A Novel Pervious Concrete Improved by Hexagonal Boron Nitride and Basalt Fiber in Mechanical Properties, Permeability, and Micro-Mechanisms

Abstract

In order to overcome the limitations of traditional pervious concrete, which is difficult to balance in terms of both mechanical properties and permeability, this study proposed a novel and effective approach to improve the performances of pervious concrete (PC) based on hexagonal boron nitride (h-BN) and basalt fibers (BF). The mechanical properties and permeability tests of PC with single-doped or double-doped h-BN and BF were conducted first. Then the influence laws of h-BN and BF content on the compressive strength, flexural strength, porosity, and permeability coefficient for PC were revealed. Finally, the micro-mechanism effects of h-BN and BF on the performances of PC were explored by using a scanning electron microscope and an energy dispersive spectrometer. The results showed that the compressive strength of PC was increased with the increase in the h-BN content, and the flexural strength, porosity, and permeability coefficient increased first and then decreased. Meanwhile, with the increase in the BF content, the compressive strength and flexural strength of PC increased first and then decreased. Moreover, the compressive strength, flexural strength, porosity, and permeability coefficient of the proposed pervious concrete were 22.8 MPa, 5.17 MPa, 18.5%, and 5.09 mm/s, respectively, which were increases of 21.9%, 19.7%, 60.9%, and 42.2%, respectively, compared with ordinary permeable concrete when the optimal admixture combination was 15% fly ash, 0.08% h-BN, and 2.25% BF. This study can avoid the limitations of traditional pervious concrete and provide an efficient alternative way for improving the mechanical and permeability properties of pervious concrete.

1. Introduction

The increasing frequency of extreme weather events globally has escalated the incidence of urban waterlogging, presenting formidable challenges to contemporary urban construction and management strategies. Within this scenario, pervious concrete (PC), renowned for its excellent drainage and rainwater purification capabilities, has emerged as a vital material in developing sponge cities. The efficacy of PC primarily hinges on the properties of its cementitious materials [1], which are crucial to its strength. Furthermore, porosity is a critical factor [2], significantly influencing both the strength and the durability of PC [3,4]. Considering these aspects, enhancing the mechanical properties and permeability of PC is not just a scientific pursuit but a necessary step toward mitigating the risks of urban waterlogging disasters. Therefore, it is essential to discover a way to improve and balance the mechanical properties and permeability of PC.

