and 80.20% for river sand SCC of their respective 28-day values. Among the four mixtures, limestone MS-SCC exhibited the highest strength at 7 days, reaching 38.82 MPa.

After 14 days of curing, all four SCC mixtures reached the designated C40 strength. With further curing to 28 and 56 days, the compressive strength (from highest to lowest) followed the order: granite MS-SCC, limestone MS-SCC, basalt MS-SCC, and river sand SCC. Among the three MS-SCC types, granite exhibited the highest ultimate strength.

3.2.2. Splitting Tensile Strength of SCC

The splitting tensile strength of SCC specimens prepared with the three lithological types of MS and with river sand was determined at different ages. Testing was conducted using the WHY-2000 computer-controlled testing machine, as shown in Figure 9. The characteristic failure modes observed during the splitting tensile tests are illustrated in Figure 10.

As the curing age increased, the splitting tensile strength of all four SCC mixtures developed progressively, as illustrated in Figure 11. At 7 days, the strengths reached approximately 74.59% for basalt MS-SCC, 77.44% for limestone MS-SCC, 71.15% for granite MS-SCC, and 69.30% for river sand SCC of their respective 28-day values. Limestone MS-SCC exhibited the highest splitting tensile strength at this age, reaching 3.33 MPa.

Throughout the curing periods (14, 28, and 56 days), the splitting tensile strength of the four specimen groups followed the order from highest to lowest: granite MS-SCC, limestone MS-SCC, basalt MS-SCC, and river sand SCC. This ranking was consistent with that observed for compressive strength.

3.2.3. Axial Compressive Strength of SCC Prisms

Uniaxial compression tests were performed on the four groups of SCC prismatic specimens. The test setup and instrumentation are shown in Figure 12, and the resulting failure patterns are illustrated in Figure 13. The failure was characterized primarily by major inclined cracks, with the angle between these cracks and the vertical axis ranging approximately from 60° to 69°.

The axial compressive strengths obtained from the four groups of specimens are summarized in

Table 7. Cubic and axial compressive strengths of the four SCC types.

Category	Cubic Compressive Strength (MPa)	Axial Compressive Strength (MPa)	Ratio
Basalt	45.20	37.47	0.83
Limestone	47.93	41.90	0.87
Granite	49.81	43.83	0.88
River Sand	43.89	35.99	0.82

. The corresponding stress–strain curves will be analyzed in the subsequent section on meso-scale simulation.

As shown in Table 7, relative to the axial compressive strength of river sand SCC, the strength differences for basalt, limestone, and granite MS-SCC are −1.48, −5.91, and −7.84 MPa, respectively. The ratios of axial to cube compressive strength for all four groups of specimens exceed the reference value of 0.76 [36], ranging from 0.82 to 0.88. These results indicate a more uniform internal structure, a balanced distribution of strength in different directions, and improved overall mechanical performance.

3.3. Phase Analysis of Four SCC Types by XRD

Cement hydration is primarily governed by the reactions of its main clinker minerals. The hydration of the four major cement phases can be described by the following equations:

$$2\left(3\mathrm{CaO}\cdot {\mathrm{SiO}}_2\right){+6\mathrm{H}}_2\mathrm{O}=3\mathrm{CaO}\cdot {2\mathrm{SiO}}_2\cdot {3\mathrm{H}}_2\mathrm{O}+3\mathrm{Ca}{\left(\mathrm{OH}\right)}_2$$

(1)

$$2\left(2\mathrm{CaO}\cdot {\mathrm{SiO}}_2\right){+4\mathrm{H}}_2\mathrm{O}=3\mathrm{CaO}\cdot {2\mathrm{SiO}}_2\cdot {3\mathrm{H}}_2\mathrm{O}+3\mathrm{Ca}{\left(\mathrm{OH}\right)}_2$$

(2)

$$3\mathrm{CaO}\cdot {\mathrm{Al}}_2{\mathrm{O}}_3{+6\mathrm{H}}_2\mathrm{O}=3\mathrm{CaO}\cdot {\mathrm{Al}}_2{\mathrm{O}}_3\cdot {6\mathrm{H}}_2\mathrm{O}$$

