Experimental Study on the Mechanical Properties of Cracked Limestone Reinforced by Modified Cement Grouting

Abstract

Grouting reinforcement is a pivotal approach to enhancing the integrity and load-bearing capacity of fractures in surrounding rock. In this study, standard limestone specimens were fractured through uniaxial compression. Then, the specimens were reinforced with grouting, using ultrafine cement paste containing varying mass fractions of enhancers and a grouting apparatus developed by the authors. After the specimens were cured under standard conditions for 28 days, CT scanning technology was used to investigate the microstructure and grouting effect characteristics of grouted bodies containing different mass fractions of enhancers from a mesoscopic perspective. Then, uniaxial compression tests were conducted on those grouted specimens. The experimental results revealed that the content of the enhancer significantly affected the post-peak characteristics, mechanical parameters, and failure modes of the grouted specimens. When the content of the enhancer increased from 2.50 wt.% to 15.00 wt.%, the uniaxial compressive strength of the grouted specimens exhibited a positive correlation with the enhancer content, with the maximum improvement rate reaching 18.10% compared to the residual strength. However, when the enhancer content ranged from 15.00 wt.% to 20.00 wt.%, the uniaxial compressive strength was negatively correlated with the enhancer content. At an enhancer content of 15.00 wt.%, the overall stability of the grouted specimens was optimal, with all mechanical parameters reaching their maximum values. Utilizing three-dimensional CT scanning and reconstruction technology, it was observed that when the enhancer content was less than 15.00 wt.%, the cracks were concentrated in the limestone matrix rather than in the grouted solid in the edge regions of grouted specimens. However, in the whole specimens, the cracks in the grouted solid exceeded that in the limestone matrix. Conversely, when the enhancer content was greater than 17.50 wt.%, the grouted solid was predominantly distributed within the edge fissures of the specimens, while the internal regions exhibited a lower volume fraction of the grouted solid. In this scenario, the volume fraction of the grouted solid in the specimens was significantly lower than that of the fissures.

1. Introduction

One of the primary challenges in the safe development and efficient utilization of deep-earth resources is controlling the stability of the surrounding rock in deep roadways. Compared to the extraction and utilization of shallow resources, deep mining is characterized by high in situ stresses and intense excavation disturbances. These conditions lead to the fracturing and damage of intact rock masses, resulting in significant deformation of the surrounding rock and the failure of support structures, as illustrated in Figure 1. Such phenomena directly impact the safe extraction of deep-earth resources, posing severe challenges to their secure, green, and efficient exploitation. Consequently, effective reinforcement measures for fractured rock are paramount in ensuring the stability of roadways and support structures with fractures in the surrounding rock.

Grouting, as a crucial method of active support in underground spaces, not only serves to seal against water but also significantly enhances the overall strength of the surrounding rock and its resistance to large deformations [1,2,3]. The complex structure of fractured rock masses means that the effectiveness of grouting reinforcement is influenced by various factors, such as the properties of the grouting materials, the grouting process parameters, and the characteristics of rock fractures. Therefore, developing high-performance grouting materials, optimizing grouting processes, and establishing a scientific evaluation system for grouting effectiveness are key to improving the reinforcement outcomes of fractured rock masses.

Numerous scholars have conducted extensive research on grouting reinforcement for fractured rock masses in recent years, yielding fruitful results. In the realm of inorganic particulate grouting materials, Liu Q et al. [4] investigated the rheological properties of ordinary Portland cement grouting materials and the influence of the water–cement ratio on flow patterns. They conducted normal and tangential mechanical tests on fractured rocks post-grouting reinforcement. With the advent of materials such as ultrafine cement [5,6], nano-SiO₂ [7,8,9], and carbon nanotubes [10,11], researchers have developed a wide variety of novel modified cement slurries and have evaluated their application effectiveness in fractured rock mass reinforcement [12,13,14]. Their research findings indicate that adding different enhancers (such as water reducers and expansion agents) to grouting slurries can significantly alter their rheological and mechanical properties [15,16,17,18]. Through laboratory experiments and numerical simulations, the proportion of enhancers can be optimized to meet the grouting requirements of complex fractured rock masses. Simultaneously, the development and application of inorganic gel grouting materials such as water glass [19], silica sol [20,21,22], and their composite materials [23,24] have also become a focal point of research.