Over the years, a variety of materials have been found to enhance the mechanical properties of concrete. These primarily include fibers [5], graphene [6], fly ash [7], and silica fume. Investigations of these materials have yielded valuable impacts, serving as foundational references for research dedicated to strengthening PC. On one hand, the research conducted by Jiang et al. [8] revealed that incorporating BF into concrete could augment the contact area between cement paste and aggregate, thereby enhancing the concrete’s mechanical properties. Complementary to this, Sun et al. [9] explored the influence of BF length and dosage on these properties. Their findings indicated an initial increase in compressive strength with rising fiber dosage, followed by a subsequent decrease. Optimally, concrete exhibited the most significant mechanical improvement with BF measuring 6 mm in length and a volume admixture of 2%. Building on these insights, Wu et al. [10] examined the impact of varying BF dosages on the compressive strength, porosity, and permeability of PC. Their results demonstrated that a BF volume dosage of 4 kg/m³ could elevate the compressive strength of PC by 24%, while concurrently reducing its porosity and permeability coefficient by 35% and 42%, respectively. In addition, Kamisetty et al. [11] added fly ash and fiber into pervious concrete individually or in combination. It was found that the combination of mixtures can not only effectively improve the mechanical properties of pervious concrete but also improve the permeability properties to a certain extent, and fly ash plays a dominant role in the improvement of mechanical properties, while the enhancement of fibers is better than fly ash in the permeability properties. On the other hand, advancements in PC performance are not limited to increasing the contact area between cement paste and aggregate. The strength of the cement matrix can be significantly bolstered by the incorporation of graphene, thereby enhancing the bond between the cement matrix and aggregate [12]. Dimitar et al. [13] reported that nanoengineering concrete with graphene reinforcement results in cement composites possessing ultrahigh strength. Similarly, Reddy et al. [14] observed a substantial increase in compressive strength with the addition of 0.05% to 0.15% graphene oxide to concrete. However, they noted a decrease in strength at a 0.20% admixture level, likely attributable to graphene oxide agglomeration. Further corroborating these findings, Kirthi et al. [15] found an optimal graphene dosage range of 0.02% to 0.1%, enhancing the concrete compressive strength by 10% to 20%. Despite these promising results, the high cost and difficulty in sourcing graphene limit its practical application in widespread engineering projects. Hexagonal boron nitride (h-BN), a material structurally akin to graphene, emerges as a promising new addition to this domain [16]. Compared with graphene, hexagonal boron nitride is not only less expensive but also more readily available, making hexagonal boron nitride a cost-effective material. Furthermore, a study has shown that the addition of h-BN to cement can effectively enhance the performance of cement-based materials [17]. In addition, Danoglidis et al. [18] investigated the effect of h-BN nanoplatelets on the cement matrix and found that the hydroxyl and carboxyl groups on the surface of the h-BN interacted with the Ca²⁺ in the calcium silicate hydrate (C-S-H), which improved the load transfer efficiency of the cement matrix to the h-BN and thus effectively improved the mechanical properties of the cement matrix. Despite these advancements, the application of h-BN in PC remains an unexplored territory, presenting a potential avenue for groundbreaking research. In addition, a large number of scholars have investigated the effect of fly ash on the mechanical properties of concrete from the 1960s to the present [19,20]. In recent years, many scholars have explored the effect of fly ash on pervious concrete on the basis of these studies. For example, research conducted by Nazeer et al. [21] revealed that the incorporation of an optimal quantity of fly ash into PC significantly fosters the formation of C-S-H within the cement matrix, thereby enhancing both the mechanical properties and the material’s durability. In contrast, Peng et al. [22] observed that substituting fly ash for cement in equal proportions in PC not only diminishes its 28-day mechanical properties but also adversely affects its permeability. Further research supports that replacing a suitable proportion of cement with fly ash not only enhances the strength of pervious concrete but also contributes to a reduction in carbon dioxide emissions associated with cement production [23]. Apart from the above materials, silica fume is another widely used additive in pervious concrete. Liu et al. [24] investigated the impact of silica fume on the properties of pervious concrete and concluded that while its addition through internal mixing does not impair the water permeability, it substantially enhances the mechanical strength. Furthermore, Tang et al. [25] explored the combined effects of silica fume and fiber in pervious concrete on its mechanical attributes. Their findings suggest that the synergy between silica fume and fiber significantly amplifies the mechanical robustness of pervious concrete. In summary, the existing research collectively demonstrates that the mechanical properties of pervious concrete (PC) can be significantly enhanced through the incorporation of various materials such as basalt fibers, graphene, and fly ash. However, while these additions are promising, there remains a need for comprehensive research to thoroughly elucidate their effects on the permeability of PC. This gap underscores the importance of continued investigation in this field.

In response to these research gaps, the current study embarked on an experimental investigation of PC augmented with either singly or doubly doped h-BN and BF. This research meticulously evaluated the impacts of varying h-BN and BF concentrations on key properties of PC, including the compressive strength, flexural strength, permeability coefficient, and porosity. To delve deeper into the underlying mechanisms, the study employed scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) analyses, offering insights into the micro-mechanisms influencing the effects of h-BN and BF on PC’s properties.

2. Materials and Methods

2.1. Materials

This study encompassed a comprehensive range of materials, including cement, silica fume, fly ash, water reducer, aggregates, h-BN, and BF. Specifically, the length of BF was 6 mm in this investigation. Detailed parameters for both the BF and h-BN are listed in

Table 1. Parameters of basalt fiber.

Diameter (μm)	Density (g/cm3)	Tensile Strength (MPa)	Fracture Strength (MPa)	Elastic Modulus (GPa)	Fracture Elongation (%)
18	2.7	4200	3200	89	3.2

and

Table 2. Parameters of hexagonal boron nitride.

Property	Specific Surface Area (m2/g)	Tap Density (g/cm3)	Particle Size (μm)	Moisture Content (%)	Carbon Content (%)
Standard Value	<1.00	0.3–0.5	5.00–8.00	<0.50	<0.0400
Actual Value	0.95	0.42	7.04	0.41	0.0235

, respectively, with the appearances shown in Figure 1.

2.2. Mixture Proportions

In this study, standard PC served as the control group, with a water-to-binder ratio fixed at 0.3 [26]. To evaluate the impact of the content of h-BN and BF on the performances of PC, the controlled variable method was applied, maintaining a consistent fly ash content of 10%. For the composite-doped experiments, an orthogonal experimental design was employed, encompassing three factors at three different levels. Specifically, the fly ash content was varied at 5%, 10%, and 15%. The concentration of h-BN was adjusted to 0.08%, 0.10%, and 0.12% relative to the cement mass. Similarly, BF was incorporated at concentrations of 0.75%, 1.50%, and 2.25% of the cement mass. The specific mixing proportions for both the single-doped and compound-doped PC are detailed in

Table 3. Mixture proportions of PC single-doped with h-BN.

