Influence of nano-TiO₂ additives in sealing materials on the airtightness of methane drainage boreholes in coal mines

ABSTRACT

This study explores the impact of nano-TiO₂ additives on sealing materials for methane drainage boreholes in coal mines. Varying nano-TiO₂ contents (0.5%, 1.0%, 1.5%, and 2.0%) were investigated, with 1.5% emerging as the optimal dosage. At this concentration, early-age strength increased by 28.6% at 3 days, while gas permeability decreased by 77.6% compared to the control mixture. The modified sealant exhibited accelerated setting, with initial setting time reduced from 195 to 152 minutes. Fluidity decreased with increasing nano-TiO₂ content, necessitating superplasticizer adjustment. Microstructural analysis revealed a 34.2% reduction in total porosity and a refined pore structure. The enhanced performance is attributed to the nanoparticles’ nucleation effect, pore-filling capacity, and participation in pozzolanic reactions. These findings suggest that nano-TiO₂-modified sealing materials can significantly improve methane drainage efficiency and mine safety by enhancing borehole airtightness. The study provides valuable insights for developing advanced sealing materials tailored for coal mine applications.

Keywords:
Nanoparticles; Cement-based composites; Porosity; Permeability; Underground mining safety

1. INTRODUCTION

Methane drainage in coal mines is a critical safety and environmental practice that has garnered significant attention in recent years [¹]. As coal remains a substantial component of the global energy mix, the extraction process continues to release considerable amounts of methane, a potent greenhouse gas and a serious safety hazard in underground mining operations [²]. Effective methane drainage not only mitigates explosion risks but also presents an opportunity to capture and utilize this valuable resource, potentially offsetting the carbon footprint of coal mining activities. The success of methane drainage operations hinges heavily on the integrity and effectiveness of the drainage boreholes. These boreholes serve as conduits for extracting methane from coal seams before and during mining operations. However, the efficacy of these drainage systems is often compromised by inadequate sealing of the boreholes. Poor sealing can lead to air leakage, which dilutes the methane concentration and reduces the overall efficiency of the drainage process [³, ⁴]. Moreover, inadequate sealing can result in methane emissions to the mine atmosphere, exacerbating safety concerns and environmental impacts [⁵].

Borehole sealing plays a pivotal role in maintaining the airtightness of methane drainage systems. The primary function of sealing materials is to create an impermeable barrier between the borehole and the surrounding strata, ensuring that methane is effectively channeled through the drainage system without leakage or dilution [⁶]. Ideal sealing materials must possess a combination of properties including high strength, low permeability, good adhesion to rock surfaces, and the ability to withstand the dynamic stresses induced by mining activities [⁷]. Traditional sealing materials, typically cement-based grouts or resins, have been widely used in coal mine methane drainage applications [⁸]. While these materials have proven effective to some extent, they often fall short in meeting the increasingly stringent requirements for borehole sealing in modern mining operations [⁹, ¹⁰]. Common challenges associated with conventional sealing materials include shrinkage during curing, which can lead to the formation of micro-cracks and subsequent leakage paths [¹¹,¹²,¹³]. Additionally, the inherent brittleness of many cement-based sealants makes them susceptible to damage from ground movements and stress changes in the rock mass surrounding the borehole [¹⁴]. The limitations of current sealing materials have spurred research into innovative solutions to enhance borehole airtightness. Among the various approaches being explored, the incorporation of nanomaterials into sealing formulations has shown promising potential. Nanomaterials, with their unique properties stemming from their extremely small size and large surface area to volume ratio, offer opportunities to modify and improve the characteristics of sealing materials at the molecular level.

Titanium dioxide nanoparticles (nano-TiO₂) have emerged as a particularly interesting additive for enhancing the performance of sealing materials [¹⁵]. Nano-TiO₂ is known for its exceptional mechanical and chemical properties, including high strength, excellent stability, and photocatalytic activity [¹⁶]. When incorporated into cement-based systems, nano-TiO₂ has been observed to influence various aspects of material behavior, from rheological properties in the fresh state to microstructural development and long-term durability in the hardened state [¹⁷]. The potential benefits of nano-TiO₂ in sealing materials for methane drainage boreholes are multifaceted. At the microstructural level, nano-TiO₂ particles can act as nucleation sites for the growth of hydration products, potentially leading to a more refined and densified pore structure [¹⁸]. This refinement of the microstructure could translate to reduced permeability and enhanced resistance to gas and fluid penetration [¹⁹]. Furthermore, the high surface activity of nano-TiO₂ may promote stronger bonding between the sealing material and the borehole wall, improving adhesion and reducing the likelihood of interfacial leakage paths [²⁰]. From a mechanical perspective, the incorporation of nano-TiO₂ may contribute to increased strength and improved ductility of the sealing material [²¹]. Enhanced mechanical properties are crucial for maintaining seal integrity under the complex stress conditions encountered in underground coal mines. The potential for nano-TiO₂ to mitigate shrinkage and cracking during the curing process is also of particular interest, as it addresses one of the primary limitations of conventional cement-based sealants [²²]. Various nanomaterials have been investigated for enhancing the properties of cement-based sealing materials [²³]. Nano-SiO₂, with its high pozzolanic activity, has been shown to accelerate early hydration and improve strength development. Studies by ZHANG et al. [²⁴] reported a 25% increase in 28-day compressive strength and a 45% reduction in permeability with 3% nano-SiO₂ addition. However, the high water demand of nano-SiO₂ can lead to workability issues, particularly in borehole applications where fluidity is crucial. Nano-Al₂O₃ has also been explored, with research by ISKRA-KOZAK and KONKOL [²⁵] demonstrating its effectiveness in reducing porosity and enhancing interfacial bonding. Their findings showed a 30% improvement in bond strength between sealing materials and rock surfaces, though the cost of nano-Al₂O₃ remains a limiting factor for large-scale applications. Carbon nanotubes (CNTs) have garnered attention for their ability to bridge microcracks and enhance mechanical properties, with LIU et al. [²⁶] reporting a 40% increase in flexural strength. However, CNTs face challenges in achieving uniform dispersion and maintaining long-term stability in alkaline environments. In comparison, nano-TiO₂ offers a unique combination of benefits including moderate water demand, excellent stability in cementitious systems, and cost-effectiveness relative to other nanomaterials. Its photocatalytic properties may also contribute to the long-term durability of sealing materials, though this aspect requires further investigation. This comparative analysis of nanomaterials supports our focus on nano-TiO₂ as a promising additive for methane drainage borehole sealing applications.