(3)

$$4\mathrm{CaO}\cdot {\mathrm{Al}}_2{\mathrm{O}}_3\cdot {\mathrm{Fe}}_2{\mathrm{O}}_3{+7\mathrm{H}}_2\mathrm{O}=3\mathrm{CaO}\cdot {\mathrm{Al}}_2{\mathrm{O}}_3\cdot {6\mathrm{H}}_2\mathrm{O}+\mathrm{CaO}\cdot {\mathrm{Fe}}_2{\mathrm{O}}_3\cdot {\mathrm{H}}_2\mathrm{O}$$

(4)

The main hydration products—calcium silicate hydrate (C-S-H) from Equations (1) and (2), and ettringite (AFt) from Equations (3) and (4)—are the primary contributors to concrete strength development. The chemical composition of the manufactured sands (Table 4) may influence these reactions by providing additional ions or altering the local chemical environment, thereby affecting the hydration product characteristics observed in the XRD analysis.

The XRD patterns of the four SCC mixtures are shown in Figure 14. All specimens exhibit diffraction peaks corresponding to quartz, calcite, albite, dicalcium silicate, calcium hydroxide (CH), and ettringite (AFt). In addition, broad humps characteristic of poorly crystalline C-S-H are observed in all patterns. Notably, the intensity of the AFt peaks and the prominence of the C-S-H hump are more pronounced in granite MS-SCC and limestone MS-SCC than in basalt MS-SCC and river sand SCC. Meanwhile, the calcite peak is strongest in limestone MS-SCC, consistent with its high calcium carbonate content.

The differences in C-S-H and AFt features among the three lithologies can be further understood by considering the chemical composition of the manufactured sands (Table 4) and their potential roles in the hydration reactions:

(1)

For limestone MS (CaO = 59.0%), the high calcium oxide content may dissolve in the pore solution, providing abundant Ca²⁺ ions that could directly participate in the hydration of C₃S and C₂S (Equations (1) and (2)), potentially promoting the formation of C-S-H gel and AFt. This is consistent with the stronger AFt peaks and more evident C-S-H hump observed in limestone MS-SCC.

(2)

For granite MS (SiO₂ = 69.4%), although quartz is largely inert, amorphous silica on particle surfaces may react with CH (a product of Equations (1) and (2)) through pozzolanic reactions at later ages, possibly generating additional C-S-H. This mechanism may account for the prominent C-S-H hump in granite MS-SCC, particularly at later ages.

(3)

For basalt MS, the presence of Fe₂O₃ (12.7%) and MgO (3.1%) may influence hydration products. Fe³⁺ can partially substitute for Al³⁺ in AFt and AFm phases derived from C₃A and C₄AF hydration (Equations (3) and (4)), potentially affecting their crystallinity; Mg²⁺ may incorporate into the C-S-H structure, altering its morphology and density. These factors may contribute to the less defined XRD features observed in basalt MS-SCC.

It should be noted that XRD peak intensities are influenced by both the amount and the degree of crystallinity of the phases; therefore, the observed differences in AFt peaks and C-S-H humps should be interpreted qualitatively. Nevertheless, the trends observed in XRD are consistent with the mechanical performance data in Section 3.2: granite MS-SCC and limestone MS-SCC, which exhibit stronger AFt and C-S-H signatures, also show higher compressive and splitting tensile strengths, while basalt MS-SCC and river sand SCC with weaker signatures have lower strengths. This consistency suggests that the lithology of manufactured sand may influence the development of hydration products, thereby affecting macro-mechanical properties.

The above experimental results demonstrate that variations in the macroscopic properties of the different SCC mixtures are governed not only by their microscopic hydration products but also by the distinct damage evolution behaviors within the material’s meso-structure under mechanical loading—particularly at the aggregate–mortar interface (ITZ). To elucidate this process mechanistically, the following section presents a meso-scale numerical simulation for in-depth analysis.