Mix Number	Weight of Ingredients (kg/m3)
	Coarse Aggregate	Fine Aggregate	Cement	Water	Silica Fume	Fly Ash	Water-Reducing Agent	h-BN
S-0-0-2	1528.8	80.5	304	91.2	16	30.4	4.56	0
BN-1	1528.8	80.5	304	91.2	16	30.4	4.56	0.25
BN-2	1528.8	80.5	304	91.2	16	30.4	4.56	0.31
BN-3	1528.8	80.5	304	91.2	16	30.4	4.56	0.37

,

Table 4. Mixture proportions of PC single-doped with BF.

Mix Number	Weight of Ingredients (kg/m3)
	Coarse Aggregate	Fine Aggregate	Cement	Water	Silica Fume	Fly Ash	Water-Reducing Agent	BF
S-0-0-2	1528.8	80.5	304	91.2	16	30.4	4.56	0
BF-1	1528.8	80.5	304	91.2	16	30.4	4.56	2.28
BF-2	1528.8	80.5	304	91.2	16	30.4	4.56	4.56
BF-3	1528.8	80.5	304	91.2	16	30.4	4.56	6.84

and

Table 5. Mixture proportions of composite-doped PC.

Mix Number	Weight of Ingredients (kg/m3)
	Coarse Aggregate	Fine Aggregate	Cement	Water	Silica Fume	Fly Ash	BF	h-BN
S-0-0-2	1528.8	80.5	304	95.76	16	30.4	0	0
Q-1-1-1	1528.8	80.5	304	95.76	16	15.2	2.28	0.25
Q-2-2-1	1528.8	80.5	304	95.76	16	15.2	4.56	0.31
Q-3-3-1	1528.8	80.5	304	95.76	16	15.2	6.84	0.37
Q-1-2-2	1528.8	80.5	304	100.32	16	30.4	4.56	0.25
Q-2-3-2	1528.8	80.5	304	100.32	16	30.4	6.84	0.31
Q-3-1-2	1528.8	80.5	304	100.32	16	30.4	2.28	0.37
Q-1-3-3	1528.8	80.5	304	104.88	16	45.6	6.84	0.25
Q-2-1-3	1528.8	80.5	304	104.88	16	45.6	2.28	0.31
Q-3-2-3	1528.8	80.5	304	104.88	16	45.6	4.56	0.37

.

2.3. Preparation of Hexagonal Boron Nitride Dispersions

It was essential to prepare the dispersion liquid of h-BN due to the agglomeration effects leading to the uneven distribution of h-BN within the pervious concrete mixture [18]. Initially, a water-reducing agent solution was prepared according to the specified mixing proportions. Subsequently, 40% of this water-reducing agent solution was taken, and a precise quantity of h-BN was added. The resulting mixture was then subjected to ultrasonic dispersion for approximately 10 min, ensuring the uniform dispersion of h-BN within the water-reducing agent solution.

2.4. Testing Methods

The WAW-1000 testing machine was employed to assess the mechanical properties. Compressive strength tests were conducted on standard cubic specimens, each measuring 150 × 150 × 150 mm (as depicted in Figure 2). Additionally, flexural strength tests were carried out using prism specimens with dimensions of 100 × 100 × 400 mm (as illustrated in Figure 3). The calculation of compressive strength and flexural strength followed the respective formulas as detailed below [27]. According to the Chinese national standard “Standard for test method of mechanical properties on ordinary concrete, GB/T 50081-2002” [28], when the size of the test specimen was a non-standard test specimen of 100 × 100 × 400 mm, it was multiplied by a size conversion factor of 0.85:

$$f_c=\frac{F}{A}$$

(1)

where f_c is the compressive strength [MPa], F is the load applied [kN], and A is the cross section area of the specimens [mm²].

$$f_{ts}=\frac{3\beta FL}{2b{h}^2}$$

(2)

where f_ts is the flexural strength [MPa]; F is the load applied [kN]; β is the conversion factor, set at 0.85; L is the length of the specimens [m]; b is the width of the specimens [m]; and h is the height of the specimens [m].

Porosity (P) tests were conducted employing the mass method, illustrated in Figure 4. The permeability coefficient (K_t) was determined using the constant head method, as depicted in Figure 5. The calculation of porosity and permeability coefficient followed the respective formulas as detailed below [27]:

$$P=(1-\frac{W_2-W_1}{\rho V})\times 100\%$$

(3)

where P is the porosity [%], W₁ is the suspended mass of the specimen in water [g], W₂ is the mass of the dry specimen [g], ρ is the density of water [g/cm³], and V is the volume of specimens [cm³].

$$K_t=\frac{L\times Q}{H\times A\times t}$$

(4)

where K_t is the permeability coefficient [mm/s]; L is the height of specimens [mm]; Q is the weight of the outflow [g]; H is the water level difference [mm]; A is the cross section area of the specimens [mm²]; and t is the time of waterflow, set at 60 s.

