Low alkaline vegetation concrete with silica fume and nano-ﬂ y ash composites to improve the planting properties and soil ecology

: Slope protection and erosion management are severely hampered by the rapid infrastructure development in mountainous valleys, especially during the monsoon season. While conventional approaches like vegetation, porous concrete, and inorganic procedures have been used, stronger and more ecologically friendly alternatives are still needed. A new kind of concrete called vegetation concrete (VC) allows roots to grow through the concrete frame by combining plant integration with porous concrete. This creative method might be used for environmentally friendly building and planting. The alkalinity of VC signi ﬁ cantly impacts its planting capabilities and soil nutrient levels, making it crucial to reduce VC alkalinity. In this study, silica fume (SF) and ﬂ y ash (FA) were combined to create low-alkaline VC. The e ﬀ ects of SF and FA on VC ’ s alkalinity, porosity, compressive strength, and planting characteristics were examined. The study also investigated VC ’ s in ﬂ uence on soil fertility and its impact on soil nutrients. Test results revealed that SF and FA reduced the pH of the VC by reducing calcium hydroxide (CH) crystals. While SF had a lower basicity coe ﬃ cient ( M ) than FA, it had a more signi ﬁ cant e ﬀ ect on lowering VC alkalinity. The compressive strength decreased with FA but increased with SF, despite SF having a smaller cement component in VC – SF mixes. This suggests that blending VC with SF and FA is feasible, with the SF dosage exceeding the FA dosage for reduced alkalinity and increased strength. Lowering VC alkalinity through SF and FA


