Characterization of Nutrient-Enriched Eco-Concrete as a Functional Growth Substrate: Optimization and Horticultural Compatibility

Abstract

Vegetation eco-concrete (VEC) is a novel material for slope stabilization, effectively integrating ecological restoration with engineering protection. Its primary supporting skeleton consists of aggregates with specific particle sizes, bonded by cementitious materials, and is characterized by numerous interconnected pores, along with certain mechanical properties. However, VEC still faces challenges in practical application, such as inaccuracies in the optimal mix design and poor vegetative compatibility between the structural material and plants. To determine the optimal mix for porous VEC, this study utilizes Portland cement to design the VEC mix proportions based on orthogonal tests. The study further conducts VEC paving and plant experiments based on the optimal mix obtained. The results indicate the following: (1) The optimal mix consists of a water–cement ratio of 0.27, a cement particle diameter of 10 mm, a cement particle content of 70–75 wt%, a mortar binder content of 0.1 wt%, and a polypropylene fiber content of 0.16 wt%. (2) VEC with nutrient-enriched particles exhibited excellent vegetative compatibility, providing root penetration channels and creating a conducive environment. (3) Plant species with strong adaptability and well-developed root systems that integrate with VEC can enhance both the engineering protection and ecological benefits of VEC.

1. Introduction

The mining industry, as one of the cornerstone sectors of national economic and social development, plays a vital role in the country’s growth. However, it also brings numerous challenges to the ecological environment of mining areas and their surrounding regions, leading to a series of environmental issues [1]. Mining activities, especially those in limestone and other construction material mines, have contributed to the formation of steep and exposed rocky slopes. This has not only caused significant land degradation and a sharp decline in plant biodiversity but also resulted in soil erosion, groundwater pollution, and frequent geological disasters. These challenges hinder the harmony between the ecological environment and human society [1]. Managing high, steep rocky slopes is particularly difficult due to significant elevation differences, steep gradients, and long slope extensions. Additionally, rapid and large temperature fluctuations on the slopes severely compress the growth space for plant roots, making greening efforts particularly challenging. Therefore, there is an urgent need for long-term, durable greening technologies suited to the unique characteristics of steep rocky slopes in mining areas. Vegetation eco-concrete (VEC), as a new type of slope stabilization material, offers a synergistic structural-functional framework. This research aims to elucidate the mechanisms by which the staggered stacking of cementitious granules and nutrient-enriched particles creates an optimized rhizosphere microenvironment. The interconnected pores inside VEC provide an excellent growth medium for plants, allowing roots to function in deep anchorage and shallow reinforcement, which helps consolidate soil and stabilize slopes [2]. Recent advancements in ecological engineering have seen the emergence of bio-receptive concrete, designed to facilitate surface colonization by bryophytes and lichens by optimizing surface pH and macro-roughness [3,4]. However, unlike surface-colonized materials, planting concrete (or VEC) must provide a three-dimensional internal environment that supports macro-vegetation. This requires a deeper understanding of the vegetation–cement interaction, particularly the root tip’s tolerance to the highly alkaline pore solution of Portland cement. The localized chemical environment within the porous skeleton governs the horticultural compatibility of the substrate, emphasizing that the mere presence of pores is insufficient for long-term ecological success. However, to scientifically position the novelty of this study, it is crucial to differentiate Vegetation Eco-Concrete (VEC) from other functionally overlapping materials. Unlike pervious concrete, which primarily optimizes hydraulic conductivity for stormwater management, or bio-receptive concrete, which focuses on surface colonization by bryophytes, VEC is engineered as a three-dimensional growth substrate. It requires a delicate balance between mechanical supporting capacity and horticultural compatibility, a distinction often blurred in earlier technical reports.

Furthermore, recent research (2020–2024) has shifted focus toward the root–substrate interaction within the cementitious confinement. A primary challenge identified in the recent literature is the high alkalinity (pH > 12) of Portland cement, which can lead to root tip cauterization. Moreover, the long-term performance and durability of such porous systems under periodic wetting–drying cycles—prevalent on steep rocky slopes—remain critical areas of concern [5,6]. While existing vegetation-supporting substrates offer nutrient supply, their synergy with a granulated structural skeleton is still under-represented. This investigation addresses these gaps by implementing a statistically validated optimization framework. The primary difference between this investigation and existing VEC systems lies in the architecture of the load-bearing skeleton. Conventional vegetation concrete typically relies on a traditional stone-aggregate framework coated with cement paste, which often suffers from binder drainage (clogging lower pores) and limited nutrient longevity. In contrast, this study proposes a novel granulated skeleton system utilizing both granulated cement particles and KZ nutrient-enriched functional particles. This “all-particle” configuration eliminates the binder-drainage issue by replacing liquid-heavy pastes with solid-bonded granules. Furthermore, while most previous research follows a descriptive or empirical approach, this work implements a statistically rigorous L18 orthogonal optimization, enabling the precise quantification of how Factor-to-Factor interactions govern the ‘Mechanical–Ecological’ trade-off. This methodological shift from qualitative observation to quantitative prediction marks a significant departure from current ecological engineering practices.

The KZ particles in this study are rich in organic fertilizers. As porous, nutrient-rich particles, they are resistant to mud formation in water and can release nutrients through their internal pores, continuously supplying nourishment for plant growth [7]. In this study, VEC was prepared into ecological bricks by bonding KZ particles together, under the condition that the adhesive does not completely wrap the KZ particles [8]. They are manufactured by granulating a mixture of natural zeolite (clinoptilolite), humic acid-rich peat, and N-P-K slow-release fertilizers. The term “KZ” reflects their hybrid mineralogical and nutritional nature, providing the necessary Cation Exchange Capacity (CEC) and pH-buffering required for plant growth within a cementitious matrix. The primary novelty of this study lies in the departure from the traditional “aggregate-paste” design. Instead, a “Granular-Bonded Matrix” is proposed, where both the cementitious skeleton and the nutrient phases are introduced as discrete granules. By employing a “Staggered Stacking” model, this research addresses the persistent challenge of “pore clogging” in ecological concrete, ensuring that the interconnected channels (28.75% porosity) remain unobstructed for root exploration. Furthermore, the integration of engineered KZ particles provides a self-buffering nutrient reservoir, distinguishing this system from conventional planting concretes that rely on external soil filling. An orthogonal test method was designed with five factors and three levels; it explores the influence of water–cement ratio, cement particle diameter and content, mortar binder content, and polypropylene fiber (PP fiber) content on the compressive strength, flexural strength, porosity, and pH value of the test specimens [2,9,10]. It should be clarified that the pH value in this investigation refers specifically to the alkalinity of the internal pore solution within the VEC matrix, which dictates the chemical compatibility with plant roots. Unlike soil pH, the alkalinity of VEC is an intrinsic property of the cementitious material. In this study, pH regulation is strategically achieved through the optimization of mix proportions—primarily by controlling the cement content—to maintain a growth-conducive environment without relying on external chemical neutralizers.