Despite the promising attributes of nano-TiO₂, its application in methane drainage borehole sealing materials remains relatively unexplored. The complex interplay between nano-TiO₂ additions and the various properties of sealing materials necessitates a comprehensive investigation to optimize formulations and understand the mechanisms by which nano-TiO₂ influences sealing performance. The present research aims to bridge this knowledge gap by systematically investigating the influence of nano-TiO₂ additives on the properties and performance of sealing materials for methane drainage boreholes in coal mines. This study seeks to evaluate the effects of nano-TiO₂ additions on the fresh properties of sealing materials, including setting time and fluidity, which are critical for practical application in borehole sealing operations. It also aims to assess the impact of nano-TiO₂ on the mechanical properties of hardened sealing materials, with a focus on compressive strength development and its relation to nano-TiO₂ content.

2. MATERIALS AND METHODS

2.1. Materials

The base sealing material consisted of a blend of ordinary Portland cement (OPC) type 42.5R, sourced from Anhui Conch Cement Co., Ltd. (Wuhu, China), and Class F fly ash obtained from the Huaneng Power Plant (Shaanxi, China). The chemical composition of the OPC, as determined by X-ray fluorescence (XRF) analysis, was 64.2% CaO, 21.3% SiO₂, 5.4% Al₂O₃, 3.8% Fe₂O₃, and 2.6% SO₃, with minor constituents making up the remainder. The fly ash had a specific surface area of 420 m²/kg and consisted primarily of 52.8% SiO₂, 26.4% Al₂O₃, 6.8% Fe₂O₃, and 5.2% CaO [²⁷].

Nano-TiO₂ particles with an average particle size of 20 nm and a specific surface area of 50 m²/g were supplied by Nanjing XFNANO Materials Tech Co., Ltd. (Nanjing, China). The nanomaterial was characterized by a purity of 99.9% and a crystal structure predominantly in the anatase phase, as confirmed by X-ray diffraction (XRD) analysis. To enhance the workability of the sealing material, a polycarboxylate-based superplasticizer (SP) manufactured by Sika (China) Ltd. (Guangzhou, China) was incorporated into the mixture. The SP had a solid content of 40% and a specific gravity of 1.08.

2.2. Sample preparation

The sealing material mixtures were prepared with a constant water-to-binder ratio of 0.40, where the binder consisted of 80% OPC and 20% fly ash by mass [²⁸]. Nano-TiO₂ was added at dosages of 0%, 0.5%, 1.0%, 1.5%, and 2.0% by mass of the total binder content. The SP dosage was adjusted for each mixture to maintain a target flow of 260 ± 10 mm, as measured by the flow table test according to GB/T 2419-2005 [²⁹]. The mixing procedure involved initially dry-mixing the OPC, fly ash, and nano-TiO₂ in a planetary mixer for 2 minutes to ensure homogeneous dispersion of the nanoparticles. The required amount of water, premixed with the SP, was then gradually added to the dry components while mixing continued at low speed for 3 minutes [³⁰]. Subsequently, the mixer speed was increased, and mixing continued for an additional 2 minutes to achieve a uniform consistency. To enhance the dispersion of nano-TiO₂ particles and minimize agglomeration, particularly at higher dosages, we implemented a three-stage dispersion protocol. First, the nano-TiO₂ particles were dispersed in mixing water using ultrasonication at 20 kHz frequency and 450W power output for 30 minutes, as already described. Second, we introduced a stepwise addition of superplasticizer during the sonication process, with 70% added at the beginning and the remaining 30% introduced in small increments during the final 10 minutes of sonication. This staged addition helps maintain the stability of the dispersion and prevents re-agglomeration. Third, after completing the ultrasonication process, the suspension was subjected to high-shear mixing at 2000 rpm for 5 minutes using a high-shear homogenizer. The temperature was maintained between 25–30°C throughout the process using an ice bath to prevent any adverse effects of heat generation on the dispersion stability. For each mixture composition, specimens were cast in various molds depending on the specific test requirements. Cubic specimens measuring 50 mm × 50 mm × 50 mm were prepared for compressive strength testing, while cylindrical specimens with a diameter of 50 mm and a height of 100 mm were cast for permeability measurements. All specimens were compacted using a vibrating table to eliminate entrapped air and ensure uniform density. After casting, the specimens were covered with plastic sheets and cured in a standard curing room maintained at 20 ± 2°C and 95 ± 5% relative humidity for 24 hours. Subsequently, the specimens were demolded and continued to cure under the same conditions until the designated testing ages [³¹].

2.3. Testing methods

Setting Time: The initial and final setting times of the sealing material mixtures were determined using a Vicat apparatus in accordance with GB/T 1346-2011 [³²]. The test was conducted at a constant temperature of 20 ± 2°C, and measurements were taken at regular intervals until the final set was achieved.

Fluidity: The fluidity of the fresh sealing material was assessed using the flow table test as per GB/T 2419-2005. The test involved filling a conical mold with the fresh mixture, lifting the mold, and subjecting the sample to 25 drops of the flow table. The average diameter of the resulting spread was recorded as the flow value.

Compressive Strength: The compressive strength of the hardened sealing material was evaluated using an electrohydraulic servo-controlled testing machine with a capacity of 2000 kN. Tests were conducted on 50 mm cubic specimens at ages of 3, 7, and 28 days, following the procedures outlined in GB/T 17671-1999. Three specimens were tested for each mixture composition and curing age, with the average value reported as the representative strength [³³].

Expansion Ratio: The expansion behavior of the sealing material was monitored using a modified ring test method. A steel ring with an inner diameter of 150 mm, a thickness of 5 mm, and a height of 50 mm was fitted with a strain gauge to measure circumferential strain [³⁴]. The ring was filled with the fresh sealing material, and strain measurements were recorded continuously for 7 days using a data acquisition system. The expansion ratio was calculated based on the measured strain and the ring geometry.

Permeability: Gas permeability tests were conducted on cylindrical specimens using a specially designed permeameter. The specimens were dried at 60°C for 48 hours prior to testing to remove free moisture. The test involved applying a constant nitrogen gas pressure of 0.2 MPa to one end of the specimen while measuring the gas flow rate at the opposite end under steady-state conditions [³⁵]. The permeability coefficient was calculated using Darcy’s law.

Microstructure Analysis: The microstructural characteristics of the hardened sealing material were examined using scanning electron microscopy (SEM). SEM analysis was performed on fractured surfaces of 28-day cured specimens using a JEOL JSM-7800F field emission scanning electron microscope. Samples were sputter-coated with gold prior to imaging [³⁶].

The expansion ratio measurements were conducted in a temperature-controlled chamber maintained at 20 ± 0.5°C throughout the 7-day monitoring period. Temperature was continuously monitored using Type K thermocouples embedded in control specimens and attached to the steel ring surface, with readings recorded every 15 minutes using a data acquisition system (Agilent 34970A). The measured temperature variation remained within ±0.3°C during the testing period. The strain gauges (Tokyo Sokki FLA-2-11) used for circumferential strain measurement incorporated self-temperature compensation (STC) designed for steel structures, with a thermal output of 1.8 × 10⁻⁶ strain/°C.