4. Analysis Based on a Two-Dimensional Mesoscopic Numerical Model

4.1. Model Establishment

The experimental results indicate that MS-SCC specimens with different lithologies exhibit significant variations in macroscopic mechanical properties. To investigate the underlying meso-scale damage evolution mechanisms responsible for these differences, a two-dimensional random aggregate model (100 mm × 300 mm) of SCC was established based on the concept of a three-phase composite material comprising mortar, aggregate, and the ITZ (Figure 15). For each SCC type, three meso-scale models with distinct random aggregate distributions were generated to account for material heterogeneity.

Given the rough and irregular surface morphology of manufactured sand, aggregate particles were simplified as circular shapes for computational efficiency. A two-grade concrete system was adopted, with coarse aggregates divided into two size ranges (5–10 mm and 10–16 mm) and occupying a total volume fraction of 28.5%. The Fuller gradation curve was used to describe the aggregate distribution. To account for the dimensional difference between two-dimensional planar aggregates and actual three-dimensional particles, the Walraven formula was applied for correction. The Fuller gradation curve is given by Equation (5), and the Walraven formula by Equation (6):

$$P=\sqrt{{\displaystyle \frac{D}{D_{\max }}}}$$

(5)

$$P\left(D<D_0\right)=P_k\left[1.065{\left({\displaystyle \frac{D_0}{D_{\max }}}\right)}^{0.5}-0.053{\left({\displaystyle \frac{D_0}{D_{\max }}}\right)}^4-0.012{\left({\displaystyle \frac{D_0}{D_{\max }}}\right)}^6-0.0045{\left({\displaystyle \frac{D_0}{D_{\max }}}\right)}^8+0.0025{\left({\displaystyle \frac{D_0}{D_{\max }}}\right)}^{10}\right]$$

(6)

where D is the aggregate particle size; D_max is the maximum aggregate size; D₀ is the mesh size; P is the proportion of aggregate with particle size D in the total aggregate; P(D < D₀) is the probability when D < D₀; and P_k is the volume fraction of aggregate in the total concrete volume.

The base of the compression specimen model was fixed in all degrees of freedom. Loading was applied under displacement control, with a stepwise displacement increment of 0.001 mm imposed at the prescribed loading point. The overall loading configuration is illustrated in Figure 15. A finite-element mesh was then generated for the meso-scale concrete model.

4.2. Material Constitutive Relations and Parameters

4.2.1. Determination of Constitutive Relationship

Two types of mortar were defined: one incorporating manufactured sand and the other incorporating natural river sand. Since the mechanical behavior of the mortar matrix resembles that of concrete [37,38,39], a piecewise damage-plasticity constitutive model was adopted. Given its similar composition, the ITZ was treated as a mechanically weakened version of the mortar and was therefore assigned the same constitutive law. Natural coarse aggregates, which exhibit high strength and typically remain intact under ordinary loading conditions, were modeled as linearly elastic materials.

4.2.2. Determination of Material Parameters

Regarding the determination of material parameters, the mechanical properties of the mortar matrix—specifically for manufactured sand self-compacting mortar, which differs from ordinary concrete mortar—were obtained directly from laboratory tests conducted by our research group. Mortar specimens of two sizes were prepared—70.7 mm cubes and 70.7 mm × 70.7 mm × 220 mm prisms—under the same curing conditions as the concrete specimens. The tests were performed in accordance with Chinese standards JGJ/T 70-2009 [40] and DL/T 5150-2017 [41]. The experimentally derived parameters were used as direct inputs for the meso-scale model.

The properties of the ITZ were defined relative to the mortar using a scaling factor, with the ITZ-to-mortar parameter ratio set to 0.8 [42]. This approach was adopted to isolate the effect of lithology as the sole variable—by keeping ITZ parameters constant across all mixtures, any observed differences in mechanical behavior can be directly attributed to the intrinsic characteristics of the manufactured sands. It is acknowledged that different lithologies may influence the actual ITZ microstructure; however, applying the same factor ensures a consistent baseline for comparative analysis. Considering computational efficiency and established practices [43], the ITZ thickness was taken as 1 mm.