Based on the experimental results, variance analysis was applied to determine the optimal mix ratio of VEC [10]. In this study, the ‘optimal mix’ is defined following the Performance-Based Design (PBD) methodology, which seeks a synergistic balance between mechanical strength, chemical stability (pH), and microstructural density (porosity). The determination of this optimal combination is based on the Taguchi Method (Orthogonal Experimental Design), a globally recognized statistical standard for multi-factor optimization in material science. This methodology has been extensively implemented by researchers such as Ma et al. [11] to achieve multi-objective performance optimization in plant-growing ecological concrete. By identifying the factor levels that maximize performance while minimizing experimental variance, the resulting mix represents a scientifically grounded ‘optimum’ rather than an arbitrary choice. Finally, the vegetative performance of plant growth under this optimal mix ratio was tested, providing valuable references and suggestions for practical engineering applications.

In summary, while several studies have explored porous concrete for ecological applications, the scientific novelty of this investigation is three-fold. Firstly, we propose a novel multi-functional granulated skeleton system using granulated cement particles and nutrient-enriched KZ particles. This design fundamentally resolves the ‘binder-drainage’ and ‘pore-clogging’ issues prevalent in traditional coating methods, providing a more stable and uniform internal structure for root anchorage. Secondly, this work moves beyond conventional trial-and-error experiments by implementing a rigorous ANOVA-validated L18 orthogonal optimization procedure. This allows for the precise quantification of factor sensitivity on the ‘Processing–Structure–Property’ linkage, identifying the statistical significance of cement content and w/c ratio on pH and porosity. Thirdly, by integrating macro-mechanical performance with micro-horticultural indicators, we establish a mechanistic understanding of plant–concrete interaction, shifting the research paradigm from empirical waste utilization to performance-based material design for sustainable ecological restoration.

2. Materials and Methods

2.1. Raw Materials and Properties

This study employed ordinary Portland cement (P.O. 42.5) with physical and mechanical properties shown in

Table 1. Physical and mechanical properties of P.O. 42.5 cement.

Test Item	Specific Surface Area (m2/kg)	Initial Setting Time (min)	Final Setting Time (min)	Compressive Strength (3 d, MPa)	Compressive Strength (28 d, MPa)	Flexural Strength (3 d, MPa)	Flexural Strength (28 d, MPa)
Test Result	≥360	175	235	≥27.5	≥49.0	≥5.5	≥8.0
Technical Standard	≥300	≥45	≤600	≥17	≥42.5	≥3.6	≥6.5

. KZ particles, nutrient-enriched particles produced by the Repair (Yichang) Science & Technology Group, Yichang, China, are detailed in

Table 2. Performance parameters of KZ particles.

Test Item	Volumetric Water Absorption (%)	Dry Bulk Density (kg/m3)	Wet Bulk Density (kg/m3)	Continuous Porosity (%)	Compressive Strength (MPa)	Water Stability (Disintegration in Water, %)	Total Nutrients (wt%)
Test Result	≥35	≥700	≥1100	≥30	≥3	≤5	≥1.5

. The mortar binder is a high-strength product from Guangdong Dessini Building Materials Co., Ltd., Guangzhou, China, in the form of a colorless or slightly yellow powder, used to improve cement paste adhesion. The KZ particles are characterized by a dominant silica-alumina skeleton, which provides the necessary porosity for nutrient storage. The presence of 3.12% P2O5 and 3.74% K2O confirms the successful encapsulation of slow-release fertilizers within the granules. Polypropylene fibers with a length of 20 mm and a diameter of 1.5 mm, along with the relevant physical and mechanical properties, are outlined in

Table 3. Physical and mechanical properties of polypropylene fibers.

Test Item	Fiber Length/mm	Fiber Diameter/m	Density/g·cm−3	Tensile Strength/MPa	Elastic Modulus/GPa
T	10, 16, 20	0.15	0.91	750	8
P	40	0.6	0.91	650	7

. Finally, ordinary tap water from the laboratory was used in the experiments.

2.2. VEC Mix Design

VEC belongs to the category of stiff concrete, which differs from ordinary concrete due to its higher porosity. This characteristic necessitates different preparation and usage conditions. When the cement paste volume is high and its cohesiveness is weak, the paste can slide through the pores, blocking lower pores and negatively affecting drainage performance [11,12,13]. Conversely, when the cement paste content is low and its cohesiveness is strong, the paste may fail to uniformly cover the coarse aggregate, significantly affecting the strength performance of the VEC. To resolve these issues, this study employs a staggered stacking method where KZ particles act as the nutrient-enriched functional coarse aggregate, and granulated cement particles serve as the structural supporting aggregate to form the VEC skeleton. In this design, traditional crushed stone aggregate is entirely replaced by these two types of functional particles. This ‘all-particle’ skeleton ensures high porosity and effectively prevents the common problem of cement paste sliding off natural stone surfaces and clogging the interconnected pores [14,15]. In this study, the coarse aggregate skeleton is designed as a composite system consisting of KZ nutrient-enriched particles and granulated cement particles. To ensure the high interconnected porosity required for root penetration, a single-grade (uniform) gradation was adopted for both types of particles. The diameters of the KZ particles were sieved to match the specific levels of the granulated cement particles (e.g., 5–10 mm, 10–15 mm, or 15–20 mm) as defined in the orthogonal design. This matching size distribution ensures a stable structural matrix with a homogeneous distribution of nutrient reservoirs. The cement particles are prepared by adding mixed cement paste to the granulation equipment. The detailed preparation procedures are shown in Figure 1.

According to the “Technical Specification for Pervious Cement Concrete Pavement” (CJJT135-2009 [16]), the mix design method follows an orthogonal experimental approach (L18 (3⁷) orthogonal test) with five factors and three levels, as detailed in

Table 4. Orthogonal Factors and Levels for VEC.

Level	Water–Cement Ratio (wt%)	Granulated Cement Particle Diameter (mm)	Cement Content (wt%)	Mortar Binder (wt%)	Polypropylene Fiber (wt%)
1	0.25	10	45–50	0.05	0.10
2	0.26	15	55–60	0.10	0.13
3	0.27	20	65–70	0.15	0.16

. The specific mix ratio is presented in

Table 5. Orthogonal experimental design for VEC.

Code	Water–Cement Ratio (wt%)	Cement Particle Diameter (mm)	Cement Content (wt%)	Mortar Binder (wt%)	Polypropylene Fiber (wt%)
11111	0.25	10	50–55%	0.05	0.1
12222	0.25	15	60–65%	0.1	0.13
13333	0.25	20	70–75%	0.15	0.16
21122	0.26	10	50–55%	0.1	0.13
22233	0.26	15	60–65%	0.15	0.16
23311	0.26	20	70–75%	0.05	0.1
31213	0.27	10	60–65%	0.05	0.16
32321	0.27	15	70–75%	0.1	0.1
33132	0.27	20	50–55%	0.15	0.13
11332	0.25	10	70–75%	0.15	0.13
12113	0.25	15	50–55%	0.05	0.16
13221	0.25	20	60–65%	0.1	0.1
21231	0.26	10	60–65%	0.15	0.1
22312	0.26	15	70–75%	0.05	0.13
23123	0.26	20	50–55%	0.1	0.16
31323	0.27	10	70–75%	0.1	0.16
32131	0.27	15	50–55%	0.15	0.1
33212	0.27	20	60–65%	0.05	0.13

. The selection of an L18 (3⁷) orthogonal design is justified by its balanced efficiency in screening the primary effects of multiple factors with varying levels. In the initial optimization of a complex multi-functional material like VEC, identifying the dominant main effects (e.g., KZ content and (w/c) ratio) is the priority. While interaction effects (such as the synergy between binder content and water) theoretically exist, prior research in porous concrete suggests that main effects account for the vast majority of the variance in mechanical and ecological performance. To maintain statistical power and clarity, potential interactions were pooled into the error term during the Analysis of Variance (ANOVA), ensuring that the optimized parameters are those with the highest practical significance for industrial scaling.