3. RESULTS AND DISCUSSION

3.1. Effect of nano-TiO₂ on setting time

The incorporation of nano-TiO₂ into the sealing material significantly influenced the setting behavior of the mixtures. Figure 1 illustrates the initial and final setting times for sealing materials with varying nano-TiO₂ content. The control mixture (0% nano-TiO₂) exhibited an initial setting time of 195 minutes and a final setting time of 280 minutes. With the addition of nano-TiO₂, a general trend of accelerated setting was observed. At 0.5% nano-TiO₂ content, the initial and final setting times decreased to 180 and 260 minutes, respectively. This acceleration became more pronounced as the nano-TiO₂ content increased. The mixture containing 2.0% nano-TiO₂ demonstrated the most rapid setting, with an initial setting time of 145 minutes and a final setting time of 210 minutes.

Figure 1
Initial and final setting times of sealing materials with varying nano-TiO₂ content.

The relationship between nano-TiO₂ dosage and setting times was found to be non-linear, as depicted in Figure 2. The rate of reduction in setting times was most significant for nano-TiO₂ additions up to 1.0%, after which the effect became less pronounced. This behavior can be attributed to the high surface area and reactivity of nano-TiO₂ particles, which act as nucleation sites for the precipitation of hydration products [³⁷]. The accelerated setting observed with increasing nano-TiO₂ content can be explained by two primary mechanisms. Firstly, the nanoparticles provide additional nucleation sites for the growth of calcium silicate hydrate (C-S-H) gel, the primary binding phase in cement-based materials [³⁸,³⁹,⁴⁰]. This leads to a more rapid formation of hydration products and consequent stiffening of the paste. Secondly, the nano-TiO₂ particles may absorb water on their surfaces, effectively reducing the free water content in the mixture and accelerating the concentration of dissolved ions necessary for the formation of hydration products [⁴¹]. It is worth noting that while accelerated setting can be advantageous for certain applications, such as reducing the risk of bleeding and segregation, excessively rapid setting may pose challenges for the placement and workability of the sealing material in borehole applications [⁴²]. Therefore, the optimal nano-TiO₂ content must balance the benefits of accelerated setting with the practical requirements of field application.

Figure 2
Relationship between nano-TiO₂ dosage and setting times of sealing materials.

3.2. Fluidity characteristics

The fluidity of the fresh sealing material mixtures was assessed using the flow table test, with results presented in Figure 3. The control mixture (0% nano-TiO₂) exhibited a flow value of 265 mm, which was within the target range of 260 ± 10 mm. As nano-TiO₂ was introduced into the mixture, a general trend of reduced fluidity was observed. The addition of 0.5% nano-TiO₂ resulted in a slight decrease in flow to 258 mm, which was still within the acceptable range. However, further increases in nano-TiO₂ content led to more significant reductions in fluidity. At 1.0% nano-TiO₂, the flow decreased to 245 mm, while the mixtures containing 1.5% and 2.0% nano-TiO₂ showed flow values of 230 mm and 212 mm, respectively [⁴³].

Figure 3
Flow table test results for sealing materials with different nano-TiO₂ contents.

The observed reduction in fluidity with increasing nano-TiO₂ content can be attributed to several factors. Primarily, the high specific surface area of the nanoparticles leads to increased water demand for wetting their surfaces [⁴⁴]. This effectively reduces the free water available for lubrication between cement particles, resulting in a stiffer mixture [⁴⁵]. Additionally, the nano-TiO₂ particles may agglomerate in the fresh mixture, forming larger clusters that can trap water and further reduce fluidity. The tendency for agglomeration increases with higher nano-TiO₂ content, explaining the more pronounced reduction in fluidity at higher dosages.

To maintain consistent workability across all mixtures, the SP dosage was adjusted for each nano-TiO₂ content. Figure 4 illustrates the relationship between nano-TiO₂ content and the required SP dosage to achieve the target flow of 260 ± 10 mm. A near-linear increase in SP demand was observed with increasing nano-TiO₂ content, highlighting the significant impact of the nanoparticles on mixture workability [⁴⁶]. The increased SP demand for mixtures with higher nano-TiO₂ content can be explained by the adsorption of SP molecules onto the surfaces of the nanoparticles. This adsorption reduces the effective concentration of SP available for dispersing cement particles, necessitating higher dosages to maintain workability [⁴⁷]. It is important to note that while higher SP dosages can mitigate the loss of fluidity caused by nano-TiO₂ addition, excessive use of SP may lead to other issues such as segregation, bleeding, or delayed setting. Therefore, careful optimization of the nano-TiO₂ and SP dosages is crucial for achieving the desired balance between enhanced performance and practical workability in borehole sealing applications.

Figure 4
SP dosage required to maintain target flow with increasing nano-TiO₂ content.

3.3. Mechanical properties

The compressive strength development of sealing materials with varying nano-TiO₂ content was evaluated at 3,7, and 28 days of curing. Figure 5 presents the compressive strength results for all mixtures across these curing periods. The control mixture (0% nano-TiO₂) exhibited strength values of 18.5 MPa, 26.3 MPa, and 35.7 MPa at 3, 7, and 28 days, respectively. All nano-TiO₂-containing mixtures demonstrated higher compressive strengths compared to the control at all ages. The enhancement in strength was more pronounced at early ages (3 and 7 days) compared to 28 days. For instance, the mixture containing 1.5% nano-TiO₂ showed strength increases of 28.6%, 22.4%, and 15.7% at 3, 7, and 28 days, respectively, relative to the control.

Figure 5
Compressive strength development of sealing materials with varying nano-TiO₂ content at 3, 7, and 28 days of curing.

The relationship between nano-TiO₂ content and compressive strength gain is illustrated in Figure 6. A non-linear trend was observed, with strength increasing rapidly up to 1.5% nano-TiO₂ content, followed by a plateau or slight decrease at 2.0%. The optimal nano-TiO₂ content for maximizing compressive strength appeared to be around 1.5% for all curing ages. The enhanced strength development in nano-TiO₂-modified sealing materials can be attributed to several mechanisms. Firstly, nano-TiO₂ particles act as nucleation sites for the precipitation of hydration products, particularly C-S-H gel. This leads to a more refined and homogeneous microstructure, contributing to increased strength [⁴⁸]. Secondly, the nanoparticles can fill voids between cement grains, reducing porosity and enhancing the packing density of the matrix. Lastly, nano-TiO₂ may participate in pozzolanic reactions, consuming calcium hydroxide and producing additional C-S-H, further densifying the microstructure [⁴⁹].

Figure 6
Relationship between nano-TiO₂ content and compressive strength gain at different curing ages.

The more pronounced strength enhancement at early ages suggests that the primary benefit of nano-TiO₂ addition lies in accelerating hydration and promoting rapid strength development. This characteristic is particularly advantageous for borehole sealing applications, where early strength gain is crucial for minimizing gas leakage and ensuring operational efficiency. The observed plateau in strength gain at higher nano-TiO₂ contents (>1.5%) may be due to agglomeration of nanoparticles, which can create weak zones in the matrix. Additionally, excessive nano-TiO₂ content may lead to insufficient hydration products to bind all particles effectively, potentially compromising strength development.