With its advantages of being non-destructive, high-resolution, and visually three-dimensional, CT scanning technology plays an increasingly important role in the study of grouting reinforcement for fractured rocks. Firstly, CT scanning can identify the internal fracture networks of fractured rocks before and after grouting [25,26,27,28,29], including the morphology, size, orientation, and connectivity of fractures. Through image processing software, the three-dimensional reconstruction and quantitative analysis of fractures can be performed, providing fundamental data for grouting scheme design. Additionally, the gray value of CT images can be used to estimate fracture apertures [30,31], and the results can be combined with numerical simulation methods. The permeability of the rock can be assessed, offering a basis for the selection of grouting materials and the optimization of grouting parameters. Secondly, CT scanning can display the filling conditions of fractures after grouting [32,33], including the distribution range and filling density of the grout, providing intuitive evidence for evaluating grouting effectiveness. By comparing and analyzing CT images of rocks before and after grouting, the repair effect of grouting on rock fractures and the improvement in mechanical properties can be quantitatively assessed. In addition, the internal structural information of rocks provided by CT scanning can offer accurate geometric models and boundary conditions for numerical simulations [34,35]. Through inversion analysis, the model parameters can be determined, enhancing the accuracy of numerical simulations. This can validate the rationality and reliability of numerical models, providing robust support for studying grouting reinforcement mechanisms.

From the above review, it is evident that significant research achievements have been made in developing new grouting materials and applying CT scanning technology to the evaluation of grouting reinforcement for fractured rocks. However, there is limited research on the impact of varying high-flow, ultra-early-strength enhancer content on the effectiveness of grouting reinforcement for fractured rocks. Based on this knowledge gap, this paper focuses on the grouting reinforcement of fractured limestone, which is commonly encountered in underground engineering, and describes research conducted through the following steps: First, intact limestone specimens were fractured using uniaxial compression to simulate the mechanical behavior of naturally fractured rock masses. Second, a self-developed manual grouting system was utilized to inject grouting slurry with different enhancer contents into the fractured specimens, achieving the reinforcement of fractured rocks. Third, CT scanning technology was employed to scan and reconstruct the reinforced specimens three-dimensionally, extracting meso-structural information according to set parameters (such as fracture distribution, grout filling rate, and grout–rock interface characteristics). Finally, uniaxial compression experiments were conducted on the grouted specimens to establish their macroscopic mechanical response. The experimental results quantify the relationship between the mechanical properties and meso-structure of the rock mass after grouting reinforcement, providing a new perspective for studying grouting reinforcement mechanisms. These research findings can offer theoretical foundations and technical support for designing and constructing grouting reinforcement for fractured rock masses in underground engineering.

2. Experimental Program Design

2.1. Specimen Selection and Preparation

Limestone, which is commonly encountered in deep underground engineering, was selected as the research subject. According to the International Society for Rock Mechanics (ISRM) standards [36], the specimen dimensions were determined to be Φ (50 ± 2) mm × L (100 ± 2) mm. Therefore, 3 specimens were designed for each test condition (including 1 backup unit), totaling 8 groups comprising 24 specimens. Heat-shrink tubing (with a treatment temperature of 120 °C) was applied to the specimens for thermoplastic protection before loading, in case of flying debris resulting from brittle failure.

2.2. Grouting Material Selection and Preparation

This study builds upon the latest research findings in the field of high-performance concrete [37,38,39,40], utilizing ultrafine ordinary Portland cement (fineness: 1000 mesh; density: 2.8 g/cm³; specific surface area: 800 m²/kg, the material was manufactured by Huaihai Zhonglian Cement Co., Ltd., Xuzhou, China) as the matrix component of high-fluidity ultra-early-strength grouting material. To enhance material performance, a composite reinforcement system was designed, consisting of glass microspheres (particle size: 10–50 μm, manufactured by Shenzhen Tongcheng New Material Technology Co., Ltd., Guangdong, China), ultrafine silica fume (particle size: 0.1–0.5 μm, manufactured by Beijing Changde Weiye Co., Ltd., Beijing, China), and nano-SiO₂ (particle size: 20 nm, manufactured by Tangshan Caofeidian Taihong Shengda New Materials Co., Ltd., Tangshan, China) at an optimized ratio (mass ratio 5:12:9).

Existing studies [41,42,43] indicate that cement-based binding systems exhibit a distinct physicochemical threshold effect regarding the incorporation of mineral admixtures. When the dosage of the reinforcing agent exceeds 20%, particle agglomeration and interference with the hydration process may lead to a significant reduction in the mechanical properties and durability of the grouting material’s hardened matrix. Conversely, when the dosage is below 2.5%, it becomes difficult to form a continuously distributed modified network of hydration products within the matrix. Notably, within the dosage range of 2.5–15%, the reinforcing agent demonstrates significant performance enhancement, with the mechanical properties and durability of the hardened matrix improving systematically as the dosage increases. Based on these findings, this study designed eight dosage levels for comparative investigation: 2.5%, 5%, 7.5%, 10%, 12.5%, 15%, 17.5%, and 20% of the total binder content, while maintaining a constant water-to-cement ratio of 0.8:1 to ensure the reliability and comparability of the experimental results.

The developed ordinary Portland cement-based grouting material, incorporating glass microspheres, ultra-fine silica fume, and nano-SiO₂ enhancers, achieved breakthrough progress in the following two aspects compared to prior studies.