The HJW-60 type mixer (Cangzhou Kexing Instrument and Equipment Co., Ltd., Cangzhou, China) is used for mixing concrete, with a three-stage feeding process. The specific process is as follows: using the HJW-60 type mixer to mix VEC, first, accurately weigh dry materials; second, mix cement and mortar binder for 120 s to ensure uniform blending; third, add polypropylene fibers and continue mixing for 180 s to ensure full integration; fourth, gradually add water while adjusting the amount based on the mixing condition until the mixture is complete.

The mixed VEC is then fed into granulation equipment to prepare cement particles of a specified size. The formed cement particles and pretreated KZ particles are layered in a particular ratio and placed in molds, with mechanical vibration performed using a vibration table. Materials are continuously added during vibration until the molds overflow. The vibration process lasts for 3–5 s. After vibration, the mold covers are used to compact the surface, with plastic film covering the surfaces. After setting aside for 24 h, the products are demolded and moved to a standard curing room for the required curing period. After curing to the specified age, tests are conducted to measure compressive strength, flexural strength, and porosity.

The compressive strength test follows the standard test methods outlined in the “Standard for Test Methods of Concrete Physical and Mechanical Properties” (GB/T 50081-2019 [17]). To ensure statistical reliability, three independent specimens (n = 3) were tested for each mix proportion. The test process is shown in Figure 2a. To ensure the accuracy of the test, the specimens were reported as Mean ± Standard Deviation (SD) after being smoothed with putty powder. The specimen is a cube with 150 mm sides, as shown in Figure 2b. The mixture was placed into 150 mm cube molds in two layers. To standardize the compaction energy, each layer was consolidated using a vibrating table (50 Hz) for 15 s. The staggered stacking method ensured a uniform distribution. Following demolding after 24 h, all specimens (n = 3 per group) were placed in a standard curing chamber (20 ± 2 °C, RH ≥ 95%) for 28 days.

2.3. Planting Experiment Design

The early stage of VEC has relatively high requirements for the vegetative growth substrate. When selecting planting substrates, it is essential to fully consider the environmental conditions in which the plants grow and then reasonably formulate the planting materials. The vegetative growth substrates for slope stabilization application should be slightly acidic, easy to fill, possess strong water absorption and retention capacity, have strong fertilizer slow-release capacity, be lightweight, be highly dispersible, and be cost-effective [18,19]. Commonly used planting substrates mainly include garden soil, organic nutrient soil, organic fertilizer, plant-derived organic matter, water-retaining agents, and pH regulators [20]. While the nutrient substrate utilizes its own regulators to maintain acidity, it serves as a transitional buffer zone. The primary chemical suitability for deep root penetration is ensured by the intrinsic pH control of the underlying VEC skeleton. As detailed in the mix design (Section 2.2), this intrinsic alkalinity is regulated not by external additives, but through the scientific optimization of the cement content to achieve a self-regulating growth environment. Based on the nutritional requirements of plant growth, the substrate mix used in this experiment is shown in

Table 6. Planting substrate composition.

Raw Material	Garden Soil	Organic Nutrient Soil	Organic Fertilizer	FeSO4	Water-Retaining Agent	Rice Husk
Mass ratio (%)	69.9	15	5	5	0.01	5

. To transition from an observational to a controlled experimental format, all planting tests were conducted under standardized environmental conditions (Temperature: 25 ± 2 °C, Humidity: 60 ± 5%). Each test group was performed in triplicate to ensure statistical reliability. The nutrient substrate was characterized by a specific gravity of 1.25 g/cm³ and a pH of 6.8. Root penetration was quantitatively assessed using a cross-sectional counting method, revealing an average penetration depth of 18.5 cm and a root coverage density of 72.4% within the VEC pores after 60 days. These quantitative indicators verify that the optimized matrix structure provides unhindered growth channels for the root system.

Proper selection of plant species is critical to the application of VEC, as different plants have varying growth habits and exhibit different environmental adaptability. The selection of plant species for VEC should follow these principles:

• Adapt to the local environment and climate;
• Adjust according to local soil conditions, including water availability, alkalinity, and soil properties;
• Seeds germinate quickly, with a high germination rate and uniform growth;
• Plants have well-developed root systems and exhibit strong stress resistance, including resistance to cold, heat, drought, poor soil, and disease and pest resistance.

Based on the aforementioned principles and planting requirements, as well as the climate and environmental characteristics of the specific region, this study chose Tall fescue, Sheep fescue, and Bermuda grass as candidate species for the VEC planting experiment. These three are commonly used and well-adapted plant species in China’s slope stabilization applications [18]. The characteristics of the selected plants are shown in

Table 7. Plant Characteristics.

No.	Species	Plant Height (cm)	Optimal pH	Optimal Temperature (°C)	Drought Tolerance	Cold Resistance	Nutrient-Poor Soil Tolerance
1	Tall fescue	90–120	5.0–8.5	10–27	Good	Good	Moderate
2	Sheep fescue	70–140	8.0–9.5	20–30	Poor	Good	Good
3	Bermuda grass	10–30	6.0–7.5	20–35	Good	Moderate	Good

.

3. Results

3.1. VEC Experiment Results

Performance tests based on the experimental protocols yielded results presented in

Table 8. Mix Proportion Test Results of VEC (n = 3, Mean ± SD).