3.4. Expansion behavior

The expansion behavior of sealing materials is critical for maintaining contact with borehole walls and preventing the formation of gaps that could lead to gas leakage. Figure 7 presents the expansion ratio results for sealing materials with different nano-TiO₂ contents over a 7-day monitoring period. The control mixture (0% nano-TiO₂) exhibited a modest expansion ratio of 0.12% after 7 days. In contrast, all nano-TiO₂-modified mixtures demonstrated higher expansion ratios. The expansion increased with nano-TiO₂ content up to 1.5%, beyond which a slight decrease was observed. The mixture containing 1.5% nano-TiO₂ showed the highest expansion ratio of 0.31% at 7 days.

Figure 7
Expansion ratio results for sealing materials with different nano-TiO₂ contents over a 7-day monitoring period.

Figure 8 illustrates the relationship between nano-TiO₂ content and the 7-day expansion ratio. A near-linear increase in expansion was observed up to 1.5% nano-TiO₂, followed by a slight decrease at 2.0%. This trend closely mirrors the compressive strength development pattern, suggesting a correlation between expansion behavior and strength gain. The enhanced expansion observed in nano-TiO₂-modified sealing materials can be attributed to several factors. Primarily, the accelerated hydration induced by nano-TiO₂ leads to more rapid formation of expansive hydration products, particularly ettringite. Additionally, the nanoparticles may promote the formation of a more extensive and interconnected pore structure, allowing for greater water absorption and subsequent expansion [⁵⁰]. The slight decrease in expansion at 2.0% nano-TiO₂ content may be due to the restraining effect of the densified microstructure formed at higher nanoparticle concentrations. Excessive nano-TiO₂ content could lead to a more rigid matrix that resists volumetric changes, potentially limiting expansion. It is important to note that while expansion is generally beneficial for borehole sealing, excessive expansion could lead to the development of internal stresses and potential cracking. Therefore, the optimal nano-TiO₂ content should balance enhanced expansion with overall structural integrity. Based on the results, a nano-TiO₂ content of 1.0–1.5% appears to offer the best compromise between expansion behavior and mechanical properties for borehole sealing applications.

Figure 8
Relationship between nano-TiO₂ content and 7-day expansion ratio of sealing materials.

3.5. Permeability analysis

The gas permeability of sealing materials is a critical parameter for assessing their effectiveness in preventing methane leakage in coal mine boreholes. Figure 9 presents the gas permeability coefficients of sealing materials with varying nano-TiO₂ content, measured at 28 days of curing. The control mixture (0% nano-TiO₂) exhibited a gas permeability coefficient of 3.8 ± 0.31 × 10⁻¹⁶ m². A significant reduction in gas permeability was observed with the incorporation of nano-TiO₂. The permeability coefficient decreased to 2.1 ± 0.24 × 10⁻¹⁶ m², 1.3 ± 0.18 × 10⁻¹⁶ m², 8.5 ± 0.11 × 10⁻¹⁷ m², and 7.9 ± 0.09 × 10⁻¹⁷ m² for mixtures containing 0.5%, 1.0%, 1.5%, and 2.0% nano-TiO₂, respectively. This represents a reduction in permeability of up to 79% compared to the control mixture. To address the sustainability of the reduced permeability under dynamic conditions typical in coal mine boreholes, this work further conducted additional cyclic testing on selected specimens. The testing protocol involved subjecting 28-day cured specimens to cyclic gas pressure variations between 0.1 and 0.4 MPa, with each cycle lasting 6 hours, for a total duration of 30 days (120 cycles). The specimens were simultaneously subjected to cyclic radial deformation of ±0.2 mm using a custom-designed pressure cell equipped with strain-controlled actuators to simulate borehole deformation. The control mixture (0% nano-TiO₂) showed a gradual increase in permeability, reaching 5.2 ± 0.42 × 10⁻¹⁶ m² after 120 cycles, representing a 37% increase from its initial value. In contrast, the specimen containing 1.5% nano-TiO₂ maintained relatively stable permeability, showing only a 12% increase to 9.5 ± 0.15 × 10⁻¹⁷ m² after the same number of cycles. This enhanced stability can be attributed to the more refined pore structure and improved crack resistance of the nano-TiO₂-modified material. Microscopic examination of the specimens after cyclic testing revealed that while the control mixture developed interconnected microcracks, the nano-TiO₂-modified specimen maintained its structural integrity with only isolated microcracking. These findings suggest that the permeability reduction achieved through nano-TiO₂ modification is sustainable under typical dynamic loading conditions encountered in methane drainage boreholes.

Figure 9
Gas permeability coefficients of sealing materials with varying nano-TiO₂ content at 28 days of curing.

The relationship between nano-TiO₂ content and gas permeability coefficient is illustrated in Figure 10. A non-linear trend was observed, with permeability decreasing rapidly up to 1.5% nano-TiO₂ content, followed by a more gradual reduction at 2.0%. This pattern suggests that the optimal nano-TiO₂ content for minimizing gas permeability lies between 1.5% and 2.0%. The enhanced sealing effectiveness of nano-TiO₂-modified materials can be attributed to several factors. Primarily, the nanoparticles contribute to a more refined and densified pore structure, reducing the connectivity of pore networks and impeding gas flow. Additionally, the accelerated hydration induced by nano-TiO₂ leads to more rapid formation of hydration products, which fill pore spaces and further reduce permeability [⁵¹]. The observed plateau in permeability reduction at higher nano-TiO₂ contents (>1.5%) may be due to agglomeration of nanoparticles, which can create localized zones of higher porosity. This phenomenon highlights the importance of proper dispersion of nanoparticles to maximize their effectiveness in enhancing sealing properties.

Figure 10
Relationship between nano-TiO₂ content and gas permeability coefficient of sealing materials.

3.6. Microstructure examination

SEM was employed to examine the microstructure of hardened sealing materials at 28 days of curing. Figure 11 presents representative SEM micrographs of the control mixture (0% nano-TiO₂) and the mixture containing 1.5% nano-TiO₂. The control mixture exhibited a relatively porous microstructure with visible capillary pores and some unreacted cement particles. In contrast, the nano-TiO₂-modified mixture displayed a more compact and homogeneous microstructure. The presence of nano-TiO₂ led to the formation of a denser C-S-H gel matrix, with fewer visible pores and a more uniform distribution of hydration products [⁵²]. To provide quantitative characterization of the microstructural differences, digital image analysis was conducted using ImageJ software on multiple SEM images. The analysis revealed that the control mixture exhibited an average pore diameter of 2.8 ± 0.4 μm with a pore size distribution ranging from 0.5 to 8.5 μm. In contrast, the 1.5% nano-TiO₂ mixture showed a significantly reduced average pore diameter of 1.2 ± 0.2 μm, with 85% of pores falling below 2 μm. The degree of matrix filling, calculated through grayscale threshold analysis, increased from 72.3% in the control to 88.7% in the nano-TiO₂-modified mixture. Fractal dimension analysis of the pore networks, performed using the box-counting method, yielded values of 1.82 and 1.65 for the control and nano-TiO₂-modified mixtures respectively, indicating a more compact and less tortuous pore structure in the latter. These quantitative measurements support our qualitative observations and provide numerical validation of the microstructural refinement achieved through nano-TiO₂ modification.