(1)

Performance breakthroughs:

The incorporation of glass microspheres significantly improves the fluidity of ordinary Portland cement-based grouting materials, addressing the poor injectability of traditional grouting materials. The synergistic effect of ultra-fine silica fume and nano-SiO₂ markedly enhances the compressive strength of fractured limestone after reinforcement, substantially improving the anchoring capability for fractured rock masses. The gradation effect of multi-scale fillers greatly reduces the internal porosity of reinforced structures.

(2)

Enhanced engineering applicability:

The initial setting time of the new grout formulation can be dynamically adjusted based on the content of enhancers, ensuring both operational feasibility and the prevention of grout leakage. The interfacial bonding strength between the material and fractured rock masses is significantly increased.

These breakthroughs enable the material to demonstrate remarkable advantages in engineering applications such as deep roadway support and fractured tunnel roof reinforcement, offering higher construction efficiency and lower rework rates compared to traditional grouting materials.

2.3. Experimental Flowchart and Specimen Parameters

An experimental flowchart detailing the processes involved in this research is shown in Figure 2 ((a) → (b) → (c) → (d) → (e) → (f) → (g) → (h) → (i)), which mainly includes the preparation of fractured limestone specimens, the preparation of grouting materials, the grouting reinforcement of fractured limestone, standard curing of the specimens (curing time: 48 h), CT scanning of the specimens, mechanical property testing of the specimens, and data analysis. The mechanical parameters of the intact limestone specimens and the preparation of the grouting slurry are detailed in

Table 1. Mechanical characteristics of intact limestone specimens and grouting material design.

Specimen Number	Peak Strength/MPa	Peak Strain/10−2	Residual Strength/MPa	Elastic Modulus/GPa	Water–Cement Ratio	Enhancer Content
A-1#	32.63	18.70	12.38	3.13	0.8:1	2.50 wt.%
A-2#	31.46	18.81	8.36	2.62
B-1#	32.87	14.28	19.16	2.81		5.00 wt.%
B-2#	30.01	13.79	18.49	3.55
C-1#	39.62	20.70	21.67	3.50		7.50 wt.%
C-2#	33.87	12.32	20.89	3.72
D-1#	33.50	22.78	21.76	2.65		10.00 wt.%
D-2#	41.60	22.67	20.43	3.61
E-1#	38.59	18.35	13.47	3.48		12.50 wt.%
E-2#	36.43	18.18	14.99	3.41
F-1#	36.10	20.00	19.64	2.83		15.00 wt.%
F-2#	29.48	19.36	18.03	2.97
G-1#	33.02	18.47	8.78	2.87		17.50 wt.%
G-2#	40.17	17.12	11.26	2.73
H-1#	32.01	16.85	7.71	3.19		20.00 wt.%
H-2#	29.52	14.18	5.89	2.88

. All of the above processes in Figure 2 were performed in the same room, which had a temperature of 20 °C and a humidity of 40–60%.

2.4. Specimen Testing Equipment

Uniaxial compression tests on the specimens were conducted using the MTS816 (MTS Systems, USA) testing machine at the State Key Laboratory of Intelligent Construction and Healthy Operation and Maintenance of Deep Engineering at the China University of Mining and Technology. The loading rate was set at 0.5 mm/min, and the displacement control mode was employed to ensure the reliability of the test data.

A manual hydraulic grouting system developed by the authors (ZJXT-01) was used to carry out the grouting operation. This system consists of a pressurization system, a pressure cylinder, gauges, and control valves, with a grouting pressure range of 0–10 MPa. The grouting rate can be precisely adjusted through the control valves, making it suitable for precise grouting operations on small-sized specimens. Referring to the actual conditions of on-site grouting, the grouting pressure in this study was designed to be 2 MPa, with a grouting holding time of 30 min.

Microstructural scanning of the grouted specimens was completed at the South China Industrial CT Scanning (manufactured by Shimadzu Corporation, Kyoto, Japan) Laboratory using the inspeXio SMX-225CT FPD HR Plus device. It has an effective pixel count of 3000 × 3000 and a minimum voxel resolution of 1 μm (corresponding to an imaging field of view of approximately φ 1 mm), enabling high-precision 2D and 3D imaging scans. The specimens were placed vertically during the scanning process, using a scan layer thickness of 0.21 mm and a scan duration of 2 h. Image reconstruction was performed using the filtered back-projection algorithm.

3. Experimental Results

3.1. Stress–Strain Curves and Post-Peak Characteristics of Specimens

The uniaxial compression stress–strain curves of both intact and fractured–reinforced limestone specimens were obtained from the experiments, as illustrated in Figure 3. The intact limestone specimens exhibited four distinct stages during the uniaxial compression process: compaction, elastic deformation, progressive fracturing, and post-peak failure. During the compaction stage, the internal primary fractures of the specimens were gradually compacted, resulting in an upward concave curve. In the elastic deformation stage, new fractures began to form within the specimens, showing a stable fracturing trend, and the curve transitioned from an approximately straight line to a curve. Some specimens exhibited noticeable multiple stress drops, resulting from local stress concentrations and micro-fracture initiation caused by several primary fractures or voids within the specimens. During the progressive fracturing stage, the internal cracks in the specimens propagated unstably, with the primary weak structural planes failing first, followed by the secondary weak structural planes, resulting in significant dilatancy in the specimens. In the post-peak stage, the internal fractures developed rapidly, coalesced, and then formed principal fracture planes, leading to complete specimen failure, which primarily manifested as X-shaped and inverted V-shaped double-slope shear failure modes.