3. Results and Discussion

3.1. PC Single-Doped with h-BN

The results of the mechanical properties and the permeability are listed in

Table 6. The mechanical properties and the permeability of PC single-doped with h-BN.

Content/%	Compressive Strength/MPa		Flexural Strength/MPa		Porosity/%		Permeability Coefficient/(mm/s)
	7 d	28 d	7 d	28 d	7 d	28 d	7 d	28 d
0	13.7	18.7	3.22	4.32	11.0	11.5	3.43	3.58
0.08	15.9	21.5	4.02	4.68	14.8	15.1	4.23	4.34
0.10	16.9	22.2	3.71	4.65	12.9	13.5	3.89	4.01
0.12	17.7	22.6	3.64	4.30	11.5	11.8	3.54	3.72

.

To investigate the impact of the h-BN content on the mechanical properties of PC, experiments were conducted to measure the compressive strength and flexural strength at varying content levels, as shown in Figure 6.

As illustrated in Figure 6, both the compressive strength of 7 days and 28 days exhibited an upward trend with an increase in the h-BN content. In other words, there was a positive correlation between the h-BN content and the compressive strength of PC. Notably, when the h-BN content reached 0.12%, the compressive strength at 7 days and 28 days peaked at 17.7 MPa and 22.6 MPa, respectively. This represented substantial improvements of 29.2% and 20.9% compared with the control group without h-BN. Additionally, the flexural strength of PC at 7 days and 28 days demonstrated an initial increase followed by a subsequent decrease as the h-BN content rose. Specifically, when the h-BN content was 0.08%, the flexural strength of 7 days and 28 days attained its highest values at 4.0 MPa and 4.7 MPa, respectively. This corresponded to increases of 24.8% and 8.3% in comparison with the control group without h-BN.

In summary, the incorporation of an appropriate amount of h-BN proved to be an effective means of enhancing the mechanical properties of PC. The ultrasonication of the dispersion liquid of h-BN served a dual purpose—it both dispersed and created -OH bonds [29]. These -OH bonds promoted the h-BN to provide the carriers for the growth of cement hydration crystals, expediting their formation and development [30]. Moreover, h-BN functioned as a bridging agent within the cementitious materials, connecting the hydration crystals [31]. However, it was noticeable that h-BN was hard to hydrolyze and tended to remain suspended in the cement paste. Excessive addition of h-BN could result in agglomeration within the cement paste, reducing oxygen-containing groups on the surface. Furthermore, agglomeration weakened the connection between cement hydration crystals and diminished the bond between the cement paste and aggregates. Consequently, selecting an appropriate content of h-BN was crucial for significantly improving the mechanical properties of PC.

To explore the influence of the h-BN content on the permeability of PC, a series of experiments were conducted to measure the porosity (P) and permeability coefficient (K_t) at varying h-BN content levels, as illustrated in Figure 7.

The analysis of Figure 7 revealed a notable trend: both the porosity (P) and the permeability coefficient (K_t) of PC exhibited an initial increase followed by a subsequent decrease as the h-BN content rose. This performance could be attributed to the primary mechanism: an optimal h-BN content enhanced the degree of cement hydration, thereby preventing excessive cement paste from sealing the pores. Consequently, interconnected porosity within PC increased, leading to an improvement in the permeability coefficient. Conversely, an excessive h-BN content weakened the bond between the cement paste and the aggregates, resulting in an increase in sealed pores and a subsequent decrease in both the porosity and the permeability coefficient. Remarkably, when the h-BN content reached 0.08%, both P and Kt at 28 days reached their peak values, measuring 15.1% and 4.34 mm/s, respectively. Compared with the control group without h-BN, this represented significant enhancements of 31.3% and 21.2%.

An optimal quantity of h-BN enhanced the adhesive strength of the cement paste, resulting in better encapsulation of aggregates and preventing excess cement paste from sealing the pores. Consequently, effective porosity within PC increased, leading to an enhancement in the permeability coefficient. However, the agglomeration effect induced by excessive h-BN addition could weaken the bond between cement paste and aggregates, leading to an increase in sealed pores and a subsequent decrease in both the porosity and the permeability coefficient. Thus, the selection of an appropriate h-BN content proved critical in significantly enhancing the permeability of PC.

3.2. PC Single-Doped with BF

The results of the mechanical properties and the permeability are illustrated in

Table 7. The mechanical properties and the permeability of PC single-doped with BF.