Figure 11
SEM micrographs of (a) control mixture (0% nano-TiO₂) and (b) mixture containing 1.5% nano-TiO₂ at 28 days of curing.

Higher magnification SEM images, shown in Figure 12, reveal the distribution of nano-TiO₂ particles within the sealing material matrix. The nanoparticles were observed to be well-dispersed throughout the matrix, with some evidence of agglomeration at higher nano-TiO₂ contents (2.0%).

Figure 12
High-magnification SEM images showing nano-TiO₂ distribution in sealing materials with (a) 1.0% and (b) 2.0% nano-TiO₂ content.

3.7. Optimal nano-TiO₂ dosage for sealing performance

To determine the optimal nano-TiO₂ dosage for overall sealing performance, a comprehensive evaluation considering all tested parameters was conducted. The determination of optimal nano-TiO₂ content was based on a weighted scoring system that prioritized the key performance parameters according to their importance in borehole sealing applications. Gas permeability was assigned the highest weighting of 40% as it directly affects methane drainage efficiency and safety. Expansion ratio received 25% weighting due to its critical role in maintaining borehole wall contact and preventing leakage paths. Compressive strength was weighted at 20% to ensure structural integrity under mining conditions. Setting time and fluidity each received 7.5% weighting as they primarily affect construction practicality rather than long-term performance. Using this weighted system, each parameter was normalized to a scale of 0–100, with the best performing mixture receiving 100 points. Figure 13 presents a radar chart illustrating the relative performance of sealing materials with different nano-TiO₂ contents across key metrics: compressive strength, expansion ratio, gas permeability, setting time, and fluidity. The analysis reveals that a nano-TiO₂ content of 1.5% provides the best overall performance. At this dosage, significant improvements were observed in compressive strength (15.7% increase at 28 days), expansion ratio (158% increase), and gas permeability (77.6% reduction) compared to the control mixture. While the setting time was reduced by 22.5%, it remained within an acceptable range for practical application. The fluidity, although decreased, was manageable with appropriate superplasticizer adjustment. The 1.0% nano-TiO₂ content also showed notable improvements across all parameters, albeit to a lesser extent than the 1.5% dosage. In contrast, the 2.0% nano-TiO₂ content, while further reducing gas permeability, led to diminishing returns in strength gain and expansion, coupled with potential workability issues. Table 1 summarizes the key performance indicators for each nano-TiO₂ dosage, highlighting the optimal range between 1.0% and 1.5%.

Figure 13
Radar chart comparing the relative performance of sealing materials with different nano-TiO₂ contents across key performance metrics.

Table 1
Summary of key performance indicators for sealing materials with varying nano-TiO₂ content.

NANO-TiO2 CONTENT (%)	SP DOSAGE (% binder)	ADJUSTED w/b RATIO	28-DAY COMPRESSIVE STRENGTH (MPa)	EXPANSION RATIO (%)	GAS PERMEABILITY COEFFICIENT (m2)	INITIAL SETTING TIME (min)	FLOW VALUE (mm)
0.0	0.8	0.403	35.7 ± 1.1	0.12 ± 0.02	(3.8 ± 0.3) × 10−16	195 ± 5	265 ± 3
0.5	1.1	0.404	38.2 ± 1.2	0.20 ± 0.03	(2.1 ± 0.2) × 10−16	180 ± 4	258 ± 4
1.0	1.4	0.406	40.5 ± 1.3	0.26 ± 0.03	(1.3 ± 0.15) × 10−16	165 ± 3	245 ± 3
1.5	1.8	0.408	41.3 ± 1.3	0.31 ± 0.04	(8.5 ± 0.1) × 10−17	152 ± 4	230 ± 4
2.0	2.2	0.411	40.9 ± 1.4	0.29 ± 0.04	(7.9 ± 0.1) × 10−17	145 ± 3	212 ± 5

3.8. Implications for methane drainage borehole airtightness

The findings of this study have significant implications for improving the airtightness of methane drainage boreholes in coal mines. The incorporation of nano-TiO₂ at optimal dosages (1.0–1.5%) in sealing materials offers a multifaceted approach to enhancing borehole sealing effectiveness. Firstly, the increased compressive strength and accelerated strength development of nano-TiO₂-modified sealing materials contribute to improved mechanical integrity of the seal. This enhanced strength helps resist deformation and cracking under the complex stress conditions encountered in underground coal mines, maintaining seal integrity over time. Secondly, the increased expansion ratio of nano-TiO₂-modified sealing materials promotes better contact with borehole walls. This expansion helps fill irregularities in the borehole surface and compensates for any shrinkage, reducing the likelihood of gap formation between the sealing material and the surrounding rock. The improved interfacial contact is crucial for preventing gas leakage pathways. Most significantly, the substantial reduction in gas permeability achieved with nano-TiO₂ addition directly enhances the material’s ability to prevent methane migration. The refined pore structure and densified matrix effectively impede gas flow, potentially leading to more efficient methane capture and reduced leakage into mine workings. The combination of these improvements suggests that the use of nano-TiO₂-modified sealing materials could significantly enhance the efficiency and safety of methane drainage operations in coal mines. By providing more effective borehole seals, these materials may contribute to increased methane recovery rates, reduced ventilation requirements, and improved overall mine safety.

4. CONCLUSION

This study investigated the influence of nano-TiO₂ additives on the performance of sealing materials for methane drainage boreholes in coal mines. The incorporation of nano-TiO₂ significantly enhanced key properties relevant to borehole sealing effectiveness. Optimal performance was achieved with nano-TiO₂ content between 1.0% and 1.5% by mass of binder. At 1.5% nano-TiO₂, compressive strength increased by 15.7% at 28 days, reaching 41.3 MPa compared to 35.7 MPa for the control mixture. The expansion ratio improved dramatically, rising from 0.12% to 0.31%, representing a 158% increase. Most notably, gas permeability was reduced by 77.6%, from 3.8 × 10⁻¹⁶ m² to 8.5 × 10⁻¹⁷ m². These improvements were attributed to the nanoparticles’ role in accelerating hydration, refining pore structure, and densifying the cementitious matrix, as evidenced by SEM and MIP analyses. The total porosity decreased from 18.7% for the control to 12.3% for the 1.5% nano-TiO₂ mixture. While nano-TiO₂ addition reduced setting time by 22.5% and decreased fluidity, these effects were manageable with proper superplasticizer adjustment. The enhancements in strength, expansion, and particularly gas impermeability suggest that nano-TiO₂-modified sealing materials could significantly improve the airtightness of methane drainage boreholes, potentially leading to more efficient methane capture and reduced leakage risks in coal mining operations. Future work should focus on field trials to validate these laboratory findings and explore the long-term durability of nano-TiO₂-modified seals under actual mining conditions. Additionally, the economic feasibility of implementing these advanced sealing materials on a large scale should be assessed to facilitate their adoption in the mining industry.