As can be seen from Figure 3, the stress–strain curves of the specimens reinforced with the eight grouting schemes exhibited characteristics similar to those of the intact limestone specimens during the pre-peak stage of uniaxial compression. This indicates that the fracture evolution and development process of the reinforced specimens was similar to that in the intact limestone specimens during the pre-peak stage. The process can be divided into the compaction stage (OA), the elastic deformation stage (AB), and the progressive fracturing stage (BC).

However, in the post-peak stage (segment CD), the stress–strain curves of the grouted fractured limestone specimens exhibited distinctly different characteristics from those of the intact limestone specimens. The intact limestone specimens primarily displayed brittle failure in the post-peak stage, with a rapid drop in stress. In contrast, the grouted specimens demonstrated better ductility and a certain level of residual strength. The content of the enhancer had a more pronounced effect on the post-peak characteristics of the reinforced specimens, a phenomenon that has rarely been reported in existing studies.

During the post-peak failure stage, strain softening occurred, and the stress–strain curve exhibited multiple “rise and fall” declines. Although the strength of the grouted specimens exceeded the peak strength, the internal fractured rock blocks and fracture channels still retained some load-bearing capacity through the bonding of the grout. This load-bearing capacity gradually decreased with increasing strain, showing evident plastic material failure characteristics rather than the typical post-peak brittle failure characteristics.

The stress–strain curve of the F specimen, as shown in Figure 3f, exhibited multiple stress fluctuations during the strain-softening phase. Within the 2.50 wt.% to 10.00 wt.% enhancer content, the post-peak stress fluctuations became more pronounced as the content increased. However, the post-peak stress fluctuations diminished when the content exceeded 10 wt.%. This means that the failure of the grouted fractured limestone specimens did not occur along a single or a few main cracks but rather through the formation of multiple fracture channels. The reasons for this behavior are analyzed as follows: (1) after the interface between the grout and the rock blocks within the specimen fails, the stress is transferred to the rock blocks themselves. The axial compressive forces and tangential friction between the rock blocks retain the capacity to resist external forces. As axial loading continues, this failure process is repeated within the specimen. (2) The grout matrix fails first, but due to its inherent strength, the fractured rock mass undergoes rolling during loading, after detaching from the interface. Once stabilized, it regains the ability to resist axial stress.

These two factors lead to the phenomenon where the stress decreases and then rapidly increases multiple times during the post-peak failure stage, demonstrating excellent ductility and toughness deformation characteristics. This indicates that grouting reinforcement not only prevents the immediate collapse of the fractured limestone roof, facilitating the formation of a regenerated roof, but also ensures that the regenerated roof exhibits certain ductile properties during secondary fracturing. This ductility helps avoid brittle fracture of the roof, thereby contributing to the stability of the surrounding rock in the working face. During the frictional phase (after point D), the grouted material retains a certain residual strength. At this stage, the specimen develops multiple macroscopic fracture surfaces, and deformation resistance is achieved through the frictional interaction between these fracture surfaces.

3.2. Variation Characteristics of the Basic Mechanical Parameters of Grouted Fractured Limestone Under Uniaxial Compression

Based on the results of the uniaxial compression tests, the basic mechanical parameters of grouted specimens with varying reinforcement contents were obtained, as illustrated in Figure 4. The improvement rate of the uniaxial compressive strength of the grouted material is defined as “η”:

$$\eta ={\displaystyle \frac{{\sigma }_{\mathit{c}}^{\prime }-{\sigma }_{\mathit{r}}}{{\sigma }_{\mathit{r}}}}\times 100\%$$

(1)

where ${\sigma }_{\mathit{c}}^{\prime }$ is the uniaxial compressive strength of the grouted specimen, and ${\sigma }_{\mathit{r}}$ is the residual uniaxial compressive strength of the intact limestone specimen.

The variation patterns of the residual strength of intact limestone and the residual strength of fractured limestone after grouting reinforcement are shown in Figure 4a, while the variation pattern of the improvement rate of compressive strength for grouted fractured limestone is illustrated in Figure 4b.