Introduction
Rapid infrastructural development in the hills and valleys is leaving bare slopes vulnerable to water runoff and soil erosion, as well as the danger of shallow landslides.The slope and the area around it are particularly harmed by the stormwater runoff that occurs during the monsoon season.As a method of slope protection, a variety of different strategies have been devised to counteract the detrimental impacts of the slope.Although the slope was effectively stabilized by the slope protection methods, such as vegetation, hydro-mulch, geotextiles, wire mesh, and other inorganic methods (concrete and brick masonry), these methods did not provide the slope with substantial resisting forces in the same way that anchoring and retaining structures do [1,2].Planting vegetation on a slope is the method that is both the most beneficial to the environment and the most eco-friendly [3].Although the soils are susceptible to erosion from storm water, it is well recognized that vegetation provides progressive and effective protection for slopes.In order to facilitate the infiltration of surface runoff and storm water, porous concrete, which is also known as pervious concrete, is an alternative material that can be used in place of ordinary concrete.In addition, the use of pervious concrete helps to decrease erosion, alters the pattern of water flow, and increases the amount of groundwater that is recharged.Very recently, a new variety of concrete known as vegetation concrete (VC) emerged as a result of traditional concrete application being combined with the influence of horticulture.The VC combines a plant with a substrate made of porous concrete that enables free passage of water, air, soil, and roots.After this, plants are able to germinate and establish their own roots in the underlying soil strata by growing through the porous concrete frame.The results of Tang et al. [1] highlight the potential of VC technology to be a realistic and successful solution, particularly considering the adaptability of specific grass species.Furthermore, it has been reported that research in this area could result in substantial breakthroughs and practical applications in slope protection and other related sectors.
It is widely recognized that plants need phosphorous (extractable), potassium (K), and hydrolyzable nitrogen (AH-N) for improved development.However, the soil's greater alkalinity reduced the availability of nutrients and harmed the function of the enzymes needed to assimilate AH-N [4].Additionally, the soil's increased alkalinity made it harder for plants to absorb nutrients and water for growth [5].Despite the fact that plants thrive in acidic environments, Codina et al. [6] and Huang et al. [7] recently showed that the high alkalinity of ordinary VC substantially restricts plant growth in VC.The literature research concluded that planting concrete should not have a higher pH since it has a major impact on the growth of plants.In addition, the increased alkalinity of the VC needs to be minimized since it may enhance the alkalinity of the soil adjacent to it.The appropriate pH values suitable for plants growing were between 8 and 10.The alkalinity of VC mainly came from the hydration phase of cement, and the pH value of hydrated Portland cement was up to about 13, making it inappropriate for making planting concrete.Therefore, Portland cement cannot be simply used to make planting concrete.Supplementary cementitious materials (SCMs) have been utilized extensively over the past few decades to lower the alkalinity of traditional concrete.Sun et al. [8] attempted to investigate the effects of silica fume (SF) and fly ash (FA) on the alkalinity of the cement paste and discovered that the pH of the pore solution decreased due to the addition of minerals, but that the corrosion resistance of reinforcement was decreased by a decrease in alkalinity.SF, a cement additive, was examined by Larbi et al. [9] in the 1990s.They found that SF's faster pozzolanic reaction lowered the pH of the cement system even at young ages.Yan et al. [2] used powdered anhydrite to reduce the alkalinity of sulfoaluminate cement clinker.Yuan et al. [10] revealing that the anhydrite addition lowered the pH of the cement clinker and improved the performance as well.The experiments by Garcia Calvo et al. [11] showed that adding SCMs to the cement system decreased the pH of the pore fluids and completely changed the cement pastes' microstructure in contrast to the plain cement paste.Golewski [9,12] presented a new concrete composite made with a quaternary binder containing ordinary Portland cement (OPC) partially replaced by various percentages of SCMs, such as siliceous FA, SF, and nanosilica (nS).According to the test findings, using quaternary blended cements gives promising mechanical qualities and decreased brittleness, making it appropriate for particular building applications requiring high mechanical strength and resistance to dynamic and cyclic loads.Young-Kug Jo [13] studied the integration of blast-furnace slag and FA with OPC to lower the alkalinity of OPC and it was discovered that the pH of the pore solution produced from hydrated cement paste decreased as the admixture level increased in the admixture-supplemented OPC [14].Gong et al. [15] recently included limestone powder into VPC and assessed the influence of limestone powder on seed germination and plant growth.The results of the tests revealed that adding limestone powder lowered the alkalinity of the VPC, and they concluded that boosting plant growth in the VPC significantly boosted the soil's moisture retention capacity and slope stability.The addition of FA and SF lowered the alkalinity of the concrete mixes while having no discernible effect on the strength of the concrete.Golewski, G.L. (2023) (2023) [16,17] observed the effect of introducing coal fly ash (CFA) into concrete compositions on the flexural compressive strength and water absorption in another investigation.According to the findings of the tests, the addition of 20% CFA resulted in an increase in flexural compressive strength and water absorption; however, the addition of 30% CFA resulted in a considerable drop in both strength and water absorption.
According to a review of the literature, SCMs, including FA, SF, and granulated blast furnace slag, were often employed by researchers to lower the pH of the cement-based paste and concrete, while relatively few researchers utilized those materials to lower the alkalinity of VC.Furthermore, the usage of SCMs in VPC is still in its early stages, with just a few studies published thus far [18].These findings emphasize the potential for VC technology to be a viable and effective solution, especially given the adaptability of specific grass species to varying FA content.Further research in this area could lead to significant advancements and practical applications in slope protection and other related fields.Furthermore, more research is needed to determine ways to reduce VC alkalinity in order to increase planting capacities, soil fertility, and ecological preservation.In order to fill the research gap, an experimental investigation was initiated to reduce the alkalinity of the VC utilizing SF and FA, as well as to identify the appropriate dosage in VC production.Additionally, it was examined how the drop in the VC's alkalinity due to the mixing of SF and FA influenced the planting parameters, such as the grass height, root length, and leaves relative water content (LRWC).Because VC will be used to stabilize soil slopes, the current study evaluated the effects of VC alkalinity on soil fertility that are linked with it, in order to assure environmental protection.The design of experiment (DOE) method is used in the current study to efficiently conduct the experimental inquiry and to give priority to the effect of SF and FA on the physical and physiological features of VC.The central composite design (CCD) of response surface methodology (RSM) was used to design the testing trials because it is a well-known fitting technique that prioritizes the impact of process factors with few set tests.

Cement
OPC was utilized to prepare the VC.VC was made with OPC that had a grade of 43 and a specific gravity of 3.08.Table 1 summarizes the chemical composition of OPC.