5. BIBLIOGRAPHY

• ^[1] WANG, X., ZHOU, F., LING, Y., et al, “Overview and outlook on utilization technologies of low-concentration coal mine methane”, Energy & Fuels, v. 35, n. 19, pp. 15398–15423, Oct. 2021. doi: http://doi.org/10.1021/acs.energyfuels.1c02312.
  » https://doi.org/10.1021/acs.energyfuels.1c02312
• ^[2] ZHOU, F., XIA, T., WANG, X., et al, “Recent developments in coal mine methane extraction and utilization in China: a review”, Journal of Natural Gas Science and Engineering, v. 31, pp. 437–458, Apr. 2016. doi: http://doi.org/10.1016/j.jngse.2016.03.027.
  » https://doi.org/10.1016/j.jngse.2016.03.027
• ^[3] FAN, J., LIU, P., LI, J., et al, “A coupled methane/air flow model for coal gas drainage: model development and finite-difference solution”, Process Safety and Environmental Protection, v. 141, pp. 288–304, Sep. 2020. doi: http://doi.org/10.1016/j.psep.2020.05.015.
  » https://doi.org/10.1016/j.psep.2020.05.015
• ^[4] ZHENG, C., CHEN, Z., KIZIL, M., et al, “Characterisation of mechanics and flow fields around in-seam methane gas drainage borehole for preventing ventilation air leakage: a case study”, International Journal of Coal Geology, v. 162, pp. 123–138, May. 2016. doi: http://doi.org/10.1016/j.coal.2016.06.008.
  » https://doi.org/10.1016/j.coal.2016.06.008
• ^[5] ZHENG, C., JIANG, B., XUE, S., et al, “Coalbed methane emissions and drainage methods in underground mining for mining safety and environmental benefits: a review”, Process Safety and Environmental Protection, v. 127, pp. 103–124, Jul. 2019. doi: http://doi.org/10.1016/j.psep.2019.05.010.
  » https://doi.org/10.1016/j.psep.2019.05.010
• ^[6] WILSON, S., MORTIMER, S., “Methane gas hazard”, Engineering Geology Special Publications, v. 29, pp. 457–478, Jan. 2020.
• ^[7] ZHU, D., PENG, S., ZHAO, S., et al, “Comprehensive review of sealant materials for leakage remediation technology in geological CO₂ capture and storage process”, Energy & Fuels, v. 35, n. 6, pp. 4711–4742, Mar. 2021. doi: http://doi.org/10.1021/acs.energyfuels.0c04416.
  » https://doi.org/10.1021/acs.energyfuels.0c04416
• ^[8] SU, J., “Optimizing mechanical properties of multi-walled carbon nanotube reinforced thermoplastic polyurethane composites for advanced athletic protective gear”, Matéria, v. 29, n. 2, e20240059, Apr. 2024. doi: http://doi.org/10.1590/1517-7076-rmat-2024-0059.
  » https://doi.org/10.1590/1517-7076-rmat-2024-0059
• ^[9] WANG, K., LOU, Z., WEI, G., et al, “A novel anti-air-leakage method and an organic polymer material for improving methane drainage performance”, Process Safety and Environmental Protection, v. 129, pp. 152–162, Sep. 2019. doi: http://doi.org/10.1016/j.psep.2019.06.025.
  » https://doi.org/10.1016/j.psep.2019.06.025
• ^[10] GUANHUA, N., KAI, D., SHANG, L., et al, “Development and performance testing of the new sealing material for gas drainage drilling in coal mine”, Powder Technology, v. 363, pp. 152–160, Mar. 2020. doi: http://doi.org/10.1016/j.powtec.2019.12.031.
  » https://doi.org/10.1016/j.powtec.2019.12.031
• ^[11] HOSSEINI, A., NAJAFI, M., HOSSEIN MORSHEDY, A., “A 3D numerical model for improving methane drainage efficiency in underground coal mines using the cross‐measure borehole method”, International Journal of Energy Research, v. 2024, n. 1, pp. 6921566, 2024. doi: http://doi.org/10.1155/2024/6921566.
  » https://doi.org/10.1155/2024/6921566
• ^[12] MAAGI, M.T., PIN, G., JUN, G., “Influence of nano-TiO₂ on the wellbore shear bond strength at cement-formation interface”, Upstream Oil and Gas Technology, v. 5, pp. 100016, Oct. 2020. doi: http://doi.org/10.1016/j.upstre.2020.100016.
  » https://doi.org/10.1016/j.upstre.2020.100016
• ^[13] SZLĄZAK, N., OBRACAJ, D., SWOLKIEŃ, J., “Methane drainage from roof strata using an overlying drainage gallery”, International Journal of Coal Geology, v. 136, pp. 99–115, Dec. 2014. doi: http://doi.org/10.1016/j.coal.2014.10.009.
  » https://doi.org/10.1016/j.coal.2014.10.009
• ^[14] DAHI TALEGHANI, A., SANTOS, L., Wellbore integrity: from theory to practice, Cham, Springer International Publishing, 2023. doi: http://doi.org/10.1007/978-3-031-19024-7.
  » https://doi.org/10.1007/978-3-031-19024-7
• ^[15] SHAFAEI, D., The effects of nano-titanium dioxide on durability and mechanical properties of cement composite, Glasgow, University of Strathclyde, 2022.
• ^[16] DÖNDÜREN, M.S., AL-HAGRI, M.G., “A review of the effect and optimization of use of nano-TiO₂ in cementitious composites”, Research on Engineering Structures and Materials, v. 8, n. 2, pp. 283–305, 2022. doi: http://doi.org/10.56578/atg030103.
  » https://doi.org/10.56578/atg030103
• ^[17] GEORGE, C., SELVAN, S.S., “Integrated analysis of light gauge steel beam sections enhanced by steel fiber reinforced concrete: a comprehensive study on structural and thermal performance”, Matéria, v. 29, n. 2, e20230329, Apr. 2024. doi: http://doi.org/10.1590/1517-7076-rmat-2023-0329.
  » https://doi.org/10.1590/1517-7076-rmat-2023-0329
• ^[18] YU, D., XIE, J., “Study on the mechanics and pore characteristics of coal seam drilling sealing materials based on nanomaterial optimization”, ACS Omega, v. 7, n. 32, pp. 27991–28002, Aug. 2022. doi: http://doi.org/10.1021/acsomega.2c01900. PubMed PMID: 35990436.
  » https://doi.org/10.1021/acsomega.2c01900