Content/%	Compressive Strength/MPa		Flexural Strength/MPa		Porosity/%		Permeability Coefficient/(mm/s)
	7 d	28 d	7 d	28 d	7 d	28 d	7 d	28 d
0	13.7	18.7	3.22	4.32	11.0	11.5	3.43	3.58
0.75	20.9	23.6	3.96	4.83	16.1	16.9	4.81	5.34
1.50	17.9	22.3	3.51	4.52	12.3	12.9	3.56	4.34
2.25	13.8	18.9	3.47	4.43	15.6	16.1	4.63	5.05

.

To explore the influence of BF content on the mechanical properties of PC, a series of experiments were conducted to measure the compressive strength and flexural strength at varying BF content levels, as illustrated in Figure 8.

Analysis of Figure 8 revealed a distinct law: both the compressive strength and the flexural strength of PC at 7 days and 28 days exhibited an initial increase followed by a subsequent decrease with the increase in the BF content. Notably, when the BF content reached 0.75%, the compressive strengths at 7 days and 28 days reached their maximum values, measuring 20.9 MPa and 23.6 MPa, respectively. Simultaneously, the flexural strengths at 7 days and 28 days also attained their peak values, measuring 3.96 MPa and 4.83 MPa, respectively. Compared with the control group, these values represented remarkable improvements of 52.6% and 26.2% and 23.0% and 11.8%, respectively.

The results demonstrated that the addition of an appropriate amount of BF could enhance the mechanical performance of pervious concrete. However, it is noteworthy that an excessive dosage of BF might diminish the rate of improvement. BF, characterized by its high tensile strength, represents a novel high-performance fiber. The incorporation of an optimal BF content in PC effectively increased the contact area between aggregates, thereby bolstering the bonding force of the concrete and subsequently improving its strength [32]. Nevertheless, the content of basalt fiber exceeded 0.75%, which could lead to uneven dispersion, resulting in agglomeration with the cement paste. This, in turn, diminished the bond between the aggregates and cement paste, potentially causing cracks between the cement paste and aggregates and affecting the function of BF in PC [33,34]. Therefore, selecting the appropriate BF content proved instrumental in significantly enhancing the mechanical properties of PC.

To investigate the impact of BF content on the permeability of PC, experiments were conducted to measure the values of porosity (P) and the permeability coefficient (K_t) at various BF content levels, as illustrated in Figure 9.

Figure 9 illustrates that the permeability of PC could be enhanced to varying degrees with the addition of BF. Notably, when the BF content was at 0.75%, both the P and K_t values peaked at both 7 days and 28 days. The incorporation of an optimal amount of BF increased the contact area between the cement paste and the aggregates. The increased contact caused more effective encapsulation of the aggregates by the cement paste. The improvement of the paste flow resulted in a reduction of closed pores, leading to an increase in the interconnected porosity, which in turn positively affected the permeability coefficient.

3.3. PC Double-Doped with h-BN and BF

To reveal the impact of the combined addition of h-BN and BF on the mechanical properties and permeability of PC, an orthogonal experiment was designed and executed. This experiment measured the 28-day compressive strength, flexural strength, porosity, and permeability coefficient of PC with varied proportions of h-BN and BF. The results are listed in

Table 8. The 28d performance of composite-doped PC.

Group	Compressive Strength (MPa)	Flexural Strength (MPa)	Porosity (%)	Permeability Coefficient (mm/s)
S-0-0-2	18.7	4.32	11.5	3.58
1. Q-1-1-1	22.3	4.76	10.6	3.47
2. Q-2-2-1	23.4	4.23	11.9	4.26
3. Q-3-3-1	21.8	4.46	14.8	4.64
4. Q-1-2-2	22.7	4.51	16.6	4.53
5. Q-2-3-2	22.9	4.95	17.0	4.87
6. Q-3-1-2	20.2	4.72	10.4	3.61
7. Q-1-3-3	22.8	5.17	18.5	5.09
8. Q-2-1-3	22.6	5.07	10.2	3.42
9. Q-3-2-3	21.5	4.53	15.4	4.53

.

The analysis of Table 8 indicates an enhancement in the mechanical properties of PC with dual admixtures over ordinary PC. Specifically, the Q-2-2-1 admixture (5% fly ash, 0.1% h-BN, and 1.5% basalt fiber) resulted in a maximum compressive strength of 23.4 MPa, representing a remarkable improvement of 25.1%. In addition, the Q-1-3-3 admixture (15% fly ash, 0.08% h-BN, and 2.25% BF) led to an optimal flexural strength of 5.17 MPa, which was 19.7% higher than the control group. Moreover, the permeability of the composite-doped PC generally improved, except in combinations 1, 6, and 8. Notably, the Q-1-3-3 combination also demonstrated the highest porosity and permeability coefficient values at 18.5% and 5.09 mm/s, respectively. These figures represent a 60.9% increase in porosity and a 42.2% increase in the permeability coefficient over ordinary PC.