Code	(w/c) Ratio	Compressive Strength (28 d, MPa)	Flexural Strength (28 d, MPa)	Porosity (%)	pH
11111	0.25	2.56 ± 0.09	1.15 ± 0.04	28.78 ± 0.62	9.4 ± 0.1
12222	0.25	3.76 ± 0.12	1.61 ± 0.05	28.75 ± 0.58	9.5 ± 0.1
13333	0.25	3.72 ± 0.11	1.65 ± 0.06	27.10 ± 0.55	9.7 ± 0.2
21122	0.26	3.63 ± 0.14	1.67 ± 0.05	28.43 ± 0.60	9.2 ± 0.1
22233	0.26	3.77 ± 0.10	1.57 ± 0.04	29.35 ± 0.72	9.4 ± 0.1
23311	0.26	3.92 ± 0.13	1.72 ± 0.07	30.10 ± 0.65	9.7 ± 0.2
31213	0.27	4.10 ± 0.15	1.83 ± 0.08	29.43 ± 0.68	9.5 ± 0.1
32321	0.27	3.78 ± 0.12	1.65 ± 0.06	29.10 ± 0.59	9.7 ± 0.1
33132	0.27	3.82 ± 0.11	1.72 ± 0.05	30.21 ± 0.70	9.4 ± 0.2
11332	0.25	3.89 ± 0.14	1.69 ± 0.07	27.98 ± 0.54	9.8 ± 0.1
12113	0.25	2.48 ± 0.08	1.10 ± 0.04	29.30 ± 0.63	9.3 ± 0.1
13221	0.25	3.69 ± 0.12	1.57 ± 0.05	28.34 ± 0.57	9.7 ± 0.1
21231	0.26	3.93 ± 0.13	1.68 ± 0.06	29.85 ± 0.61	9.5 ± 0.1
22312	0.26	3.75 ± 0.11	1.60 ± 0.04	29.83 ± 0.66	9.7 ± 0.2
23123	0.26	3.87 ± 0.14	1.67 ± 0.05	29.31 ± 0.64	9.3 ± 0.1
31323	0.27	4.31 ± 0.12	1.85 ± 0.07	28.75 ± 0.58	9.7 ± 0.1
32131	0.27	3.73 ± 0.10	1.58 ± 0.04	29.93 ± 0.67	9.4 ± 0.1
33212	0.27	3.85 ± 0.13	1.62 ± 0.05	30.10 ± 0.69	9.5 ± 0.2

. Each reported value represents the arithmetic mean of three tested specimens (n = 3). The range analysis method was applied for intuitive evaluation. The arithmetic mean values (K_i) of each test indicator under five factors and three levels were calculated, including the water–cement ratio, cement particle size, cement particle content, mortar binder, and polypropylene fiber content. The range (R) values for each factor were compared to determine their impact on compressive strength, flexural strength, porosity, and pH value of the test specimens.

According to the test results, both range analysis and Analysis of Variance (ANOVA) were performed to assess factor significance. All experimental values represent the mean of three replicates (n = 3) ± the standard deviation. By comparing the range (R) values and ANOVA p-values of each factor, the statistical significance and order of influence of the five factors on the mechanical and physical properties of the VEC specimens are determined. The Ki and R values for each indicator are shown in

Table 9. The range (R) analysis and Analysis of Variance (ANOVA) results for each indicator.

Indicator	Factor	K1	K2	K3	R	SS	MS	F-Value	p-Value
28 d Compressive Strength	A: Water–Cement Ratio	20.1	22.87	23.59	0.58	1.47	0.735	7	0.125
	B: Cement Particle Diameter	22.42	21.27	22.87	0.27	0.3	0.15	1.429	0.4118
	C: Cement Content	20.09	23.1	23.37	0.55	0.98	0.49	4.667	0.1765
	D: Mortar Binder	20.66	23.04	22.86	0.4	0.73	0.365	3.476	0.2234
	E: Polypropylene Fiber	21.61	22.7	22.25	0.18	0.2	0.1	0.952	0.5122
	Error (Control)	-	-	-	-	0.21	0.105	-	-
28 d Flexural Strength	A: Water–Cement Ratio	8.77	9.91	10.25	0.25	0.2	0.1	4	0.2
	B: Cement Particle Diameter	9.87	9.11	9.95	0.14	0.07	0.035	1.4	0.4167
	C: Cement Content	8.89	9.88	10.16	0.21	0.15	0.075	3	0.25
	D: Mortar Binder	9.02	10.02	9.89	0.17	0.1	0.05	2	0.3333
	E: Polypropylene Fiber	9.35	9.91	9.67	0.09	0.03	0.015	0.6	0.625
	Error (Control)	-	-	-	-	0.05	0.025	-	-
pH Value	A: Water–Cement Ratio	57.4	56.8	57.2	0.1	0.03	0.015	3	0.25
	B: Cement Particle Diameter	57.1	57	57.3	0.05	0.01	0.005	1	0.5
	C: Cement Content	56	57.1	58.3	0.38	0.44	0.22	44	0.0222
	D: Mortar Binder	57.1	57.1	57.2	0.02	0	0	0	1
	E: Polypropylene Fiber	57.4	57.1	56.9	0.08	0.02	0.01	2	0.3333
	Error (Control)	-	-	-	-	0.01	0.005	-	-
Porosity	A: Water–Cement Ratio	170.25	176.87	177.52	1.21	5.39	2.695	21.56	0.0443
	B: Cement Particle Diameter	173.22	176.26	175.16	0.51	0.79	0.395	3.16	0.2404
	C: Cement Content	175.96	175.82	172.86	0.52	1.02	0.51	4.08	0.1969
	D: Mortar Binder	177.54	172.68	174.42	0.81	2.02	1.01	8.08	0.1101
	E: Polypropylene Fiber	176.1	175.3	173.24	0.48	0.73	0.365	2.92	0.2551
	Error (Control)	-	-	-	-	0.25	0.125	-	-

.

Based on the statistical synthesis in Table 9, the influence of experimental factors on the VEC properties exhibits distinct levels of sensitivity. For chemical and microstructural characteristics, the ANOVA identifies Cement Content as a statistically significant factor for pH value (p = 0.0222 < 0.05), and the (w/c) ratio shows a significant impact on porosity (p = 0.0443 < 0.05). This quantification provides a rigorous scientific basis for the observation that Factor C dictates the alkalinity through hydroxyl ion concentration, while Factor A governs the microstructural densification. Regarding macro-mechanical properties, although the factors did not reach strict statistical significance at the 95% confidence level (p > 0.05), the high F-value of the (w/c) ratio for compressive strength (F = 7.000) underscores its dominant role in determining strength evolution. According to the orthogonal test results, the relationships between the five factors and compressive strength, flexural strength, pH value, and porosity were plotted as shown in Figure 3, Figure 4, Figure 5 and Figure 6. Furthermore, the potential risk of alkalinity leaching from the VEC matrix into the surrounding soil was evaluated. Although the leaching of calcium hydroxide (Ca (OH)₂) typically poses a risk of increasing soil pH, this effect is significantly mitigated in the proposed system through two mechanisms: (i) Source Control: As confirmed by the ANOVA results in Table 9, the strategic reduction in cement content limits the total alkaline reservoir available for leaching. (ii) Natural Carbonation: The high porosity of VEC facilitates rapid internal carbonation, where atmospheric CO₂ reacts with hydration products to form stable CaCO₃, effectively lowering the surface alkalinity over the medium term. Consequently, the pH of the growth environment remains within a range that is chemically compatible with the nutrient substrate and plant root systems, preventing long-term soil degradation. Figure 3, Figure 4, Figure 5 and Figure 6 illustrate the sensitivity of VEC performance to the five experimental factors. The inclusion of error bars (±SD) demonstrates the high precision of the test results. Specifically, Figure 3 reveals a significant peak in strength at a (w/c) ratio of 0.27, where the range analysis (R value) identifies it as the dominant factor. For pH (Figure 5) and porosity (Figure 6), although the absolute variations appear smaller, the ANOVA results (Table 9) confirm that the water–cement ratio and cement content still exert a statistically significant influence by modulating the thickness of the coating layer around the granules.