When the enhancer content ranged from 2.50 wt.% to 15.00 wt.%, the uniaxial compressive strength of the grouted material exhibited a positive correlation with the reinforcement content. As the content increased from 2.50 wt.% to 15.00 wt.%, the average improvement rate of compressive strength for grouted fractured limestone rose from 8.89% to 18.10%. With a 15.00 wt.% content (Group F specimens), the average residual strength of intact limestone under uniaxial compression was 18.84 MPa, while the average uniaxial compressive strength after grouting reinforcement increased to 22.24 MPa. However, when the content ranged from 15.00 wt.% to 20.00 wt.%, the uniaxial compressive strength of the grouted material showed a negative correlation with the reinforcement content. As the reinforcement content increased to 20.00 wt.%, the average improvement rate of compressive strength decreased from 18.10% to 12.10%. With a 20.00 wt.% content (Group H specimens), the average residual strength of intact limestone was 6.80 MPa, and the average uniaxial compressive strength after grouting reinforcement increased to 7.62 MPa, with an improvement rate of only 12.10%.

The content of the enhancer significantly influences both the elastic modulus ($E^{\prime }$) and the residual strength (${\sigma }_{\mathit{r}}^{\prime }$) of the grouted material, with their variation patterns closely resembling the improvement rate of compressive strength, as shown in Figure 4c,d. When the enhancer content was 15.00% wt.%, both the elastic modulus ($E^{\prime }$) and the residual strength (${\sigma }_{\mathit{r}}^{\prime }$) of the grouted material reached their maximum values at 2.03 GPa and 14.41 MPa, respectively. However, when the enhancer content increased to 20.00 wt.%, both the elastic modulus ($E^{\prime }$) and the residual strength (${\sigma }_{\mathit{r}}^{\prime }$) decreased to their minimum values at 0.60 GPa and 3.88 MPa, respectively.

The analysis reveals that the reinforcement effect of grouting materials on fractured rock mass is primarily influenced by the properties of the materials and the rock mass itself. In this study, the bonding strength at the grout–rock interface plays a decisive role in the macroscopic mechanical characteristics of the specimens. The fluidity and bonding strength of the grouting material are the main factors affecting the bonding strength at the grout–rock interface. When the enhancer content is low, the grouting material exhibits high fluidity and strong permeability, resulting in a wide grouting range. However, the bonding strength at the grout–rock interface after solidification is weak.

In contrast, when the enhancer content is high, the presence of nano-SiO₂ particles in the enhancer may lead to agglomeration effects due to high van der Waals forces between molecules [44,45]. This increases the viscosity of the grout and weakens its penetration into rock joint surfaces [7], thereby reducing the effectiveness of grouting reinforcement. Determining the optimal content of the enhancer is crucial for enhancing the reinforcement effect of fractured limestone specimens. Based on the above analysis, the overall stability of the reinforced specimens was optimal when the enhancer content reached 15%, with all mechanical parameters achieving their maximum values.

3.3. Failure Modes of Grouted Fractured Limestone

Under uniaxial compression conditions, the failure patterns of the eight types of reinforced specimens are shown in Figure 5. The specimens primarily exhibited the following three composite failure modes:

(1)

Shear–Local Fracture Failure

Under uniaxial compression, the original fractured blocks of the specimen undergo significant shear deformation along the grout–rock interface, with the shear deformation direction parallel to the loading direction. Simultaneously, partial fracture and failure of the original fractured rock blocks occur, with fracture surfaces that are approximately perpendicular to the loading direction, leading to overall specimen failure. This failure mode primarily occurs in specimens with lower enhancer content, as shown in Figure 5a. After removing the protective film, the specimen is mainly fragmented, as illustrated in Figure 5d–f. The shear local fracture failure is mainly controlled by the shear strength of the grout–rock interface. When the vertical stress reaches 65%, the shear stress exceeds the shear strength of the grout–rock interface, causing initial separation and sliding at the interface. With increased sliding distance, partial structural failure occurs, resulting in block fracture.

(2)

Shear–Sliding Failure

Under uniaxial compression, the specimen undergoes gradual shear failure along the grout–rock interface, with the fractured blocks exhibiting slow sliding and dislocation. The shear dislocation direction is parallel to the loading direction, causing bulging at the lower part of the specimen. This failure mode primarily occurs in specimens with slightly higher enhancer content, as shown in Figure 5b. After failure, the specimen breaks into numerous elongated blocks, as depicted in Figure 5g–i.

(3)

Tensile–Bulging Failure

Under uniaxial compression, the upper part of the specimen exhibits slow but significant bulging deformation. After reaching a certain deformation threshold, tensile cracks develop along the grout–rock interface in the upper region of the specimen. The failure mode for specimens with higher enhancer content is shown in Figure 5c. After failure, the specimen contains larger rock blocks, and the number of blocks is significantly reduced, as illustrated in Figure 5j,k.