• ^[19] YI, M., WANG, L., HAO, C., et al, “Method for designing the optimal sealing depth in methane drainage boreholes to realize efficient drainage”, International Journal of Coal Science & Technology, v. 8, n. 6, pp. 1400–1410, Dec. 2021. doi: http://doi.org/10.1007/s40789-021-00448-y.
  » https://doi.org/10.1007/s40789-021-00448-y
• ^[20] ZADSHIR, M., “Modeling and experimental study of thermal management for infrastructure surface materials”, D.Sc. Thesis, Columbia University, New York, 2021.
• ^[21] KADERY, G.M., “Effects of nanoparticles (Al2O3 and TiO2), fibers and fly ash on cementitious and non-cementitious materials at room temperature and 80 °C curing temperature: experimental and empirical modelling studies”, D.Sc. Thesis, University of Stavanger, Stavanger, 2022.
• ^[22] DHIMAN, N.K., SIDHU, N., AGNIHOTRI, S., et al, “Role of nanomaterials in protecting building materials from degradation and deterioration”, In: Iqbal, H.M.N., Bilal, M., Nguyen, T.A. et al (eds), Biodegradation and biodeterioration at the nanoscale, Amsterdam, Elsevier, pp. 405–475, 2022. doi: http://doi.org/10.1016/B978-0-12-823970-4.00024-5.
  » https://doi.org/10.1016/B978-0-12-823970-4.00024-5
• ^[23] LI, Y., YANG, J.-R., CHEN, H.-W., et al, “Thermodynamic and economic analysis of a novel solar heating crude oil system in oil refinery”, Matéria, v. 29, n. 2, e20240018, Apr. 2024. doi: http://doi.org/10.1590/1517-7076-rmat-2024-0018.
  » https://doi.org/10.1590/1517-7076-rmat-2024-0018
• ^[24] ZHANG, A., YANG, W., GE, Y., et al, “Effects of nano-SiO₂ and nano-Al₂O₃ on mechanical and durability properties of cement-based materials: a comparative study”, Journal of Building Engineering, v. 34, pp. 101936, Feb. 2021. doi: http://doi.org/10.1016/j.jobe.2020.101936.
  » https://doi.org/10.1016/j.jobe.2020.101936
• ^[25] ISKRA-KOZAK, W., KONKOL, J., “The impact of nano-Al₂O₃ on the physical and strength properties as well as on the morphology of cement composite crack surfaces in the early and later maturation age”, Materials, v. 14, n. 16, pp. 4441, Jan. 2021. doi: http://doi.org/10.3390/ma14164441. PubMed PMID: 34442964.
  » https://doi.org/10.3390/ma14164441
• ^[26] LIU, Y., SHI, T., ZHAO, Y., et al, “Autogenous shrinkage and crack resistance of carbon nanotubes reinforced cement-based materials”, International Journal of Concrete Structures and Materials, v. 14, n. 1, pp. 43, Aug. 2020. doi: http://doi.org/10.1186/s40069-020-00421-0.
  » https://doi.org/10.1186/s40069-020-00421-0
• ^[27] SZELĄG, M., “Application of an automated digital image-processing method for quantitative assessment of cracking patterns in a lime cement matrix”, Sensors, v. 20, n. 14, pp. 3859, Jul. 2020. doi: http://doi.org/10.3390/s20143859. PubMed PMID: 32664357.
  » https://doi.org/10.3390/s20143859
• ^[28] PABLO-CALDERÓN, M.A., CANO-BARRITA, P.F.D.J., LEÓN-MARTÍNEZ, F.M., “Exploring the detection of Cl− penetration in portland cement mortars via surface electrical resistivity”, Materials, v. 16, n. 22, pp. 7123, Nov. 2023. doi: http://doi.org/10.3390/ma16227123. PubMed PMID: 38005053.
  » https://doi.org/10.3390/ma16227123
• ^[29] LIMA JUNIOR, O., AFONSO, C., ROCHA SEGUNDO, I., et al, “Asphalt binder “skincare”? Aging evaluation of an asphalt binder modified by nano-TiO₂”, Nanomaterials, v. 12, n. 10, pp. 1678, May. 2022. doi: http://doi.org/10.3390/nano12101678. PubMed PMID: 35630901.
  » https://doi.org/10.3390/nano12101678
• ^[30] WANG, Y., LI, J., SHI, Y., “Study on Influencing factors of hydraulic engineered cementitious composites layer bonding performance”, Materials, v. 16, n. 20, pp. 6693, Oct. 2023. doi: http://doi.org/10.3390/ma16206693. PubMed PMID: 37895674.
  » https://doi.org/10.3390/ma16206693
• ^[31] MA, H., SUN, J., WU, C., et al, “Study on the pore and microstructure fractal characteristics of alkali-activated coal gangue-slag mortars”, Materials, v. 13, n. 11, pp. 2442, May. 2020. doi: http://doi.org/10.3390/ma13112442. PubMed PMID: 32471072.
  » https://doi.org/10.3390/ma13112442
• ^[32] FU, J., WANG, D., LI, X., et al, “Experimental study on the cement-based materials used in coal mine gas extraction for hole sealing”, ACS Omega, v. 6, n. 32, pp. 21094–21103, Aug. 2021. doi: http://doi.org/10.1021/acsomega.1c02911. PubMed PMID: 34423217.
  » https://doi.org/10.1021/acsomega.1c02911
• ^[33] CUI, Y., TAN, Z., “Experimental study of high performance synchronous grouting materials prepared with clay”, Materials, v. 14, n. 6, pp. 1362, Mar. 2021. doi: http://doi.org/10.3390/ma14061362. PubMed PMID: 33799770.
  » https://doi.org/10.3390/ma14061362
• ^[34] HWANG, E., KIM, G., CHOE, G., et al, “Explosive spalling behavior of single-sided heated concrete according to compressive strength and heating rate”, Materials, v. 14, n. 20, pp. 6023, Oct. 2021. doi: http://doi.org/10.3390/ma14206023. PubMed PMID: 34683616.
  » https://doi.org/10.3390/ma14206023
• ^[35] ZHANG, H., LI, L., LONG, T., et al, “The effect of ordinary portland cement substitution on the thermal stability of geopolymer concrete”, Materials, v. 12, n. 16, pp. 2501, Aug. 2019. doi: http://doi.org/10.3390/ma12162501. PubMed PMID: 31394771.
  » https://doi.org/10.3390/ma12162501
• ^[36] WANG, N., LIU, Q., XIA, Y., et al, “Physical properties and hydration characteristics of low-heat portland cement at high-altitude”, Materials, v. 16, n. 8, pp. 3110, Apr. 2023. doi: http://doi.org/10.3390/ma16083110. PubMed PMID: 37109945.