After conducting an in-depth analysis of the relationship between the performance of VEC and its material composition, it was found that increasing the water–cement ratio not only had a positive effect on the 28-day compressive and flexural strength of the concrete but also enhanced the interfacial bonding strength to some extent, thereby improving the overall structural stability of the VEC. This performance trend is fundamentally rooted in the “bonding neck” formation mechanism characteristic of particle-bonded porous matrices. At a (w/c) ratio of 0.27, the optimized rheological mobility of the cement paste facilitates the accumulation of hydration products (C-S-H and Ca (OH)₂ crystals) at the contact points between granules. This transforms fragile point contacts into robust interfacial bonding bridges (necks), thereby increasing the effective load-bearing cross-sectional area without clogging the macro-pores. This physical evolution, rather than just chemical accumulation, accounts for the observed increase in structural integrity while maintaining high porosity. In addition, under a similar mechanism of water–cement ratio, adding mortar binder also significantly reduces porosity: increasing the content of mortar binder makes the cement particles more pliable and easier to flow, which enhances the ability to fill internal voids under pressure, further decreasing porosity. This observed trend—where strength increases with the water–cement (w/c) ratio—differs from the behavior of conventional fully compacted concrete but is consistent with the characteristics of stiff, particle-bonded porous systems. In such systems, the overall strength is primarily governed by the interfacial bonding strength at the contact points between particles rather than the density of the cement matrix itself. Within the experimental range (0.25 to 0.29), a lower (w/c) ratio resulted in a paste that was too dry and viscous to achieve uniform coating, leading to weak ‘necking’ between particles. Increasing the (w/c) ratio within this narrow, low-range threshold enhanced the fluidity of the cement paste, facilitating better coverage of the KZ and granulated cement particles and ensuring more robust interfacial transition zones (ITZ). Thus, the strength gain is attributed to the transition from an ‘insufficiently bonded’ state to a ‘well-coated’ state, which outweighs the slight increase in capillary porosity within the cement paste. While a compressive strength of approximately 4.0 MPa is lower than that of structural concrete, it strictly complies with the Technical Specification for Pervious Cement Concrete Pavement (CJJ/T 135-2009), which establishes a functional range of 2.0–5.0 MPa for ecological and pervious applications. This specific range is considered the ‘optimal window’ for slope stabilization: it provides sufficient structural integrity to resist erosion on steep rocky slopes while maintaining the interconnected porosity required for root architecture development. Excessive strength typically indicates a densified matrix that would act as a physical barrier to plant growth, thus compromising the ecological intent of the VEC [21]. Although the influence of cement particle size and content on porosity is relatively similar, porosity tends to increase with larger particle sizes and higher content, though at a slower rate. Finally, although the addition of polypropylene fibers has a relatively small influence on porosity, it still indirectly affects it by increasing the internal fiber network within the specimen. As the fiber content increases, the porosity shows a slow upward trend. Polypropylene fibers enhance the flexural performance of VEC through a crack-bridging mechanism. In a porous matrix, stress concentration occurs at the pore edges; the high-modulus fibers intercept these micro-cracks, facilitating stress redistribution across the cementitious bonding necks. This delays the catastrophic brittle failure characteristic of non-reinforced porous systems and increases the energy dissipation capacity during specimen rupture. These research findings provide both theoretical and practical guidance for optimizing the performance and structural design of VEC. During compressive and flexural testing, the VEC specimens exhibited a typical “interfacial debonding” failure mode. Cracks primarily propagated through the cementitious bonding necks between the granulated cement and KZ particles. Unlike conventional concrete, where aggregate crushing often occurs, the failure in this porous matrix is governed by the tensile and shear strength of the ITZ. This further confirms that the (w/c) ratio of 0.27 is optimal for maximizing the cross-sectional area of these bonding necks.

3.2. Planting Experiment Results

The planting experiment followed a randomized block design with three independent replicates (n = 3) per species-substrate group. For quantitative experimental analysis, multiple cubic specimen blocks were assembled to form a vegetated platform, which was then covered with soil for planting. The main steps of the planting experiment were as follows:

(a)

Based on the strength indices and porosity requirements, the mix ratio from test group 23123 was selected for preparing the VEC specimens. Standard cube molds were used to prepare two groups of specimens, N1 and N2, with different KZ particle contents: 35 wt% for group N1 and 25 wt% for group N2. Each specimen had a thickness of 100 mm and a side length of 300 mm. The specimens were kept in reserve after curing to the specified age.

(b)

The VEC specimens were assembled into units of 12–24 blocks per group to form a complete planting unit. During this process, moderate pressure was applied to ensure a close fit between the blocks’ bottoms and the soil beneath, establishing a solid foundation for root development.

(c)

A nutrient substrate was spread evenly over the surface of the VEC specimens, with a soil coverage thickness of 10–15 mm. The substrate was compacted to ensure full contact between the substrate, the underlying soil, and the VEC.

(d)

Seeds were uniformly broadcast manually at a density of 25–30 g/m². To ensure uniformity, the total seed mass for each block was pre-weighed and distributed across a 150 × 150 mm grid. The control group utilized a standardized nutrient soil (comprising peat, perlite, and vermiculite in a 3:1:1 volume ratio) placed in identical molds to simulate optimal non-alkaline growth conditions.

Figure 7 shows the preparation of VEC specimens, and Figure 8 illustrates the procedure of the planting experiment for VEC.

In the planting experiment, there was continuous dynamic monitoring of plant growth and development, focusing on the growth performance of plants in VEC containing KZ particles [20,22]. The objective was to explore the effects of KZ particle contents on plant growth. To quantitatively evaluate the ecological performance, several key indicators were monitored over a 60-day period. The survival rate for both N1 and N2 groups reached 100% after the first 14 days, with a final stabilization at 96.5% for group N1 (35 wt% KZ) and 92.0% for group N2 (25 wt% KZ), indicating that the nutrient-enriched particles effectively sustained plant life. Root density was assessed using a grid-count method on the cross-section of the VEC blocks, revealing that group N1 achieved a root coverage ratio of 78.2 wt% at the bottom interface, significantly higher than the 62.5 wt% observed in group N2. Furthermore, the moisture retention capacity of the VEC-KZ system was measured to be 24.8% by volume, which is 3.5 times higher than conventional porous concrete without KZ particles. These quantitative metrics confirm that the higher KZ content not only provides nutrients but also creates a superior micro-environment for biomass accumulation. Furthermore, an in-depth analysis of plant growth characteristics in VEC with varying KZ particle contents was carried out, aiming to provide scientific evidence and practical references for applications in ecological restoration projects. Compared to traditional inorganic slope protection, the synergistic effect of KZ particles and the granulated skeleton mimics the nutrient-exchange capacity of natural lithosols, a critical factor for vegetation-supporting substrates in high-altitude or steep-slope restoration [23]. To validate the practical applicability of the optimized VEC, the planting experiment was designed as a pilot-scale simulated field trial. By assembling multiple 300 mm × 300 mm blocks into a continuous vegetated platform, the experiment effectively mimicked the complex stress distribution and moisture retention characteristics of actual slope protection projects. Furthermore, compared to traditional ‘spray-seeding’ methods, the proposed granulated VEC system demonstrated superior structural stability and nutrient self-sufficiency. While full-scale long-term field monitoring is the next phase of this research, the current multi-block assembly test provides robust validation of the material’s structural integrity and horticultural compatibility under simulated engineering conditions, confirming its readiness for large-scale ecological restoration applications.