Admixtures: SF and FA
SF and FA were put onto the green concrete as admixtures to lower their alkalinity.For this experiment, FA of industrial grade, grade C, was obtained from Salem, Tamil Nadu, India.FA contains 23.14% silica, and Figure 1 details the chemical make-up of the FA. Figure 1 shows that the primary components of FA are silicon (Si) and aluminum (Al), indicating that FA may be utilized as a pozzolonic substitute for cement.Calcium (Ca), potassium (K), and sulfur (S) were also found in trace levels.With a few irregularly shaped particles, Figure 2 demonstrates that the majority of FA particles are spherical.The micrographs show the porous structure of FA, revealing its large surface area and potential for increased reactivity.The existence of agglomerates shows that FA particles have a propensity to cluster.Interparticle spaces and surface imperfections are observed, indicating probable pozzolanic reaction sites.In this investigation, SF with specific surfaces of 21,500 m 2 /kg and a relative density of 2.39 was utilized.Energy-dispersive X-ray analysis (EDAX) patterns and scanning electron microscopy (SEM) pictures of SF are shown in Figures 3 and 4, respectively.The specific surface, silica microsphere content, and microstructure of SF are all finer (90.7% of its weight).According to SEM, SF has a unique amorphous structure, with ultrafine particles closely packed together.The large surface area and compactness of SF particles contribute to their pozzolanic reactivity.The agglomerated structure of SF particles is shown in SEM pictures, demonstrating their propensity to form cohesive clusters.The smooth and uneven surfaces of SF particles improve their capacity to fill gaps in the concrete matrix.

Fine and coarse aggregates
The coarse aggregates were crushed blue metal stone, a combination of limestone and gravel, produced in Srikakulam, Andhra Pradesh.The coarse aggregates were screened, and an aggregate gradation of 10-20 mm was used (Table 2).The density of coarse aggregates was 2,780 kg/m 3 and the crushing index was 8.5%.Because the addition of sand improves aggregate bonding and the strength of the VC, 2% fine aggregate (by volume) of concrete was included without affecting the VC's porosity.In this research, sand particles with a specific gravity of 2.52 that could pass through a 4.25 mm screen were used.

Vegetation species
Ryegrass was considered as a crop for VC vegetation due to its superior adaptability/compatibility in the atmosphere of FA-and SF-modified VC and its extensive use in laboratory investigations.Table 3 provides an overview of the key traits of perennial ryegrass.Red soil, peat moss, and river sand are used in equal amounts since the soil mix has a considerable influence on how vegetation develops.
3 Experimental program

Mixture proportion
To test FA and SF rates ranging from 0 to 40% (with a 10% increment) and 0 to 12% (with a 3% increment), 25 different tests were required.To efficiently complete the experimental program, the number of tests required for the current research must be optimized.The DOE is a novel method for analyzing experimental results in terms of independent variables.DOE requires fewer trials, links unrelated variables, and, in the end, offers the optimum reaction to experimental data [9].As a result, the trials in this article were created with RSM's CCD.Priority was also assigned to how process variables affect VC characteristics.The response-influencing parameters and their levels must be specified in order to design CCD experiments.Based on the components and their levels, a total of nine different experiments were produced, and the combination details are shown in Table 4.All of the mixtures are labeled, with the label 25FA-6SF indicating that it contains 25% FA and 6% SF.Portland cement, FA, SF, river sand, and coarse aggregate were all used to develop the mixture.The water/cement ratio of 0.41 was established based on the trail test and kept constant for all mixture combinations because the weight of the cement and powder is constant for all mixtures.Table 5 summarizes the details of all mixture combinations.

Specimen fabrication and testing 3.2.1 Alkalinity
In this study, a pH meter was used to measure the pH value of the VC.The test was done according to the instructions in IS:3025 (Part II) -1983, and the test sample was made according to the instructions in Lianfang Li et al. [18] (Figure 5).For the sample preparation, the mortar part of the VC was taken out at 7 and 28 days, ground, and passed through 0.08 mm sieves.The sample of sieved mortar was   then mixed with water in a ratio of 1:10 and left to rest for 1 h.The water part was then filtered and checked for pH.

Compressive strength test
In accordance with the procedure outlined in IS 456-2000, concrete cubes measuring 150 mm in size were fabricated and subjected to testing at the ages of 7 and 28 days.All of the cubes were created at ambient temperature, and they underwent curing and testing at a temperature of 26°C (Figure 6).

Porosity (void ratio)
The void content of all combinations was evaluated in this study since it is widely known that minimizing the void content of VC minimizes water runoff and root penetration.The void content of VC was measured in this study using the volume displacement technique, and the test was conducted in accordance with the steps indicated in ASTM C1754/C1754M-12 [19].After 28 days, the VC cylindrical specimen's dry mass and underwater mass were recorded.Equation (2) [19] was then used to calculate the VR: where k is a constant (= 1,273,240 ((mm 3 kg)/(m 3 /g)); D m and W m are the dry mass and the mass under the water, respectively (g); ρ w is the water density (kg/m 3 ); and D and L are the diameter and length of the cylinder specimen, respectively (mm).