Coarse aggregate, modeled as ordinary crushed stone, was assigned an elastic modulus of 70 GPa and a Poisson’s ratio of 0.2. The key mechanical parameters for each meso-scale component are summarized in

Table 8. Mechanical parameters of the mesoscopic three-phase materials.

Parameter	Mortar				Interfacial Transition Zone				Coarse Aggregate
Lithology	Basalt	Limestone	Granite	River Sand	Basalt	Limestone	Granite	River Sand	Crushed Stone
Elastic Modulus (GPa)	23.93	24.11	24.89	23.67	19.14	19.29	20.91	18.94	70
Poisson’s Ratio	0.20	0.20	0.20	0.20	0.20	0.20	0.20	0.20	0.20
Dilatancy Angle	30	30	30	30	30	30	30	30	30
Compressive Strength (MPa)	45.7	46.80	51.80	44.2	36.56	37.44	41.44	35.36
Tensile Strength (MPa)	3.85	3.96	4.28	3.70	3.08	3.17	3.42	2.96

.

Based on the plastic damage model and the mechanical parameters listed in Table 8, the resulting stress–strain relationships for both mortar and the ITZ are illustrated in Figure 16.

4.3. Validation of the Meso-Scale Numerical Model

4.3.1. Comparison of Simulated and Experimental Stress–Strain Curves

Based on the mechanical tests presented in Section 3.2, uniaxial compression simulations of the four SCC types were performed using the parameters listed in Table 8. The simulated stress–strain curves were then compared with the corresponding experimental results, as shown in Figure 17.

As shown in Figure 17, the stress–strain curves of the four SCC types are closely aligned in the ascending branch and follow comparable trends in the descending branch, all exhibiting marked brittle behavior. However, they are not identical. This indicates that the random distribution of aggregate positions exerts a limited effect on the macroscopic ascending branch of the stress–strain curve but significantly influences crack propagation, resulting in variability in the descending branch.

Table 9. Comparison of axial compressive strength for the four SCC types (MPa).

Serial Number	Basalt	Limestone	Granite	River Sand
Simulated Value	39.87	40.78	44.74	36.57
	40.36	41.54	44.33	36.28
	40.04	41.29	44.41	36.36
Simulated Average Value	40.09	41.20	44.49	36.40
Experimental Value	37.47	41.90	43.83	35.99
Error	6.9%	1.7%	1.5%	1.1%

presents a comparison of the axial compressive strengths for the four types of SCC. As shown, the errors between the simulated and experimental axial compressive strengths are 6.9% for basalt MS-SCC, 1.7% for limestone MS-SCC, 1.5% for granite MS-SCC, and 1.1% for river sand SCC. These results indicate that the material parameters assigned in the meso-scale concrete model established in this study are close to those of the actual specimens, and the simulation results show good agreement with the experimental outcomes.

The comparison between simulation results and experimental data for the prismatic MS-SCC specimens validates the reliability of the established two-dimensional meso-scale model.

4.3.2. Comparison of SCC Failure Modes Between Simulation and Experiment

The simulated compression failure modes obtained from the meso-scale model for basalt, limestone, and granite MS-SCC, as well as for river sand SCC, are compared with their corresponding experimental failure patterns and crack propagation characteristics in Figure 18.

As shown in Figure 18, the experimental failure of the SCC prism specimen typically appears as a single dominant inclined crack. In contrast, the meso-scale simulation yields an X-shaped cracking pattern. This difference arises from the model’s ability to comprehensively account for initial defects within the concrete’s internal components (aggregates and mortar) and to track the initiation and growth of micro-cracks in multiple directions. In a macroscopic test, however, only the most critical initial defect aligns with the principal stress direction and develops into a through-thickness inclined crack during the final loading stage. Once this dominant crack forms and the specimen undergoes unloading, other micro-cracks that have not yet reached macroscopic visibility cease to develop. As a result, only a single inclined crack is observed experimentally.