The research results indicate that the failure modes of grouted specimens are primarily influenced by the spatial structural morphology and mechanical parameters of the grout matrix, which are controlled by the content of the enhancer. When the enhancer content is low (2.50 wt.%–7.50 wt.%), the grout exhibits low viscosity and high fluidity, resulting in excellent permeability. A thin layer of grout matrix material almost entirely covers the internal surfaces of the fractured rock blocks. However, due to the weak bonding force and friction between the fractured blocks, the skeletal support effect of the grout matrix is insufficient, making structural failure more likely to occur initially. As the enhancer content increases, the thickness of the grout matrix gradually increases, and the bonding force and friction between the fractured blocks also increase. The degree of block fragmentation significantly decreases, while the number of larger blocks increases noticeably. When the enhancer content is relatively high (10.00 wt.%–15.00 wt.%), the grout exhibits higher viscosity and lower fluidity, reducing its permeability. The area of the grout matrix on the surfaces of the fractured blocks tends to decrease. However, the bonding force and friction between the fractured blocks increase significantly. The increased thickness of the grout matrix enhances its skeletal support effect, considerably improving the load-bearing capacity of the surrounding areas. The failure mode of the specimens becomes similar to that of intact specimens, being dominated by shear failure. The integrity of the fractured blocks increases noticeably with higher enhancer content. When the enhancer content increases (17.50 wt.%–20.00 wt.%), the grout exhibits even higher viscosity and lower fluidity, resulting in poor permeability. It becomes difficult for the grout to penetrate the internal regions of the fractured specimens, and only the surface fractured areas are filled with viscous grout, which has high strength. During uniaxial loading, when stress is transferred to the upper regions of the specimen, the ungrouted internal rock blocks exhibit significant dilation. However, due to the high overall strength of the external areas, the upper part of the specimen undergoes bulging deformation, accompanied by a certain number of tensile cracks.

Leveraging the spatial imaging characteristics of CT technology and the benefits of digital image quantitative analysis, researchers have established quantitative relationships between macroscopic mechanical strength and mesoscopic crack density, achieving significant results [46,47,48,49,50,51,52]. Compared with traditional grouting reinforcement evaluation methods, the CT scanning technique employed in this study demonstrates significant advantages. Conventional approaches (e.g., mechanical property testing, wave velocity detection, core drilling, and empirical formula evaluation) are inherently limited by low detection efficiency, the destructive nature of testing, and limited representativeness. In contrast, CT scanning enables the nondestructive and precise identification of the 3D spatial distribution characteristics of grout diffusion, offers intuitive 3D reconstruction to visualize the spatial morphology of grout vein networks, and facilitates the rapid evaluation of grouting effectiveness and optimization of design parameters, based on quantitative analysis. This technique overcomes the limitations of traditional methods and provides a more scientific and efficient detection and evaluation approach for grouting reinforcement engineering.

For this paper, we conducted CT scans on grouted limestone specimens to observe the distribution of grout within the rock under different enhancer contents. Using CT image processing techniques, the internal fracture characteristics, grouted fracture features, and mechanical variation patterns of specimens with different enhancer contents were quantitatively analyzed. This revealed the relationship between mesoscopic structural evolution and macroscopic mechanical properties, providing a theoretical foundation for grouting reinforcement mechanism research and engineering applications.

In this study, CT slice scanning was performed on cured grouted limestone specimens, with the slicing direction being perpendicular to the axial direction of the specimens (as shown in Figure 6a). The slice thickness was 0.21 mm, resulting in 749 slices per specimen. Taking specimen A1 as an example, the 2D slices are shown in Figure 6b. In the grayscale images, the red-marked regions represent the grout matrix, while the blue-marked regions indicate fractures. Using the three-dimensional image reconstruction technology in the Avizo software(Avizo 3D 2022.2, Avizo XLVolume Extension include user protection under license for Landmark U.S. Patent Numbers 6, 765, 570.), the 749 two-dimensional images were superimposed to obtain three-dimensional scanned images of the bedrock, grout matrix, and fractures. Figure 6c shows the three-dimensional scanned image of specimen A1 along the axial direction, while Figure 6d,e displays the three-dimensional scanned images of fracture distribution and grout matrix distribution along the axial direction, respectively. Through quantitative analysis, parameters such as the volume fraction and spatial distribution characteristics of fractures and the grout matrix were extracted, providing essential mesoscopic structural data for subsequent research on grouting reinforcement mechanisms.

In this study, the median filtering method was employed to denoise the CT images of the specimens. Median filtering technology effectively overcomes the image detail blurring caused by linear filtering. Its processing is equivalent to filtering a two-dimensional data sequence, thereby achieving noise reduction. For the denoised images, based on their grayscale, color, and texture characteristics, threshold segmentation technology was used to divide the images into non-overlapping regions. The Otsu algorithm embedded in the Avizo software was applied for threshold segmentation to construct images identifying the internal structure of the specimens, as shown in Figure 6. The volume fractions of pores and the grout matrix in the specimens could be accurately extracted through threshold segmentation. The volume fractions of the fractures and grout matrix for the eight types of reinforced specimens are illustrated in Figure 7.