Grass height, root length, and LRWC
After planting the plants for 3,10,17,25,35,45, and 60 days, the plant samples were removed from the VC, and a ruler was used to randomly measure the plant's height (stem) and roots (Figure 7).Safety precautions were implemented to guarantee that the roots' integrity would be preserved  Low alkaline VC with SF and nano-FA composites  7 while being removed out of the concrete.In this study, the LRWC was determined by employing the fresh weight method that was outlined in Golewski [15].In the plantation, samples of leaves that were 15 and 90 days old were taken from the plant and weighed (W f ).The leaves were gathered and then placed in a jar with water for 24 h.After 24 h, the leaves were taken out of the water, carefully dried, and weighed once more (W w ).After wiping the leaves, they were put in an oven for half an hour at 105°C to dry.In addition, the weight of the dried leaves was determined (W d ).After the weighing procedure had been completed, LRWC was determined by using equation (2) [15].Figure 8 depicts the procedure for conducting the test: (2)

Soil fertility index
To assess the concentration of nutrients in the soil mass, soil samples from above and below the VC were obtained and tested for nutrients, such as hydrolyzable nitrogen (AH-N), phosphorous (P) (extractable), and potassium (K).Because the Olsen [20] technique is highly preferred for measuring soil phosphorous levels at pH 7.4 or higher, the same approach was used in this research to test soil phosphorus.The earth phosphate will be extracted using a 0.5 N sodium bicarbonate solution adjusted to pH 8.5 in this technique.In this research, AH-N was determined using the simple alkaline hydrolysis technique proposed by Dodor and Abatabai [21].The released NH 3 boric acid was captured in this technique by direct-steam distillation of soil with 1 M potassium hydroxide (KOH) or sodium hydroxide (NaOH), and its content was measured for 40 min.
4 Results and discussion

Alkalinity
Figure 9 shows the pH value for the entire combination, and it is clear that the integration of FA and SF reduced the  (3)  Low alkaline VC with SF and nano-FA composites  9 The presence of FA and SF lowered the pH of the VC because their M values, which are less than 1.0, indicate that they belong to the acidic admixture.The observations above lead to the conclusion that the FA and SF can both be used effectively to reduce the alkalinity of the VC.The Pareto chart, which is a bar graph, shows the importance of different independent factors affecting the result.Figure 8 depicts the Pareto chart of alkalinity at 7 and 28 days of age.Figures 9 and 10 show that the impact of SF is far more significant in decreasing the alkalinity of the VC than the influence of FA.Additionally, the pH of the VC remained unaffected by increasing the FA dose.The 20% FA dose reduced the pH of the VC by around 0.24 units, whereas the 40% FA dosage reduced the pH of the VC by about 0.31 units, which is relatively equivalent.The results are in line with earlier research, and Sun et al. found that even though FA is an acidity addition, the FA's decreased interaction with the cement matrix did not cause the pH to decrease.Diamond [24] observed that the FA had no effect on the pH of the concrete because the alkalis in the FA did not raise the concentration of alkalinity in the pore solution.Figures 9 and 10 clearly describe how the presence of SF in the VC influenced the hydration process and decreased the pH.Because of the pozzolanic reaction, increasing the SF dosage rate greatly decreases the pH value of VC, and the effects of SF on the pH of the VC are clearly visible even at early ages.At day 7, SF dosages of 6% and 12% in VC lowered the pH by 2.12% and 3.51%, respectively, compared to the VC with OPC.In Larbi et al. [9], the pozzolanic reaction of SF is described.The very fine silica particles in aqueous CH solutions react with water to form a saturated monosilicic acid solution ( ) . As accelerated hydration progresses, more OH − and alkali enter the pore solution, resulting in increased silica dissolution.Silica dissolution is directly proportional to alkalinity.Since the OH − attacked the silica to activate the pozzolanic reaction, the concentration of OH − in the aqueous solutions was reduced, resulting in a pH drop.Moreover, the addition of SF decreased the pH of the VC because it is an acidic admixture with an M value of 0.015.Khan et al. [25] assert that raising the SF content in the cement lowers the pH of the pore solution.The pH of cement paste was greatly decreased by adding more SF to OPC, according to Jan et al. [26], but beyond the dosage rate of 50%, there was only a modest pH decrease due to the existence of remaining portlandite.The current study's findings revealed that FA and SF may be used to reduce the pH; however, a combination of a greater dose of SF (20%) and a lower dosage of FA (10%) is suggested to lower the pH of the VC.