To further validate the simulation results, the inclination angles of the final main cracks were measured from the damage contour plots for each lithology, as annotated in Figure 18. The simulated crack angles range from 60° to 61°, which falls within the experimentally observed range of 60–69° reported above. This quantitative agreement indicates that, although the simulation predicts X-shaped cracking while experiments show single inclined cracks, the fundamental shear failure mechanism along a critical plane is consistently captured. The similarity in crack inclination angles provides additional confidence that the meso-scale model accurately represents the failure behavior of SCC under uniaxial compression.

Therefore, the failure modes observed in the meso-scale simulations and the corresponding experiments for the SCC prism specimens are generally consistent. This agreement confirms that the meso-scale model can effectively represent the macroscopic failure behavior of the material, thereby validating its reliability. Furthermore, in the actual specimens, variations in the position of coarse aggregates lead to differences in the final number and propagation paths of cracks—an effect that is also captured in the simulations. These observed variations demonstrate that the meso-scale heterogeneity of the material directly governs its macroscopic failure pattern.

In summary, the close agreement between the simulated stress–strain curves, failure modes, and experimental results confirms the validity of the meso-scale numerical model developed in this study.

4.4. Analysis of Damage Evolution in SCC Prisms Using the Mesoscopic Model

Based on the validated model, this section analyzes the complete damage evolution process of SCC prism specimens with different lithologies under uniaxial compression. The simulations reveal that, although all specimens follow a similar failure sequence—damage initiates at the interface, propagates steadily, and culminates in macroscopic fracture—their damage evolution characteristics differ significantly.

4.4.1. Comparison of Damage Evolution Characteristics for Different Lithologies

As shown in Figure 19, Figure 20, Figure 21 and Figure 22, the meso-scale simulation results reveal that differences in the interfacial bond strength between aggregates and the cement matrix—resulting from lithological variations among the four SCC types—influence the location and distribution of microcrack initiation. Moreover, notable differences were also observed in the timing of damage onset across the SCC types. The ITZ of granite MS-SCC demonstrated the highest resistance to damage initiation, with microcracking beginning when the applied stress reached 35.1% of the peak stress. In contrast, damage initiated earlier in basalt MS-SCC, occurring at 31.7% of the peak stress. Limestone MS-SCC exhibited intermediate behavior, with damage initiating at 33.2% of the peak stress. This progression indicates that, among the materials studied, granite MS-SCC possesses the strongest resistance to microcrack initiation.

The differences in damage evolution described above directly contribute to the variations in macroscopic mechanical performance observed in Figure 8 and Figure 11. Granite MS-SCC, with its most delayed onset of damage and slowest propagation rate, achieves the highest compressive and splitting tensile strengths, and its stress–strain curve is characterized by a relatively gentle post-peak descending branch, indicating better ductility. Basalt MS-SCC, due to its earlier damage initiation and rapid propagation, exhibits the lowest strength and a more abrupt failure process, accompanied by faster post-peak load degradation. Limestone MS-SCC displays intermediate characteristics, consistent with its macroscopic performance ranking.

4.4.2. Quantitative Analysis of Damage Evolution

To quantitatively characterize the damage evolution process, the cumulative damage area was extracted from the simulation results. Figure 23 presents the relationship between the cumulative damage area and loading step for granite MS-SCC as a representative example.

As shown in the figure, with increasing loading steps, the cumulative damage area exhibits distinct stage-wise evolution characteristics:

(1)

Damage initiation stage: the cumulative damage area remains zero, indicating that the specimen remains in an undamaged state.

(2)

Rapid growth stage: damage initiates and accumulates rapidly. The cumulative damage area increases sharply from 20.08 mm² at step 16 to 3476.9 mm² at step 100, corresponding to the initiation, propagation, and coalescence of micro-cracks within the specimen. This stage reflects the transition from stable micro-crack growth to unstable crack propagation.

(3)

Macroscopic failure stage: the growth rate of the cumulative damage area gradually decreases, and the curve tends to flatten, eventually reaching 6561.9 mm² at loading step 376, indicating that damage evolution has approached saturation and macroscopic failure has occurred.