A range analysis was conducted to evaluate the impact of combining h-BN and BF on the mechanical properties and permeability of PC. This analysis focused on determining the K values (representing the average effect of a factor at a given level) and the R values (indicating the range of variation for each factor) for the 28-day compressive strength, flexural strength, porosity, and permeability coefficient of the PC. The specific findings from this range analysis are detailed in

Table 9. Results of range analysis.

Test Metrics	Element	K1	K2	K3	R
Compressive strength (MPa)	Fly ash	22.50	21.93	22.30	0.57
	h-BN	22.60	22.97	21.17	1.80
	BF	21.70	22.53	22.50	0.83
Flexural strength (MPa)	Fly ash	4.48	4.73	4.92	0.44
	h-BN	4.81	4.75	4.57	0.24
	BF	4.85	4.42	4.86	0.44
Porosity (%)	Fly ash	12.43	14.67	14.70	2.27
	h-BN	15.23	13.03	13.53	2.20
	BF	10.4	14.63	16.77	6.37
Permeability coefficient (mm/s)	Fly ash	4.12	4.34	4.35	0.22
	h-BN	4.36	4.18	4.26	0.18
	BF	3.50	4.44	4.87	1.37

1. The values K_i (i = 1, 2, 3) in the table are the sums of experimental data for a certain factor at a particular level, averaged. 2. R denotes the range of a factor, calculated as the maximum K_i value minus the minimum K_i value for that factor. A larger R value indicates a more significant influence of that factor on the test parameters [35].

.

An analysis of Table 9 reveals the different impacts of h-BN, BF, and fly ash on various properties of PC. For the 28-day compressive strength, the R values indicated that h-BN content (1.8) had the most significant influence, followed by BF (0.83), and then fly ash (0.57). In addition, for the 28-day flexural strength, both BF and fly ash showed equal sensitivity (R values of 0.44), with h-BN (0.24) being less influential. Regarding the 28-day porosity, the BF content (6.37) emerged as the most impactful, followed by fly ash (2.27) and h-BN (2.20). Last, in terms of the 28-day permeability coefficient, the BF content (1.37) again showed the greatest impact, while the effects of fly ash (0.22) and h-BN (0.18) were comparatively lesser.

To further analyze the impact of combined h-BN and BF content on the mechanical properties and permeability of PC, the average K values (representing the average effects of content combinations on performance indicators) of PC are plotted in Figure 10. These figures illustrate the variations in PC performance across different h-BN and BF content combinations.

From Figure 10a,b, it can be observed that the 28-day compressive strength of the PC with dual admixtures initially increased and then decreased with the increase in the h-BN content, as well as with the increase in the BF content. This similarity in the influence laws of h-BN and BF on the composite-doped PC’s compressive strength suggested a common underlying mechanism. An optimal number of admixtures appeared to strengthen the bond between the cementitious matrix and aggregates, thereby enhancing the compressive strength. However, the admixtures beyond a certain threshold would weaken this bond, leading to a reduction in the compressive strength.

Figure 10c,d offered a deeper understanding of the permeability performance in PC with composite admixtures. It revealed an inverse relationship between the effects of h-BN and fly ash on permeability compared with their impacts on the 28-day compressive strength, a trend not observed with BF. Enhanced permeability, indicative of a high number of connected pores within the PC, generally corresponded with reduced mechanical properties. While interconnected porosity is crucial for permeability, an excessive degree of it could lead to an incomplete internal structure. This heightened porosity made the PC more susceptible to cracking, consequently diminishing its mechanical strength.

In summary, this study demonstrated that the permeability performance and mechanical strength of PC could be effectively and simultaneously enhanced through the judicious incorporation of h-BN and BF.

To quantitatively analyze the influence of inaccuracies on the impact of the combined content of h-BN and BF on the mechanical properties and permeability of PC, an analysis of variance (ANOVA) was conducted to calculate the variance of the performance indicators for 28 days of PC. The results are presented in

Table 10. Result of ANOVA.

Performance Indicator	Item	Degree of Freedom	Sum of Squares	Mean Square	F	p	Significance
Compressive strength (MPa)	Fly ash	2	0.496	0.248	6.027	0.142	-
	h-BN	2	5.429	2.714	66.027	0.015	**
	BF	2	1.336	0.668	16.243	0.058	*
Flexural strength (MPa)	Fly ash	2	0.291	0.146	43.148	0.023	**
	h-BN	2	0.096	0.048	14.155	0.066	*
	BF	2	0.373	0.186	55.188	0.018	**
Porosity (%)	Fly ash	2	10.127	5.063	12.25	0.075	*
	h-BN	2	7.98	3.99	9.653	0.094	*
	BF	2	63.007	31.503	76.218	0.013	**
Permeability coefficient (mm/s)	Fly ash	2	0.095	0.048	3.847	0.206	-
	h-BN	2	0.049	0.024	1.972	0.336	-
	BF	2	2.933	1.467	118.177	0.008	***

“*” indicates the significance of the impact on the performance indicators. Specifically, the number of “*” indicates the level of significance, with more “*” implying a more pronounced impact [36].