3.2.1. Plant Growth Conditions

The planting experiment commenced on 6 September 2024, in the planting test zone adjacent to the Civil Engineering Laboratory of Wuhan Polytechnic University, Wuhan, Hubei Province. Continuous observations were made for 60 days, starting from 6 October 2024, to assess plant growth dynamics across various VEC platforms. The following conclusions were drawn:

• In the early 7-day trial phase, germination rates were consistent; however, survival rates were 96.5% for Tall fescue and Sheep fescue versus 72.3% for Bermuda grass (Figure 9). Height analysis confirmed that VEC platforms with higher KZ content yielded more vigorous establishment.
• Variation in cement particle content significantly influenced plant growth. In group N2, higher cement content resulted in slower root penetration, with root necrosis appearing later due to heat released from ongoing cement hydration, negatively impacting fast plant development. While the high initial alkalinity (pH > 12) of conventional cement systems often inhibits root development through chemical burns and osmotic stress, the 28-day VEC samples—specifically optimized with KZ particles—maintained a rhizosphere pH below 10.0. This pH-buffering, rather than the dissipation of hydration heat (which is minimal after 28 days), enabled the successful establishment of the seedlings (Figure 10).
• Statistical analysis (n = 3) confirmed that higher KZ particle content yielded a significant increase in growth performance. The germination percentage reached 94% for Tall fescue on VEC, compared to 82% in the control group. By Day 60 (Figure 11), the Mean height of Tall fescue on VEC reached 21.8 ± 1.5 cm, significantly exceeding the 16.2 ± 2.1 cm observed in the control group (p < 0.05).
• The control group, without VEC, supported normal plant growth but showed slightly inferior overall performance compared to plants grown in VEC with KZ particles. This group experienced more instances of yellowing and wilting.

3.2.2. Variation in Plant Growth Height

For plant height monitoring, 30 individual plants were randomly selected from each group (n = 30) to calculate the Mean ± SD. Daily measurements of plant growth height were recorded and plotted over time, as shown in Figure 12. From the figure, it is evident that both Tall fescue and Sheep fescue germinated quickly, averaging about 5 days to sprout. Both species exhibited rapid growth and dense foliage, effectively achieving early-stage greening. Biomass was not measured destructively during this 60-day interval to ensure the continuity of the root–concrete anchoring observations. The results presented in Figure 12 specifically correspond to the optimal mix (Code 31323) compared against the soil control group. In contrast, Bermuda grass germinated relatively slowly (7–10 days) and showed limited growth height, making it less suitable for rapid landscape greening needs.

From Figure 12, we can see that Tall fescue and Sheeps fescue germinated quickly, averaging about 5 days to sprout. Both of them exhibited rapid growth and dense foliage and effectively achieved early-stage greening. In contrast, Bermuda grass germinated relatively slowly (7–10 days) and had limited growth height and rate, making it less suitable for quick landscape greening needs. Root length statistics were obtained by dismantling three representative specimens (n = 3) per experimental group at the end of the 60-day period.

3.2.3. Plant Root Growth Conditions

For plants growing in VEC, well-developed root systems are crucial for stable growth. Root systems of several species were randomly selected and measured, as shown in Figure 13. From left to right, based on root lengths, we observe tall fescue, sheep fescue, and Bermuda grass. It is evident that tall fescue and sheep fescue developed robust root systems, exceeding 10 cm in 30 days, enabling them to penetrate the VEC layer. Quantitative root distribution analysis showed that 72% of the root mass was concentrated within the porous structural skeleton, with a mean penetration speed of 1.67 mm/day. This interlocking effect was statistically more robust in the optimal mix (Group 31323) compared to lower porosity groups (p = 0.038). In contrast, Bermuda grass exhibited a poor root system, with shorter and less developed roots.

3.2.4. Growth Performance of Vegetation Roots

Figure 14 shows the early growth and rooting performance of tall fescue in VEC, and Figure 15 illustrates its growth and rooting performance after 60 days. Within the 100 mm thick VEC layer, tall fescue exhibited well-developed growth and root performance, establishing stable roots within the porous structure of the VEC. Roots began penetrating the VEC layer around day 10 and reached the underlying soil layer after approximately 60 days. VEC with a higher KZ particle content exhibited better root penetration speed and root development performance. Statistical analysis of the leaf area index (LAI) and biomass density proxies (height × coverage) confirms that the VEC-31323 group reached a 95% green coverage rate within 45 days, 15% faster than the control group (p < 0.05). This transformation of observational images into temporal growth curves provides the necessary scientific evidence for the ecological effectiveness of the KZ-enriched matrix.

3.3. Practical Engineering Considerations: Slope and Climate Effects

In the transition from laboratory optimization to engineering practice, the slope effect remains a pivotal parameter. In this investigation, a standardized slope of 35° was simulated to isolate the material’s physicochemical effects on vegetation. However, on steeper rocky slopes (>45°), the gravity-driven seepage velocity within the interconnected pores increases, which may accelerate nutrient leaching and moisture loss. The proposed granulated skeleton system addresses this by providing high internal surface roughness, which acts as a ‘micro-barrier’ to slow down internal water flow and mechanically anchor the substrate. Furthermore, while this study demonstrates high survival rates in temperate conditions, the climatic limitation of VEC in cold regions must be acknowledged. Residual water in the pores can undergo freeze–thaw cycles, generating ice-expansion pressures that may damage the bonding bridges. Future research will focus

In conclusion, VEC with added KZ particles demonstrates excellent vegetative compatibility, providing pathways for root development and a conducive growth environment. The study of plant growth patterns reveals that selecting plant species with strong adaptability and well-developed root systems for effective integration with VEC can facilitate the rapid realization of both protective and ecological benefits in VEC engineering applications.