This quantitative analysis confirms that granite MS-SCC exhibits a clear damage initiation threshold and a progressive damage development process. The same methodology can be applied to other lithologies to comprehensively compare the damage evolution rates and toughness differences among the various MS-SCC mixtures.

In summary, the lithology of manufactured sand fundamentally governs the macroscopic damage and failure behavior of SCC by influencing both the microstructure of the mortar matrix—as evidenced by variations in hydration products (XRD)—and the performance of the meso-scale aggregate–mortar interface. Essentially, SCC with higher mechanical strength and enhanced macro-performance exhibits greater resistance to damage initiation and slower damage propagation at the meso-scale. The meso-scale simulation analysis thus directly elucidates the intrinsic material failure mechanisms that account for the observed differences in macroscopic mechanical behavior.

5. Discussion

This study examined the influence of manufactured sand lithology on the performance of SCC through macroscopic experiments and meso-scale simulations. The macroscopic experimental results showed that granite-based MS-SCC exhibited the best overall performance: slump flow of 701 mm, T500 time of 5 s, and 28-day compressive and splitting tensile strengths of 49.81 MPa and 4.61 MPa, respectively—values that were 3.8% and 6.7% higher than those of limestone-based MS-SCC, and 9.3% and 7.8% higher than those of basalt-based MS-SCC. Granite was characterized by low early strength but high medium-to-late strength, whereas limestone exhibited high early strength followed by a more moderate increase in the medium-to-late stage.

The observed differences in macroscopic performance can be attributed to the physical characteristics of the manufactured sands. As described in the literature [44,45], an appropriate amount of stone powder can enhance both workability and mechanical properties by reducing porosity through three mechanisms: physical water reduction, lubrication, and water retention and thickening. Based on the data in Table 4, the stone powder content of granite MS was 6.7%, while that of basalt MS was 9.5%. In comparison, the relatively higher stone powder content of basalt MS increases paste viscosity, disrupts the optimal dense packing structure, and may hinder the normal hydration of cement, ultimately resulting in lower workability and compressive strength. In contrast, limestone MS had a stone powder content of 3.3%, which is below the optimal range, and its performance fell between the two extremes.

From the perspective of methylene blue (MB) value, granite exhibited the lowest MB value (0.5), indicating a very low content of expansive clays, which is beneficial for ITZ quality and cement–aggregate bonding. Basalt had the highest MB value (1.3); in such cases, clay minerals tend to coat aggregate particles, hindering cement hydration and weakening the ITZ. Limestone had an intermediate MB value (0.7), with performance lying between the two extremes.

Regarding water absorption, basalt MS exhibited higher water absorption compared to granite and limestone, which further reduces the free water available for lubrication and exacerbates its poor workability. In contrast, the lower water absorption of granite MS helps maintain the effective water content in the mixture, contributing to its superior workability among the manufactured sands. Furthermore, compared to river sand, all manufactured sand particles have rougher surfaces and greater angularity, which increases particle–paste friction and reduces flowability. The fineness moduli of the various manufactured sands were similar, indicating that gradation was not the primary factor causing the performance differences.

At the microstructural level, the strength development characteristics of the three lithologies can be further explained by their chemical composition and hydration behavior, as revealed by XRD analysis. The high early strength of limestone MS-SCC is attributed to its high calcium content of 59% (Table 4), which promotes cement hydration. During the initial hydration process, calcium accelerates the formation of C-S-H between the fine aggregate and cement paste, filling microscopic pores and enhancing the compactness of the concrete. This finding is consistent with the observation in the literature [46] that “a comparison of limestone with siliceous aggregates shows that the former typically provides higher compressive strength at the same degree of hydration”.

Granite MS-SCC exhibits low early strength but high medium-to-late strength. This behavior can be attributed to two factors: firstly, its dense structure and low water absorption may not be fully engaged during the initial stage, leading to a relatively slower early hydration rate; secondly, as hydration progresses, these characteristics—dense structure, good compatibility with cement paste, and low water absorption—promote continued hydration, thereby contributing to strength development in the medium-to-late stages. This is consistent with the conclusion in reference [47] that “granite manufactured sand has relatively low water absorption, thus offering better stability, durability, and compressive strength”.