The volume fractions of the fractures and grout matrix at the upper and lower edges of the specimen slices are significantly greater than those in the middle region under all conditions. During the loading process, the contact areas at the loading end experience greater damage compared to the middle region of the specimen, leading to more extensive crack propagation and development at the edges. When the grout is injected into the specimen, the fractures at the edges are the most accessible. As the path lengthens and the viscosity of the grout increases, the amount of grout penetrating the internal fractured rock mass gradually decreases. The grout injection volume “Q” can be derived based on Darcy’s law and the continuity equation:

$$Q={\displaystyle \frac{\mathit{k}·\mathit{A}·\Delta \mathit{P}}{\mu ·\mathit{L}}}$$

(2)

where Q is the flow rate of the grout (m³/s), k is the permeability of the fractured rock mass (m²), A is the cross-sectional area of the fractures in the rock mass (m²), ΔP is the pressure difference (Pa), μ is the dynamic viscosity of the grout (Pa·s), and L is the length of the flow path (m).

As shown in Figure 7a, when the enhancer content in the grouting material is 2.50 wt.%, the CT slices of the different regions show a sudden increase in the volume fraction of the grout matrix, indicating that the grout can effectively penetrate more minor fractures and pores. Due to the short pressure-holding time and low viscosity of the grout, the grout attached to larger fractures, especially in the end regions of the specimen, tends to be lost, resulting in a higher volume fraction of fractures in these areas. The grout matrix mainly adheres to minor fractures and pores. From Figure 7b–f, it can be seen that as the content of the enhancer increases, the volume fraction of the grout matrix gradually increases. In contrast, the volume fraction of fractures correspondingly decreases. Moreover, the volume fraction of the grout matrix is consistently greater than that of the fractures. When the enhancer content reaches 17.50 wt.% and 20.00 wt.%, the volume fraction of the grout matrix becomes significantly smaller than that of the fractures. The CT two-dimensional scanning results reveal that the grout matrix is only present in the outer edge regions of each slice, with a relatively uniform distribution, while the internal regions of the specimen are dominated by a network of fractures, as shown in Figure 7g,h.

The above analysis also provides a better explanation for the variation in failure modes of the reinforced specimens as the content of the enhancer increases.

To quantitatively analyze the influence of the volume fraction of internal fractures and grout matrix on the peak strength and residual strength of the grouted specimens, this study calculates the average volume fraction of internal fractures (V_f) and the average volume fraction of the grout matrix (V_p) based on the volume fractions identified from CT scanning. The calculation formulas are as follows:

$$V_{\mathit{f}}={\displaystyle \frac{\sum V_{\mathit{f}}^{\prime }}{\mathit{n}}}$$

(3)

$$V_{\mathit{p}}={\displaystyle \frac{\sum {\mathit{V}}_{\mathit{p}}^{\prime }}{\mathit{n}}}$$

(4)

where $V_{\mathit{f}}^{\prime }$ is the volume fraction of fractures in each CT slice of the specimen, ${\mathit{V}}_{\mathit{p}}^{\prime }$ is the volume fraction of the grout matrix in each slice, and n is the number of CT slices of the specimen.

The average volume fraction of internal fractures, average volume fraction of the grout matrix, uniaxial compressive strength, and residual strength of the reinforced specimens are summarized in

Table 2. Statistical table of the average volume fraction of internal fractures and the grout matrix, and the strength of the grouted specimens.

Specimen Number	A	B	C	D	E	F	G	H
Average Volume Fraction of Fractures (%)	1.47	3.36	0.95	0.73	0.23	0.66	14.69	7.97
Average Volume Fraction of Grout Matrix (%)	6.86	5.57	5.42	5.69	7.72	13.91	3.77	3.46
Peak Strength (MPa)	11.29	20.64	23.41	23.52	16.27	22.24	11.53	7.62
Residual Strength (MPa)	4.27	6.26	9.11	10.26	10.30	14.41	4.16	3.88

. Based on Table 2, the relationships among these four parameters are then plotted in Figure 8. As the mass fraction of the enhancer increases, the average volume fraction of internal fractures initially decreases slowly and then increases, while the average volume fraction of the grout matrix first increases and then decreases. For grouted specimens with 15.00 wt.% enhancer (Group F specimens), the volume fraction of the grout matrix reaches its maximum, and the volume fraction of fractures is only 0.66%. The uniaxial compressive strength and residual strength of these specimens are relatively high, which is consistent with their failure modes. In contrast, the volume fraction of the grout matrix in Group G and Group H specimens is relatively small, while the volume fraction of fractures is relatively large. This results in lower uniaxial compressive strength and residual strength for these two groups of specimens.

This means that adding an appropriate amount of enhancer to ultra-fine cement particles results in a larger specific surface area and faster hydration reaction rates. The consolidation process forms a denser cementitious structure, while the glass microspheres in the enhancer enhance the fluidity of the ultra-fine cement particles. This enables the grouting material to more effectively fill the fractured surfaces of the rock and penetrate micro-fractures and other structural voids [53,54,55]. Consequently, a well-bonded interface is formed between the grout matrix and the fractured rock surfaces, exhibiting higher shear strength. This supports new foundational theories and technological innovations for the grouting reinforcement of fractured rock masses.