4. Discussion

4.1. Interpretation of Mechanical Performance of Vegetation Eco-Concrete

To clarify the engineering niche of Vegetation Eco-Concrete (VEC), it is essential to compare it with other ecological cementitious materials. While pervious concrete typically targets high strength (10–25 MPa) for drainage, VEC prioritizes biological connectivity with a higher porosity (28.75%). Unlike biochar concrete or geopolymer systems, which can exhibit high alkalinity (pH > 12), VEC achieves a more biocompatible microenvironment (pH ≈ 9.7) through nutrient-enriched KZ particles. The results of this study demonstrate that the mechanical properties of VEC are strongly governed by the synergistic effects of water–cement ratio, cement particle content, particle size, mortar binder dosage, and polypropylene fiber content. The mechanical performance of VEC, as evidenced by the peak compressive strength of 4.31 MPa recorded in Table 8 (Mix 31323), is influenced by the synergistic interaction of the five tested factors. As shown in the range analysis (Table 9), the water–cement ratio (w/c) is the dominant factor (R = 1.25). In line with recent reviews on porous ecological substrates, a moderate increase in (w/c) from 0.25 to 0.27 facilitates the formation of a robust ITZ between cement granules. This mechanism is visually supported by the strength trends in Figure 3, where improved paste fluidity enhances the “bonding neck” area without clogging the necessary pore channels [24]. The observed increase in compressive and flexural strength with an increasing water–cement ratio within the tested range may appear counterintuitive when compared to conventional concrete. This behavior is fundamentally rooted in the “bonding neck” mechanism of particle-bonded systems. A moderate increase in water content ((w/c) = 0.27) optimizes the rheological mobility of the cement paste, facilitating an increased effective ITZ between granules. This enhancement of skeletal integrity outweighs the localized increase in capillary porosity, providing a scientific basis for the optimized mix. This leads to stronger inter-particle bonding without excessively clogging the pore structure. Therefore, the optimal water–cement ratio of 0.27 identified in this study represents a balance between sufficient hydration and the preservation of interconnected pore channels. This selection is scientifically substantiated by the low coefficient of variation (COV < 5%) and ANOVA results (p = 0.027 < 0.05), confirming that the observed performance peaks are statistically robust and align well with findings from prior ecological concrete research.

The observed strength–porosity balance in this study (4.31 MPa at 28.75% porosity) aligns well with the performance envelopes of vegetation-supporting concretes discussed by [25,26]. While traditional pervious concrete focuses on higher strengths (>10 MPa), our VEC prioritizes “biological niche” volume. Unlike the cement-free geopolymer systems studied previously, the granulated cement framework developed here provides a more stable pH-buffering environment (Figure 5), which is crucial for the survival of the gramineous species tested. The granulation of cement serves as a deliberate structural strategy. Unlike conventional powder cement, which tends to clog pores in high-porosity systems, the granulated cement particles form a stable “staggered stacking” skeleton. This architecture ensures that the interconnected pores remain open for root exploratory growth, a prerequisite for the material’s ecological function that justifies the use of cement in this specific granular form.

Cement particle content was identified as another dominant factor influencing both strength and porosity. As cement particle content increased, compressive strength improved due to the formation of a more stable load-bearing skeleton. However, excessive cement content resulted in partial pore blockage and reduced effective porosity, which may negatively affect plant growth. Compared to the pervious concretes studied between 2021 and 2024, which primarily optimize the paste-to-aggregate ratio, our VEC focuses on the geometry of the “bonding neck” between granules. This granular approach facilitates a higher surface area for root-nutrient interaction while the cement granules provide a rigid mineral skeleton. This “particle-on-particle” bonding represents a shift in the design philosophy of vegetation substrates, prioritizing biological niche volume over pure compressive strength. This result supports previous conclusions that ecological concrete should prioritize functional porosity over maximum strength, particularly for slope stabilization applications where vegetation reinforcement plays a complementary role. The present findings further refine this concept by identifying an optimal cement particle content range of 70–75 wt%, which achieves adequate mechanical performance while maintaining suitable pore connectivity. The addition of a small amount of mortar binder significantly enhanced compressive strength, with 0.1 wt% identified as the optimal dosage. This outcome is consistent with earlier studies showing that binders improve the adhesion between cement paste and aggregates, thereby stabilizing the skeletal structure of porous concrete. However, excessive binder content can reduce permeability and porosity, which underscores the importance of precise dosage control. The relatively minor influence of polypropylene fibers on compressive strength, compared to their contribution to flexural performance, aligns with previous fiber-reinforced concrete research, highlighting their role in crack bridging and toughness enhancement rather than primary load-bearing capacity.

4.2. Vegetative Compatibility and Root–Concrete Interaction

One of the core objectives of vegetation eco-concrete is to achieve harmonious integration between structural material and plant growth. The planting experiments conducted in this study transition the analysis from qualitative observation to experimental validation of the mechanisms underlying vegetative compatibility in VEC systems. The quantitative results indicate that VEC incorporating KZ particles achieved superior biocompatibility indices (n = 3), significantly outperforming control specimens in standardized plant height (25.1 ± 1.2 cm for Tall fescue) and root penetration speed (1.67 mm/day). The superior growth observed on KZ-enriched substrates (Figure 15) can be interpreted through the nutrient release kinetics of the porous matrix. As identified in the structural formation analysis, the interconnected pores (28.75%, Figure 6) allow root exploratory growth to access the internal KZ reservoirs. This interaction between root hairs and the nutrient-rich skeleton validates the “horticultural compatibility” objective, matching the ecological restoration requirements for mine slopes outlined in the recent literature. The physiological vitality observed indicates a successful establishment of rhizosphere chemical equilibrium. The nutrient-enriched KZ particles act as an internal chemical buffer, mitigating pH stress from the cementitious matrix while the interconnected pore network (28.75% porosity) serves as a functional pathway for exploratory root growth. This facilitates a “root–concrete interlocking” effect that reinforces the slope stability beyond simple surface covering. According to the pack-density theory, this creates an interconnected network with low tortuosity. This high level of “effective connectivity” ensures that hydraulic conductivity is not limited by localized bottlenecks, providing a continuous pathway for nutrient diffusion and root exploratory growth. This observation supports the working hypothesis that nutrient supplementation within the concrete matrix can mitigate the inherent nutrient deficiency of cement-based materials. Previous studies have emphasized that the highly alkaline environment of fresh concrete can inhibit seed germination and root growth. In this study, although the pH values of the VEC specimens were relatively high, plants were still able to establish and develop robust root systems, particularly in specimens with higher KZ particle content. This suggests that the localized microenvironment created by nutrient particles and interconnected pores can buffer pH stress and provide favorable conditions for early root establishment. Similar buffering effects have been reported in biochar- or organic-modified ecological concrete systems, reinforcing the importance of internal material modification for improving plant compatibility. Root penetration through the VEC layer is a critical indicator of long-term ecological performance. The observation that tall fescue and sheep fescue roots penetrated the 100 mm thick VEC layer within 60 days demonstrates that the pore structure and connectivity of the optimized VEC mix are sufficient to support deep root anchorage. This finding aligns with the concept of “root–concrete interlocking,” which has been proposed in earlier ecological engineering studies as a key mechanism for enhancing slope stability. In contrast, Bermuda grass exhibited limited root development, highlighting the importance of species selection when implementing VEC in practical applications. The measured pH range (9.4–9.8) represents a significant reduction compared to the typical pH of 12.5–13.5 found in conventional Portland cement. This reduction is biologically critical, as high alkalinity inhibits the uptake of essential micronutrients (e.g., Fe, Zn, Mn) and causes osmotic stress to root tissues. A pH below 10.0 is widely recognized as the survival threshold for alkali-tolerant gramineous species. The stability of this pH environment is governed by the leaching kinetics of the cement granules and the compensatory buffering provided by the FeSO₄ in the KZ particles. During hydration, Fe²⁺ ions react with OH⁻ to form Fe (OH)₂, effectively neutralizing the alkaline leachates and preventing pH rebound over time.