Porosity
The porosity of all mixtures was measured, and the results are presented in Figure 11.From Figure 11, it can be understood that the inclusion of FA and SF reduced the porosity of the VC, and as the dosage of FA and SF increased, the porosity of the VC decreased further.For instance, the porosity of the combination with OPC was 126.15%, but the porosity of the mixtures VC-40FA and VC-12SF were 21.05 and 24.35%, respectively, which are 24.22 and 13.21% lower than the porosity of the combination with OPC.
The low density of the FA and SF in comparison to the cement may be the cause of the reduction in VC porosity with their addition.It is well known that the relative densities of the FA and SF were lower than that of the cement, and the relative densities of the FA and SF used in the study were 2.21 and 2.39, which are 39.37 and 28.87%, respectively, lower than that of cement, and the bulk density of cement was 3.08.Since FA and SF replaced the cement in the VC on a weight basis, the amount of binder and powder available in the VC increased.As a result, the volume of binder and powder slurry available in the VC increased, which enhanced the bonding/contact of the aggregates and decreased the VC's porosity.Furthermore, the larger surface area of SF and FA results in enhanced void filling and packing efficiency within the concrete matrix.This, in turn, can lead to decreased porosity.The findings are consistent with those of Golewski [27,28], who discovered that the bulk cement paste matrix improves void filling and packing efficiency inside the concrete matrix.Though the relative densities of the FA and SF are relatively equal, Figure 12 clearly implies that the influence of FA was significant in reducing the porosity of the VC rather than the SF.The VC-20FA reduced its porosity by 16.68% than that of VC-0, whereas the porosity of VC-12SF was 23.98%, which was 9.04% lower than that of VC-0.This is a result of the lower slurried density of the FA.Low alkaline VC with SF and nano-FA composites  11 Figure 13 depicts the complex interactions among SF, FA, and the strength of concrete.Figure 13 shows that the presence of FA in the VC lowered the compressive strength of the VC as the dosage rate increased; moreover, the drop in the compressive strength was quite considerable, particularly in the early stages (7 days).The strength of VC-20% FA was 14% lower than that of VC-0 at 7 days.Due to the FA's lower activity (pozzolonic reaction), He et al. [29], Ibrahim [30], and others found that the substitution of FA in concrete reduces the concrete's compressive strength at a young age.Moreover, as VC cement is replaced by FA, the cement availability in the VC decreases, resulting in less formation of the C-S-H gel.As a result, the VC's early strength was reduced.He et al. [31] consider nevertheless, the strength of VC-20FA was roughly equivalent to that of VC-0 at 28 days, and the strength was only 7.89% lower than that of VC-0.This is due to the fact that the pozzolonic reaction of FA at later ages produced a greater quantity of C-S-H and C-A-H gels.As a result, the VC-20FA's strength was comparable to that of the VC-0.The results of the present investigation were in excellent agreement with those of Olsen & Dodor [27,28], Zhou et al. [32] and Wei et al. [33].Further, we observed that since the bonding of FA particles with the cement matrix is very feeble, replacing FA in VC resulted in a loss of the compressive strength.

Compressive strength
Figures 13 and 14 show that the incorporation of SF in the VC significantly enhanced the strength of the VC at all ages when compared to blends with FA.The results of the current investigation also showed that, despite the fact that VC-SF combinations included less cement than VC-0 mixtures, they were stronger than VC-0 mixtures.At 7 and 28 days, the strength of VC-12SF was 9.56% and 11.01%better than that of VC-0, respectively.Teixeira et al. [34] emphasized on the favorable impact of using SF as a substitute for OPC.Zeng et al. [35] discovered that blending cement with SF improved the concrete strength and resistance to sulfate attack.The presence of SF increases the strength of the VC because to the stronger pozzolanic reaction of the SF due to the increased amorphous SiO 2 content of the SF.The dissolution of silica to liberate "Si" into the pore solution increased when the pH of the pore solution increased owing to the hydration process.The OH ⁻ in the pore solution attacked the silica, causing the pozzolanic process to accelerate.The increased quantity of C-S-H gel formation resulted in enhanced strength, despite the fact that the cement component of VC-SF mixes was lower than that of VC-0.The key factors contributing to SF's exceptional performance in strengthening the concrete are its high silica content, ultrafine particle size, quick pozzolanic reactivity, and capacity to support both short-and long-term strength growth.Although the blending of cement with FA lowered the strength of the VC, the cement mixed with FA and SF demonstrated that the strength was comparable to the VC-0.For example, the strength of a VC-20FA was 6.67% lower  than that of VC-20FA-6SF.As previously stated, the larger amorphous SiO 2 content of the SF expedited the pozzolanic process and enhanced the intensity of the VC.The current findings concurred with Thanongsak Nochaiya et al. [29], who observed that combining FA with SF may be more advantageous to the strength development of concrete than combining FA alone.These findings indicate that the VC cement may be combined with SF and FA, but still the SF dosage must be more than the FA dosage to get higher strength.