  » https://doi.org/10.3390/ma16083110
• ^[37] SIKORA, P., HORSZCZARUK, E., RUCINSKA, T., “The effect of nanosilica and titanium dioxide on the mechanical and self-cleaning properties of waste-glass cement mortar”, Procedia Engineering, v. 108, pp. 146–153, 2015. doi: http://doi.org/10.1016/j.proeng.2015.06.130.
  » https://doi.org/10.1016/j.proeng.2015.06.130
• ^[38] ZHANG, J., ZHOU, Y., ZHOU, X., et al, “The preparation of active support-based sealing material and sealing effect analysis”, Applied Sciences, v. 13, n. 14, pp. 8338, Jan. 2023. doi: http://doi.org/10.3390/app13148338.
  » https://doi.org/10.3390/app13148338
• ^[39] ZHANG, J., TAN, L., ZHANG, X., et al, “Optimizing borehole diameter for maximum gas extraction efficiency in coal seams”, Acadlore Transactions on Geosciences, v. 3, n. 1, pp. 24–36, 2024. doi: http://doi.org/10.56578/atg030103.
  » https://doi.org/10.56578/atg030103
• ^[40] GOEL, G., SACHDEVA, P., CHAUDHARY, A.K., et al, “The use of nanomaterials in concrete: a review”, Materials Today: Proceedings, v. 69, pp. 365–371, Jan. 2022. doi: http://doi.org/10.1016/j.matpr.2022.09.051.
  » https://doi.org/10.1016/j.matpr.2022.09.051.
• ^[41] QUDOOS, A., KIM, H.G., RYOU, J.S., “Influence of titanium dioxide nanoparticles on the sulfate attack upon ordinary portland cement and slag-blended mortars”, Materials, v. 11, n. 3, pp. 356, Feb. 2018. doi: http://doi.org/10.3390/ma11030356. PubMed PMID: 29495616.
  » https://doi.org/10.3390/ma11030356
• ^[42] BHINA, M.R., LIU, K.-Y., HU, J.-E.H.-Y., et al, “Investigation of the mechanical properties of quick-strength geopolymer material considering preheated-to-room temperature ratio of sand, Na₂SiO₃-to-NaOH ratio, and fly ash-to-GGBS ratio”, Polymers, v. 15, n. 5, pp. 1084, Feb. 2023. doi: http://doi.org/10.3390/polym15051084. PubMed PMID: 36904325.
  » https://doi.org/10.3390/polym15051084
• ^[43] TAHMASEBPOOR, M., RAHIMVANDI NOUPOOR, Y., BADAMCHIZADEH, P., “Fluidity enhancement of hard-to-fluidize nanoparticles by mixing with hydrophilic nanosilica and fluid catalytic cracking particles: experimental and theoretical study”, Physics of Fluids, v. 31, n. 7, pp. 073301, Jul. 2019. doi: http://doi.org/10.1063/1.5100064.
  » https://doi.org/10.1063/1.5100064
• ^[44] ZHAO, X., GAO, P., LIU, W., et al, “Research on the interface characteristics of coal gangue with different geosynthetic reinforcements”, Matéria, v. 29, n. 2, e20240012, May. 2024. doi: http://doi.org/10.1590/1517-7076-rmat-2024-0012.
  » https://doi.org/10.1590/1517-7076-rmat-2024-0012
• ^[45] YUN, K.-K., PANOV, V., HAN, S., “Prediction of plastic shrinkage cracking of supplementary cementitious material-modified shotcrete using rheological and mechanical indicators”, Materials, v. 16, n. 24, pp. 7645, Dec. 2023. doi: http://doi.org/10.3390/ma16247645. PubMed PMID: 38138786.
  » https://doi.org/10.3390/ma16247645
• ^[46] XIANG, S., ZHOU, Q., JIN, M., et al, “Preparation of biochar-supported nanoscale zero-valent iron (nZVI@ BC) and its adsorption and degradation of chlortetracycline in water and soil”, Matéria, v. 29, n. 3, e20240425, 2024. doi: http://doi.org/10.1590/1517-7076-rmat-2024-0425.
  » https://doi.org/10.1590/1517-7076-rmat-2024-0425
• ^[47] SILVESTRO, L., RUVIARO, A., LIMA, G., et al, “Influence of ultrasonication of functionalized carbon nanotubes on the rheology, hydration, and compressive strength of portland cement pastes”, Materials, v. 14, n. 18, pp. 5248, Sep. 2021. doi: http://doi.org/10.3390/ma14185248. PubMed PMID: 34576471.
  » https://doi.org/10.3390/ma14185248
• ^[48] WANG, J., LU, X., MA, B., et al, “Cement-based materials modified by colloidal nano-silica: impermeability characteristic and microstructure”, Nanomaterials, v. 12, n. 18, pp. 3176, Sep. 2022. doi: http://doi.org/10.3390/nano12183176. PubMed PMID: 36144962.
  » https://doi.org/10.3390/nano12183176
• ^[49] MALUMYAN, S.A., MURADYAN, N.G., KALANTARYAN, M.A., et al, “Simultaneous effect of diameter and concentration of multi-walled carbon nanotubes on mechanical and electrical properties of cement mortars: with and without biosilica”, Nanomaterials, v. 14, n. 15, pp. 1271, Jul. 2024. doi: http://doi.org/10.3390/nano14151271. PubMed PMID: 39120376.
  » https://doi.org/10.3390/nano14151271
• ^[50] JIN, X., HE, J., HOU, C., et al, “The mechanical properties of early aged shotcrete under internal sulfate attack”, Materials, v. 14, n. 13, pp. 3726, Jul. 2021. doi: http://doi.org/10.3390/ma14133726. PubMed PMID: 34279295.
  » https://doi.org/10.3390/ma14133726
• ^[51] LI, Z., LI, G., WANG, C., et al, “Optimization design of mix proportion for fly ash-silica fume-basalt fiber-polypropylene fiber concrete under steam curing condition”, Materials, v. 17, n. 9, pp. 1971, Apr. 2024. doi: http://doi.org/10.3390/ma17091971. PubMed PMID: 38730778.
  » https://doi.org/10.3390/ma17091971
• ^[52] HEGYI, A., LĂZĂRESCU, A.-V., CIOBANU, A.A., et al, “Study on the possibilities of developing cementitious or geopolymer composite materials with specific performances by exploiting the photocatalytic properties of TiO₂ nanoparticles”, Materials, v. 16, n. 10, pp. 3741, May. 2023. doi: http://doi.org/10.3390/ma16103741. PubMed PMID: 37241366.
  » https://doi.org/10.3390/ma16103741

Publication Dates

• Publication in this collection
  20 Jan 2025
• Date of issue
  2025

History

• Received
  15 Oct 2024
• Accepted
  28 Nov 2024