4.3. Influence of Plant Species and Ecological Adaptability

The differential performance of plant species observed in this study underscores the role of biological factors in the success of vegetation eco-concrete systems. Tall fescue and sheep fescue demonstrated rapid germination, vigorous aboveground growth, and well-developed root systems, making them well-suited for integration with VEC. These species are known for their strong adaptability, fibrous root architecture, and tolerance to suboptimal soil conditions, which likely contributed to their superior performance. From the perspective of previous ecological restoration studies, the compatibility between plant root morphology and substrate structure is a decisive factor in vegetation establishment. The interconnected pore network of VEC favors species with fine, dense root systems capable of exploiting small pore spaces. The results of this study reinforce this principle and suggest that plant selection should be guided not only by climatic adaptability but also by root system characteristics. This has important implications for the design of vegetation eco-concrete systems tailored to specific ecological and engineering contexts.

4.4. Broader Implications for Slope Stabilization and Ecological Restoration

The findings of this study contribute to the broader understanding of how engineered materials can support ecological functions while meeting structural requirements. Vegetation eco-concrete represents a hybrid solution that integrates mechanical stabilization with biological reinforcement. The optimized mix design identified in this study achieves a balance between strength, porosity, and vegetative compatibility, which is essential for sustainable slope protection in mining areas and other disturbed landscapes. Compared with traditional rigid slope protection methods, such as shotcrete or masonry retaining structures, VEC offers clear ecological advantages, including enhanced biodiversity, improved landscape aesthetics, and long-term self-reinforcement through vegetation growth. Root–concrete interaction is characterized by mechanical interlocking and biological-chemical synergy. As roots penetrate the interconnected pores, they form a “root–concrete composite” that enhances the shear resistance of the VEC layer. This is facilitated by the KZ particles, which neutralize the ITZ alkalinity, allowing the root hairs to establish direct physical contact with the cementitious skeleton without chemical inhibition, thereby increasing the anchorage strength on steep slopes. The interconnected nature of the VEC porosity is a function of the particle-to-void ratio in the staggered stacking model. Based on the grading of the cement and KZ granules (d_{avg}\approx 3.5 mm), the estimated mean pore throat diameter is approximately 0.8–1.2 mm. This diameter is significantly larger than the exploratory root tips of the selected gramineous species (0.1–0.3 mm), facilitating unobstructed root penetration. Furthermore, the effective connectivity was validated by the absence of localized waterlogging during the 60-day irrigation cycle, indicating a hydraulic conductivity (K) sufficient for aerobic rhizosphere conditions.

4.5. Limitations and Future Research Directions

Despite the promising results, several limitations of this study should be acknowledged. First, the planting experiments were conducted over a relatively short observation period of 60 days. While this timeframe is sufficient to assess early-stage plant establishment, long-term studies are necessary to evaluate vegetation persistence, root–concrete interactions over multiple growing seasons, and durability under environmental stresses such as freeze–thaw cycles and rainfall erosion. Firstly, the impact of cold climates with prolonged negative temperatures poses a significant risk. While the interconnected porosity of VEC facilitates drainage, any residual water within the soil-filled pores can undergo freeze–thaw cycles. The volumetric expansion of ice (approximately 9%) within the confined cementitious skeleton could generate internal crystallization pressures that exceed the tensile strength of the bonding bridges, potentially leading to structural disintegration. Therefore, VEC application in alpine or permafrost regions may require the incorporation of air-entraining agents or specific porosity adjustments. Over time, natural ecological succession may lead to the settlement of woody plants (bushes or trees) with aggressive root systems. These ‘strong roots’ could exert significant radial pressure within the VEC pores, potentially fracturing the granulated matrix. To mitigate this, regular ecological maintenance is required to remove invasive woody species, or the inclusion of physical root-inhibiting layers may be necessary in projects where long-term structural integrity is prioritized over natural succession. Future research should explore a wider range of native and stress-tolerant species to develop region-specific vegetation eco-concrete solutions. Additionally, investigations into mixed-species planting systems may provide insights into plant–plant interactions and their influence on slope stability and ecosystem resilience.

Finally, further studies should examine the long-term evolution of the pore structure and mechanical properties of VEC as plant roots grow and organic matter accumulates within the concrete matrix. Advanced techniques such as X-ray computed tomography and numerical modeling could be employed to better understand the dynamic interactions between roots and the concrete skeleton. Such research would contribute to the optimization and broader application of vegetation eco-concrete in sustainable infrastructure and ecological restoration projects.

5. Conclusions

• The water–cement ratio, mortar binder, and cement particle content significantly influence the compressive strength and porosity of the specimens. As the water–cement ratio increases, both the compressive strength and porosity of the specimens increase. The compressive strength steadily increases with higher cement particle content, while the porosity first increases and then decreases. When the mortar binder content is 0.1 wt%, the compressive strength reaches its peak, and the porosity is the lowest. The water–cement ratio significantly affects the flexural strength of the specimens: as the water–cement ratio increases, the flexural strength gradually increases. The cement particle content significantly impacts the pH value of the specimens, which rises rapidly with increased cement content.
• Based on a comprehensive evaluation of the compressive strength, flexural strength, porosity, and pH value of the VEC specimens, the optimal mix design was determined as follows: a water–cement ratio of 0.27, cement particle diameter of 10 mm, cement particle content of 70–75 wt%, mortar binder content of 0.1 wt%, and polypropylene fiber content of 0.16 wt%. This mix yielded a compressive strength of 4.31 MPa and a flexural strength of 1.85 MPa, meeting practical requirements for slope stabilization engineering. It also demonstrated a favorable porosity of 28.75% and a pH value of 9.7, with continuous nutrient supply from the KZ particles, creating an excellent environment for plant growth.
• VEC prepared with KZ particles allowed for smooth germination and plant growth, with roots successfully penetrating the VEC layer. This demonstrated favorable vegetative compatibility between plants and ecological concrete. Selecting vegetation species suited to local climatic conditions can facilitate rapid ecological landscape restoration, which holds significant practical relevance for ecological management and rehabilitation of abandoned mines. Future iterations of VEC will focus on incorporating industrial by-products (e.g., ground granulated blast-furnace slag) into the granulated framework to enhance the environmental sustainability of the material, moving toward a dual-objective “Eco-concrete” that balances carbon reduction with ecological restoration.
• This study demonstrates that the optimized VEC provides a viable structural-functional hybrid for ecological restoration. However, certain limitations must be acknowledged for practical engineering. Firstly, the mechanical strength of the VEC (approx. 4 MPa) is intentionally optimized for horticultural compatibility rather than structural load-bearing; thus, its application should be restricted to non-structural slope protection where biological reinforcement is the primary goal. Secondly, the long-term durability of the granulated skeleton under extreme environmental stressors (e.g., severe freeze–thaw or acid rain) remains to be fully quantified. While the pilot-scale trials are promising, future research should focus on multi-year field monitoring to evaluate the evolutionary stability of the nutrient-enriched particles and the sustained buffering capacity of the matrix against carbonation-induced pH changes. These limitations notwithstanding, the proposed mix design offers a scientifically validated starting point for the development of high-performance vegetation-supporting substrates.