.

An analysis of Table 10 confirms the consistency between the ANOVA results and the range analysis, lending scientific credibility and accuracy to this study’s findings. Notably, the mixture combination of 15% fly ash, 0.08% h-BN, and 2.25% BF emerged as optimal in the experiment for the 28-day evaluation of dual-doped PC. This combination showed significant improvements over ordinary PC, with increases of 19.7% in the flexural performance, 60.9% in the porosity, and 42.2% in the permeability coefficient. Moreover, a 21.9% enhancement in the 28-day compressive strength was observed compared with ordinary PC. These results highlighted the effectiveness of this specific admixture combination in simultaneously enhancing the mechanical properties and permeability of PC.

4. Micro-Mechanism Analysis of Pervious Concrete

To elucidate the micro-mechanisms underlying the influence of different additives on PC, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) [36] analyses were conducted. These techniques were applied to examine the microstructural characteristics of 28-day-old samples of ordinary PC, as well as samples single-doped with either h-BN or BF.

4.1. Micro-Mechanism Analysis of Ordinary Pervious Concrete

An SEM analysis was performed on 28-day PC specimens to examine their microstructural characteristics. This analysis specifically focused on revealing the intricacies of the boundaries, pores, and surfaces of ordinary PC. The detailed microstructural observations are shown in Figure 11 and Figure 12.

Figure 11 shows the boundary between the cement and the aggregate in the control group of this study. As can be seen from Figure 11a, the gap at the boundary between the cement and the aggregate in the sample was relatively wide and not sufficiently compact. Figure 11b reveals a height difference between the cement and aggregate, indicating that the cement paste had not completely enveloped the aggregate.

As shown in Figure 12a, the hydration products in conventional Portland cement typically include amorphous calcium silicate hydrate (C-S-H). It also contains plate-like crystals of calcium hydroxide (CH) and columnar ettringite (AFt) [37,38]. The hydration products of cement can fill the micro-pores in permeable concrete, thus creating a tight connection in the concrete, making it less susceptible to damage under compression. However, as these hydration products fill the pores, they reduce the connected porosity in the permeable concrete, consequently lowering its permeability.

Figure 12b depicts the hydration products on the surface of the permeable concrete in the control group, specifically in areas A, B, and C. According to Figure 12, the hydration crystals in the permeable concrete of the control group were arranged in a disordered manner, resulting in an overall structure that was overly loose. Additionally, the surface layer contained a certain amount of fly ash particles. There were gaps between crystals, and a small amount of flocculent gel products could be observed around these crystals. This could be attributed to the early reaction between the reactive silica on the surface of smaller-particle-size fly ash and Ca(OH)₂, where the reactivity of the fly ash within the permeable concrete had not been fully activated, leading to an insufficient amount of gel product formation. An EDS elemental analysis was conducted on areas A, B, and C of Figure 12b, with the results presented in

Table 11. EDS elemental analysis results of ordinary pervious concrete.

Element	Weight/%	Atomic/%	Error Rate/%
O	56.71	73.77	9.03
Al	3.64	2.81	22.59
Si	11.60	8.60	12.15
S	1.97	1.28	65.50
Ca	26.07	13.54	12.91

and Figure 13. The results shown in the Table 11 are the average of the measurements in areas A, B, and C.

Combining the analysis results from Table 11 and Figure 13, the hydration products on the surface of the control group contained elements such as Si, O, Al, and Ca, thereby confirming the presence of hydration products like SiO₂, Al₂O₃, and C-S-H. The Ca/Si ratio is 2.24, indicating a lower degree of crystallinity [39], which resulted in a lower compressive strength of the ordinary permeable concrete with the fly ash admixture.

4.2. Micro-Mechanism Analysis of PC Single-Doped with h-BN

An SEM analysis was conducted on 28-day PC specimens with a 0.12% admixture content of h-BN. The microscopic structures of the boundaries, pores, and surfaces are illustrated in Figure 14, Figure 15 and Figure 16.

Comparing Figure 11 and Figure 14, it is observed that after the incorporation of hexagonal boron nitride (h-BN), the gap between the cement paste and aggregates became significantly tighter, with the cement paste enveloping the aggregates more effectively. This enhanced the interconnection between the aggregates, thereby improving the performance of the permeable concrete.