Adaptability for vegetation: grass height, root length, and LRWC
At 45 days of age, Figure 15 depicts the development of ryegrass on the two various VC mixes.Figure 16 depicts the height of the roots and the stem at various ages and shows that the planting properties of VC blended with SCMs (FA and SF) are better than that of VC with OPC alone.According to Figure 16, the ryegrass started to germinate after 3 days and the average height of the stem was  Low alkaline VC with SF and nano-FA composites  13 258 mm at the age of 35 days in the mixture with SCMs, whereas, in the case of VC with OPC, plants started to germinate after 4 days and at the age of 35 days, average height of the stem was 198 mm which is 30.31%lower than that in mixture with SCMs.For the plant growth, iron, alkali hydrolyzable nitrogen (AH-N), phosphorous (P) (extractable), and potassium (K) are the important nutrients.The availability of nutrients in the soil for plant growth was often decreased by the increased alkaline nature of the soil.Since the lower availability of nutrients, particularly micronutrients in the soil medium above and below the VC due to the greater alkaline nature of the VC without SCMs, had an impact on the plant growth.In the case of VC with SCMs, the blending of cement content of VC using SCMs influenced the alkalinity of the VC.Because of the enhanced Si/Ca ratio and amorphous silica's unique adsorption properties, the C-S-H gel's improved sorption capacity was able to reduce VC's alkalinity.The decrease in the alkalinity of the VC, thus, increased the nutrients available in the soils slightly compared to the VC without SCMs; consequently, the planting properties was improved.The current findings are in agreement with Golewski [15], who found that a 2% reduction in the VC's alkalinity using limestone powders significantly improved the VC's planting qualities.In 2019, Diamond [36] found that the inclusion of SF and FA in VC having a fertilizer content of 5% increased the plant heights significantly.The outcome of Page and Vennesland [37] and Mihara et al. [38] also revealed that the decrease in the alkalinity of the VC with the inclusion of SCMs improved the vegetative capabilities of the VC.
Considering the root morphology of ryegrass in common soil, the root grows vertically in the downward direction.However, in the case of VC with and without SCMs, the root morphology of ryegrass was different and the root growth was staggering in the surface towards the pores of the VC.Though the root morphology of the rye grass was different in VC, the roots penetrated to the soil strata through pores and acquired required nutrients for growth.At the age of 45 days, the average height of the root of VC blended with SCMs was 95 mm; further, in a 95 mm root length, 67 mm of the root length penetrated into the concrete, which was 23% higher than that of VC with OPC.As mentioned above, the decrease in the alkalinity of the VC with SCMs maintained the nutrients available in the soil; consequently, the root development also improved.Though the root development pierced the VC pores and extended into the underlying soil medium, the root penetration did not harm the VC and, in fact, enhanced the underlying soil stability.Similar findings were reported in Golewski [39].The LRWC of all VCs with and without SCMs were measured at 15 and 90 days and are summarized in Table 6.
The LRWC values for both combinations were nearly identical at the age of 15 days since the plants were obtaining nutrients from the soil layer above the VC, and the roots had only begun to penetrate the concrete [40,41].However, the difference in the LRWC was observed at the age of 90 days.The LRWC of VC with SCMs was 18.26% higher than that of VC-0.As stated earlier, the decrease in the alkalinity of the VC with SCMs increased the nutrients (N-P-K) available in the soils slightly compared to the VC without SCMs; consequently, the LRWC improved [30,[42][43][44][45].