The excellent performance of granite MS-SCC observed in this study is consistent with the recent findings of Wang et al. [48], who reported that, under the same mix proportion, granite-based mixtures exhibit better mechanical properties than limestone-based mixtures. This further confirms that the dense structure and low water absorption of granite MS are conducive to sustained hydration and strength development. Regarding the role of stone powder content, the optimal range identified in this study falls within the beneficial range of 5–15% reported in the literature [49]. The performance comparison between granite (6.7% stone powder) and basalt (9.5% stone powder) further supports the view that excessive stone powder is detrimental to concrete performance. The dominant role of lithology in determining strength differences is also supported by the study of Liu et al. [4], which indicated that mix proportion parameters have a limited effect on the mechanical strength of high-strength manufactured sand self-compacting concrete. This implies that material characteristics—such as lithology—may play a more decisive role in strength development. Furthermore, the observed relationship between MB value and workability aligns with the research findings of Xu Zhihua et al. [50], who systematically investigated the influence of the MB value of manufactured sand fines on concrete performance and concluded that the MB value significantly affects both workability and strength development.

The meso-scale simulation provided an intuitive mechanical explanation for the macroscopic differences described above. Although the failure of SCC consistently follows the basic pattern of interfacial damage initiation, stable propagation, and macroscopic coalescence, the damage evolution laws vary with lithology. Granite MS-SCC exhibited the latest damage initiation and the slowest propagation, which corresponds directly to its high macroscopic strength and relatively good ductility. In contrast, damage in basalt MS-SCC developed rapidly, resulting in the lowest macroscopic strength and a stress–strain curve with the steepest descending branch after the peak. The simulation results clearly indicate that lithology influences the macroscopic failure mode and load-bearing capacity by affecting meso-scale behavior.

It should be noted, however, that the two-dimensional meso-scale model adopted in this study represents a simplification of self-compacting concrete, which is inherently a three-phase, three-dimensional material. Although two-dimensional meso-scale simulation has been widely used in the literature [51,52,53,54] and has proven effective in capturing key damage mechanisms and failure trends, it cannot fully replicate the complex aggregate packing and crack propagation paths under three-dimensional conditions. In this context, the reasonable consistency observed between the simulated and experimental stress–strain curves and damage patterns (as described in Section 4.3) suggests that the two-dimensional approach provides a useful basis for the comparative analysis of lithology effects conducted in this study.

Based on the findings presented above, this study clarifies the suitability of limestone, basalt, and granite manufactured sands for the workability and mechanical properties of SCC. In engineering practice, when high strength and excellent workability of SCC are required, granite manufactured sand should be prioritized. If early strength is a primary concern, limestone manufactured sand is a more suitable choice. Furthermore, the comparative verification framework combining macroscopic testing and meso-scale simulation developed in this study provides a reliable methodology for the rational selection of manufactured sand and the optimization of SCC performance.

Based on the discussion above, this paper has elucidated the key mechanisms by which lithology influences the performance of SCC. Nevertheless, the following limitations of this study should be acknowledged and warrant further investigation:

(1)

This study selected only three representative lithologies—basalt, limestone, and granite. Although they represent the three main rock types (basic, carbonate, and acidic), they do not cover all rock varieties, such as metamorphic rocks and clastic sedimentary rocks.

(2)

Idealized spherical aggregates were used in the meso-scale simulation, which differ from the angular morphology of actual manufactured sand.

(3)

Although this study focused on the fresh performance, mechanical properties, and meso-scale damage evolution of different manufactured sand self-compacting concretes, the long-term durability of these materials remains to be investigated.

It should be noted that the use of idealized spherical aggregates eliminates the mechanical interlocking effect and removes the sharp edges that act as stress concentration sources. Studies have shown that replacing irregular polyhedral aggregates with spherical ones can reduce the compressive strength by approximately 7% [55]. Therefore, the model may provide a conservative estimate of interfacial strength and lead to a slightly delayed prediction of damage initiation. Furthermore, this simplification may affect the accuracy of stress transfer and potentially underestimate lithology-related differences—for example, basalt typically exhibits a more angular morphology than granite, which would enhance mechanical interlocking in practice. However, since the same simplification was applied uniformly across all simulated lithologies, the observed relative differences among granite, limestone, and basalt remain valid in the context of this comparative analysis.