3.4. Engineering Application Analysis

Based on these research findings, the optimal ratio of the reinforcing agent was determined. The identified critical content of 15% of reinforcing agent can be applied to grouting reinforcement in fractured rock mass zones. A grouting strategy based on CT scanning enabled the establishment of a matching model between fracture volume fraction and reinforcing agent content, which could then be utilized for the reinforcement of fractures in the surrounding rock in tunnels. By locating fracture-concentration areas through CT scanning and by adopting gradient grouting, the differential settlement was controlled within the specified limits, exceeding the design requirements. The contributions of this study to engineering practice are mainly reflected in the following three aspects:

(1)

Material ratio optimization: The systematic investigation of the effects of different reinforcing agent contents on the reinforcement performance of ordinary Portland cement-based grouting materials in fractured rock masses provides a scientific basis for the mix design and parameter optimization of grouting materials.

(2)

Innovation in experimental methods: Breaking through the limitations of traditional grouting studies on prefabricated fractures, this study adopted an experimental approach involving the uniaxial compression-induced damage of intact rock samples, followed by grouting reinforcement, which more realistically simulates the grouting conditions of fractured rock masses in engineering practice. The research results provide reliable technical support for the rapid evaluation and prediction of grouting reinforcement effects in fractured rock masses.

(3)

Application of detection technology: The innovative application of CT scanning technology for the three-dimensional visual analysis of grouting reinforcement effects enables the precise identification and quantitative characterization of the spatial distribution characteristics of grout veins, offering new technical means for grouting quality assessment and parameter optimization.

These research findings have significant practical guiding value for improving the scientificity, reliability, and construction efficiency of grouting reinforcement projects in fractured rock masses.

This study achieved certain research results through laboratory experiments, but the following limitations remain. Firstly, laboratory conditions (e.g., temperature, humidity, confining pressure, and loading rate) cannot fully replicate the complex environment of underground engineering (e.g., in situ stress fields, groundwater seepage, and dynamic disturbances). Secondly, there are scale differences in the grouting process parameters (e.g., grouting pressure and grout diffusion range) between the scaled-down laboratory models and field conditions, leading to deviations in mechanical performance evaluation. Thirdly, this study focused solely on limestone, without comparing the grouting reinforcement effects of other lithologies (e.g., sandstone and granite). Thus, the conclusions may vary for rock masses with different mineral compositions or structural characteristics. Additionally, this study only examined short-term mechanical properties (e.g., compressive strength and elastic modulus), without considering long-term performance degradation caused by chemical corrosion at the grout–rock interface, freeze–thaw cycles, or stress fatigue. Finally, due to challenges in rock sample collection, preparation, and experimental costs, the limited sample size may affect the generalizability of the conclusions.

4. Conclusions

This study focuses on the grouting reinforcement of fractured limestone using a developed enhancer and ultra-fine cement as the grouting materials. By employing CT scanning technology, the reinforcement effects of specimens with varying enhancer contents were investigated, leading to the following conclusions:

(1)

Under uniaxial compression, the post-peak failure stage of the reinforced fractured limestone specimens exhibits strain-softening behavior. The stress–strain curve shows multiple “rise-and-fall” drops, demonstrating enhanced ductility and a certain level of residual strength. The enhancer content significantly influences the mechanical parameters of the specimens. When the enhancer content reaches 15 wt.%, the overall stability of the reinforced specimens is optimal, and all mechanical parameters achieve their maximum values: the UCS of the grouted specimen is equal to 22.24 MPa, and the increase rate reaches 18.10%.

(2)

As the enhancer content increases, the failure mode of the reinforced fractured limestone specimens under uniaxial compression gradually transitions from shear–local fracture failure to shear–sliding failure and tensile–bulging failure. The occurrence of these three mixed-failure modes is primarily influenced by the spatial structural morphology and mechanical parameters of the grout matrix, which are controlled by the enhancer content.

(3)

The three-dimensional CT scanning and reconstruction results indicate that when the enhancer content is low, the edge regions of the fractured limestone specimens exhibit a higher volume fraction of fractures and a lower volume fraction of the grout matrix. However, the overall volume fraction of the grout matrix is greater than that of the fractures. When the enhancer content is high, the grout matrix is mainly distributed in the fractures at the edges of the specimens. At the same time, the internal regions have a lower volume fraction of the grout matrix. In this case, the volume fraction of the grout matrix is significantly smaller than that of the fractures. When the mass fraction of the enhancer is 15.00 wt.%, the volume fraction of the grout matrix inside the specimen reaches its maximum (13.91%), with the volume fraction of fractures being only 0.66%.

(4)

In this paper, the influence of the content of the enhancer on the reinforcement effect of cement-based grouting materials is mainly studied and discussed. However, the action mechanism has not been thoroughly analyzed in this paper. In subsequent studies, the strengthening mechanism can be explored further from a microscopic perspective, utilizing X-ray diffraction, scanning electron microscopy, and molecular dynamics with numerical simulations.