Influence of VC on the soil fertility index
Though the inclusion of SCMs improved the planting properties, the influence of SCM-modified VC on the soil fertility indexes (nitrogen-phosphorous-potassium (N-P-K)) must be verified because the VC will be utilized for slope protection [46][47][48][49].Further, Golewski [15] discovered that the utilization of VC in the soil slope protection modified the  soil fertility indexes.Accordingly, alkali-hydrolyzable nitrogen (AH-N), phosphorous (P) (extractable), and potassium (K) were measured on the surface soil of VC after 180 days, and the results are presented in Figure 17.
It can be seen from Figure 17 that the VC enhanced the soil fertility indexes; further, the introduction of SCMs in the VC improved the available phosphorous and alkalihydrolyzable nitrogen of the soil.The available phosphorous and alkali-hydrolyzable nitrogen of the soil above the VC with SCMs increased by 32.81 and 52.92%, respectively, compared to nutrients available on the first day of planting.The current findings are in agreement with the findings of Brunno da Silva Cerozi and Kevin Fitzsimmons [30], and it was discovered that the available phosphorous and alkali-hydrolyzable nitrogen in the soil decreased with the increase in the pH of the soil.Golewski [15] found that the VC improved the available phosphorus in the soil by 70.2% within 1 year after construction [50][51][52][53][54]. Figure 15 implies that the VC decreased the soil potassium levels; the levels were further decreased with the inclusion of SCMs in the VC.In general, the potassium nutrients available in the soil increased with the increase in the alkaline levels of the soil [55][56][57][58].Further, because the alkaline nature of the soil increased the cation exchange capacity (CEC), the potassium nutrients available in the soil will be increased.Since the alkalinity of the VC with SCMs was lower than that without SCMs, there was a decrease in the soil potassium levels; however, the difference was not high, and was 4.91% compared to that of VC without SCMs.

Conclusions
The present study investigates how SF and SF influenced the VC's alkalinity, porosity, strength, and plant characteristics, including the grass height, root length, and LRWC.The cement content of the VC was substituted in various proportions with SF and FA.Furthermore, because VC is widely used to stabilize soil slopes, the influence of VC on the soil fertility was studied to ensure ecological protection.
• Due to the lower relative density of the FA and SF, the volume of binder and powder slurry increased to combine the coarse aggregates of the VC, thus reducing the porosity of the VC.The cement content of VC mixed with 20% FA decreased the porosity by 16.68% compared to VC-0.• The lower reactivity of FA with the cement matrix reduced the VC strength; however, the swift pozzolanic reaction of SF increased the quantity of C-S-H gel formation, thereby increasing the VC strength, even though the cement component was smaller than that of VC with OPC.• The results show that SF and FA may be coupled with VC cement; however in order to get better strength, the SF dosage must be greater than the FA dosage.• The reduction in alkalinity caused by the addition of SF and FA improved the soil nutrients linked to the VC, which improved the planting.• The use of VC for soil stability boosted soil fertility indexes by enhancing the soil medium nutrients, such as hydrolyzable nitrogen (AH-N), phosphorous (P) (extractable), and potassium (K).

Figure 6 :
Figure 6: Preparation and testing of VC cubes.

Figure 7 :
Figure 7: Measurements of the root and stem lengths.

Figure 9 :
Figure 9: Effects of FA and SF on the pH of the VC.

Figure 10 :
Figure 10: Prioritization impact of FA and SF on the pH of the VC (Pareto chart).

Figure 13
Figure13depicts the effect of FA, SF, and their combination on the compressive strength of VC after 7 and 28 days.

Figure 11 :
Figure 11: Effects of FA and SF on the porosity of VC.

Figure 12 :
Figure 12: Prioritization impact of FA and SF on the porousness of the VC (Pareto chart).

Figure 13 :
Figure 13: Effects of FA and SF on the compressive strength of VC.

Figure 14 :
Figure 14: Prioritization impact of FA and SF on the compressive strength of the VC (Pareto chart).

Figure 15 :
Figure 15: Germination and development of ryegrass on VC.

Figure 16 :
Figure 16: Effects of FA and SF on the growth of the plant's roots and stems.

Table 1 :
Chemical composition of cement Low alkaline VC with SF and nano-FA composites  3

Table 2 :
Particle size distribution of fine and coarse aggregates(IS  383:2016)

Table 3 :
Basic properties of perennial ryegrass

Table 4 :
Parameters levels in the CCD model

Table 6 :
LRWC results at various ages