Future research directions may include the following aspects:

(1)

Expand the scope of lithology research: adopt a more refined approach to the classification of MS lithology and the characterization of other raw material properties, including parameters such as particle size distribution, shape, and surface roughness, and analyze their influence on the performance of SCC. This would contribute to a deeper understanding of how different lithologies of MS affect the mechanical properties of concrete.

(2)

Develop realistic aggregate shape modeling: employ CT scanning or three-dimensional laser scanning technology to capture the morphology of real aggregates, reconstruct the geometric features of aggregates using methods such as spherical harmonics, and incorporate realistic particle morphology into meso-scale simulations. This would enable a more comprehensive capture of the synergistic effects of lithology and morphology, as well as their influence on stress transfer and fracture mechanisms.

(3)

Advance three-dimensional meso-scale simulation: gradually develop three-dimensional meso-scale models, as computational resources permit, to more realistically simulate the spatial distribution of aggregates, the three-dimensional structure of the ITZ, and crack propagation paths, thereby providing more reliable predictive tools for engineering applications.

(4)

Investigate long-term durability and its impact on mechanical properties: for instance, high-calcium limestone MS-SCC may be more susceptible to sulfate attack, as calcium carbonate can react with sulfates to form gypsum and expansive ettringite; therefore, comparative studies between limestone and siliceous aggregate systems under sulfate exposure are necessary. Furthermore, the dense structure and low water absorption of granite MS suggest it may possess good frost resistance; systematic freeze–thaw cycle tests should be conducted to verify whether granite MS-SCC exhibits superior durability in cold environments.

(5)

Establish a more extensive database: collect and analyze research and experimental data on MS-SCC from different regions and with various lithologies to construct a comprehensive database, thereby facilitating more in-depth statistical analysis and theoretical research.

6. Conclusions

This study investigated the influence of basalt, limestone, and granite manufactured sands, as well as natural river sand, on the performance of SCC through an integrated approach combining macroscopic experiments and meso-scale numerical simulation. The main conclusions are as follows:

(1)

Regarding macroscopic performance, SCC prepared with different fine aggregates exhibited distinct characteristics. Workability decreased in the order of river sand, granite, limestone, and basalt. In contrast, mechanical strength followed the order of granite, limestone, basalt, and river sand. Granite MS achieved the most favorable overall balance between strength and workability.

(2)

The established two-dimensional meso-scale random aggregate model effectively captured the failure process of SCC. The simulated ascending branches of the stress–strain curves closely matched the experimental data, with peak stress errors ranging from 1.1% to 6.9%. The simulated failure modes, characterized by inclined cracks initiating from the ITZ, were consistent with experimental observations, thereby validating the reliability of the model.

(3)

Meso-scale simulation further elucidated the underlying mechanisms responsible for the differences in macroscopic strength. Different lithological MS types primarily influence the macroscopic strength and failure mode of SCC by altering the performance of the ITZ, thereby controlling the damage initiation threshold and propagation rate. The highest-strength granite SCC exhibited the latest damage initiation (at 35.1% of peak stress) and slowest propagation, indicating superior ITZ quality and resistance to damage development. In contrast, the lower-strength basalt SCC showed earlier damage initiation and more rapid propagation, corresponding to the steepest post-peak descent in its macroscopic stress–strain curve. These findings demonstrate that the lithology of manufactured sand governs the macroscopic failure mode and load-bearing capacity by fundamentally influencing the mechanical behavior of this critical meso-scale phase.

(4)

Based on a comprehensive evaluation, granite MS is recommended for structures requiring high flowability and ultimate strength (e.g., densely reinforced members). Limestone MS is preferable for applications demanding high early strength, such as precast components or winter construction, while basalt MS requires special attention due to its high water absorption, which can be mitigated through mix adjustments or pre-wetting.