According to the literature research [40], the microstructure of hexagonal boron nitride (h-BN) usually appears in circular or elliptical flake-like structures and is prone to agglomeration, thereby tending to stack and form clustered formations. An SEM (scanning electron microscopy) analysis was performed on a selected area of the surface of permeable concrete with a single admixture of h-BN, as indicated in area A in Figure 15a. Figure 15b shows that the surface of the permeable concrete with h-BN admixture featured clusters of hexagonal boron nitride. An EDS elemental analysis was conducted on Figure 15b, with the results presented in

Table 12. EDS elemental analysis results of PC single-doped with h-BN.

Element	Weight/%	Atomic/%	Error Rate/%
N	3.05	4.29	14.19
O	52.90	65.10	3.72
Al	0.74	0.54	13.29
Si	41.73	29.25	1.96
B	0.15	0.09	28.57
S	0.17	0.10	95.24
Ca	1.27	0.62	18.24

and Figure 17.

Comparing Table 11 and Table 12, it is evident that the hydration products on the surface of the permeable concrete with a single admixture of h-BN (hexagonal boron nitride) not only contained Si, O, Al, and Ca but also included N and B elements, which are not present in ordinary permeable concrete. This confirms the presence of h-BN and its hydrates on the surface of the permeable concrete. Additionally, after the incorporation of h-BN, the content of Si significantly increased, while the content of Ca noticeably decreased.

As shown in Figure 16, the hydration crystals in the h-BN permeable concrete were disordered in arrangement, but the overall structure was compact, with only a few gaps between crystals. This was primarily because h-BN could react with Ca+ in cement to enhance the interface bonding strength of cement, thereby providing higher strength to the concrete. This finding is largely consistent with the conclusions of the literature [41,42].

4.3. Micro-Mechanism Analysis of PC Single-Doped with BF

An SEM analysis was conducted on 28-day PC specimens with a 0.75% admixture content of BF. The purpose was to elucidate the micro-mechanism performance of the PC after the addition of BF. The microscopic structures are illustrated in Figure 18 and Figure 19.

As can be seen from Figure 18, after incorporating basalt fibers into the permeable concrete, although minor gaps were produced at the cement boundaries, they did not lead to significant cracks as shown in Figure 11. This suggests that the introduction of basalt fibers enhanced the mechanical properties of the permeable concrete. The fibers achieved this by increasing the contact area between the aggregates and the cement paste through a bridging action. Furthermore, as observed in Figure 18, the cement hydrates on the right side of the fiber tightly enveloped the basalt fibers, enabling the fibers to provide a more effective bridging action at the interface [43].

Figure 19 demonstrates that basalt fibers filled the micro-pores in permeable concrete. A large number of micro-pores can lead to easier crack formation in permeable concrete under external loads, leading to structural failure. When basalt fibers are added, they fill these pores and thereby inhibit crack formation, consequently enhancing the mechanical properties of the permeable concrete. However, as the fibers fill these pores, they reduce the permeability of the permeable concrete [44].

The microscopic structural analysis above thoroughly confirms the micro-mechanisms through which both h-BN and BF effectively enhance the mechanical and permeable properties of PC.

5. Conclusions

This study revealed the influence of hexagonal boron nitride (h-BN) and basalt fiber (BF) content on the mechanical properties and permeability of pervious concrete. Additionally, it explored the micro-mechanisms through which h-BN and basalt fibers affect the performance of PC. The research findings provide a theoretical foundation for expanding the practical application of PC in engineering. The main conclusions are summarized as follows:

(1)

In the single-doped experiment, the appropriate amount of hexagonal boron nitride or basalt fibers can improve the mechanical properties and water permeability of permeable concrete, on which the basalt fibers have the most obvious effect. In the double-doped experiment, the optimal combination for the composite pervious concrete was a mixture of 15% fly ash, 0.08% hexagonal boron nitride, and 2.25% basalt fiber. The 28-day compressive strength, flexural strength, porosity, and permeability coefficients were 22.8 MPa, 5.17 MPa, 18.5%, and 5.09 mm/s, respectively, representing increases of 21.9%, 19.7%, 60.9%, and 42.2%, respectively, compared with ordinary pervious concrete.

(2)

The significance of the influence of hexagonal boron nitride and basalt fiber on the 28-day compressive strength of composite pervious concrete increased and then decreased with the increase in the content. The trend of the influence of hexagonal boron nitride (h-BN) content on the 28-day permeability of dual-doped pervious concrete was opposite to the trend of the 28-day compressive strength.

(3)

The scanning electron microscopy and energy dispersive spectrometer results indicated that hexagonal boron nitride and basalt fiber could promote the degree of cement hydration and increase the contact area between cement and aggregates, thus enhancing the bond strength between cement and aggregates, ultimately improving the mechanical and permeable properties of pervious concrete.