Effect of Curing on Micro-Physical Performance of Polypropylene Fiber Reinforced and Silica Fume stabilized Expansive Soil Under Freezing Thawing Cycles

This study presents the micro-physical investigation of polypropylene (PP) fiber-reinforced, and silica fume (SF) stabilized expansive soil (BC) subgrade. The coupling effect of soil, PP fiber, and SF has been evaluated under the freezing-thawing (F-T) cycle to assess the durability of treated BC Soil. The curing method and duration staggeringly influence the strength of SF treated BC soil; therefore, three different curing method, i.e., moisture-controlled curing (MC), gunny bag curing (GC), and water submerged curing (SC) to a period of 7, 14, and 28 days were considered. The BC soil has been reinforced with 0.25%, 0.50%, and 1.00% PP fiber and stabilized with 2%, 4%, 6% and 8% SF. The physical, chemical, and microstructural properties were determined before and after 2,4,6,8,10 F-T cycles. With the increase in SF content, the unconfined compressive strength of the expansive soil has been increased due to the formation of Calcium Silicate Hydrate (C-S-H) gel. The chemically inert, hydrophobic, non-corrosive nature, and higher tensile strength of PP fiber, it has a higher potential to reinforce the BC soil for durability under tensile failure. This research confirms the possibility of incorporating SF and PP Fiber in road work applications, with significant environmental benefits.

The infrastructure development like paved structure, building, dams involves exhaustive use of natural resources and has a serious concern to the environment 1 . Industrialization is proliferating in developing countries such as India; hence, the disposal of industrial waste became a severe threat 2 . A paved structure (highway, airfields, lawns) is the composite structure that usually consists of a surface layer, base layer, and subgrade layer. The durability of paved structures majorly affected by the performance of the subgrade material. The soft soil is found in the semi-arid and arid regions of the world. The soft soil is also termed as expansive soil or black cotton soil. About 3.50 million square kilometers of vertisols, clayey soils rich in smectite, are spread worldwide over 60 countries and regions 3,4 . Expansive soil is found in Australia (0.80 million square kilometers), India (0.73 million square kilometers), China (0.60 million square kilometers), Sudan (0.50 million square kilometers), southern and western states of United States (0.13 million square kilometers), Chad, Cuba, Egypt, Ethiopia, Ghana, Puerto, Rico, and Taiwan 5 .
The layered structure of the clay minerals and cations adsorbed for the charged equilibrium induced swelling-shrinkage in expansive soil 6 . A higher nonrecurring swelling-shrinkage behavior of expansive soil culminate damages to the built infrastructure 7 . The damages caused by the expansive soil can be observed around the world; approximately $7-$ 9 billion per year economic losses have been reported alone in the United States 8 . The major deterioration was espied on the canals, pathways, buried pipelines, and paved structures. The numerous remedies were successfully demonstrated to improve the engineering behavior of the expansive soil, such as chemical alternation additive [9][10][11][12][13] , moisture control 14,15 , application of adequate surcharge pressure 16 , Biocementation 17 , biochemical [18][19][20] , and Geosynthetic [21][22][23] .
The chemical stabilization is a substantiate quick-fix method to improve the shear strength and also reduces the swelling shrinkage nature of expansive soil [24][25][26][27] . Nonetheless, there are substantial problems for the long-term

Materials and methods
Material. The soil used in this study was collected from the Malwa plateau site inside the Indian Institute of Technology Indore, Madhya Pradesh, located in the central part of India. The samples have been excavated at a depth of 1.5 m to 2.0 m from ground level. The major constitutes of soil clay (71.5%), silt (24.5%), and sand (4.0%) were observed as per grain size distribution. The Atterberg's limits of the expansive soil were also determined (liquid limit at 89%, plastic limit at 47%, and shrinkage limit at 11%). As per the Unified Soil Classification System (USCS), the soil has been classified as inorganic clay with high plasticity (CH). The maximum dry density (MDD) and optimum moisture content (OMC) have been found to be 17.65 kN/m 3 and 19.20%, respectively. A 120% Free Swell Index (FSI) has been observed for 7 days; this indicates the higher potential of swelling in soil. Table 1 summarizes the results of the index properties of expansive soil considered in the present study. The index properties of the expansive soil have been evaluated as per Bureau of Indian Standard IS 2720.
PP fiber is produced from a polymeric material and waste plastics. The PP fiber is highly acid, and salt resistance. Its higher tensile strength shows a higher potential to use the material as microfine reinforcement. The chemically inert, hydrophobic, and non-corrosive nature of PP fiber also indicates its effective utilization in the soil reinforcement due to low susceptibility of absorption or reaction with soil moisture or leachate. The polypropylene (PP) fibers of an average length of 12 mm have been considered. The technical specification of the considered PP fiber is tabulated in Table 2.
The cementation material used in this study is silica fume (SF) and its chemical compositions and mechanical properties are tabulated in Table 3. It is an industrial waste derived from silicon metal or ferrosilicon alloy. The SF majorly consists of amorphous silica (SiO 2 ). The higher specific surface area, cementing effect, and silica fume pozzolanic activity exhibit a higher potential to use as a cement and lime alternative.
Sample preparation. The four types of samples have been prepared to investigate the effect of curing methods on freezing-thawing cycle of PP fiber-reinforced, and silica fume stabilized expansive soil: (1) an untreated sample; (2) an SF stabilized sample; (3) a PP Fiber-reinforced samples, and (4) a PP fiber-reinforced and SF stabilized sample. The expansive soil has been reinforced with 0.25%, 0.50%, and 1.00% PP fiber and stabilized with 2%, 4%, 6%, and 8% SF as reported by   63 . The expansive soil samples were air-dried until a stable state was reached. Several mixtures were prepared by mixing the various percentages of water to achieve a uniform mixture of all ingredients.
The behavior of BC soil is affected by optimum moisture content (OMC) and maximum dry density (MDD) 65 . The specimen was therefore prepared at the MDD and OMC. The 4.75 mm passed dry mixture of 105-110 °C oven drived BC soil, SF (2.0%, 4.0%, 8.0%), and PP fiber (0.25%, 0.50%, 1.00%) was prepared. The dry mix was maintained at 27 ± 2 °C and 65 ± 5% humidity in the environmental chamber to preserve the soil mass constant temperature and uniform moisture content.
The 38 ± 2 mm diameter 76 ± 4 mm length specimens of the various mix have been papered with the statically simple compaction method. The compaction of the sample was carried out using an automatic soil compactor to avoid the change in energy level of a hammer. The statically compacted specimens have been kept for three different curing methods, i.e., MC, GC, SC; each to a period of 7 14, and 28 days. The maximum duration for the curing has been finalized since the various studies reported that the basic hydration reaction reached stable in 28 days curing period 66 . Figure 1 shows the schematic plan of the sample preparation method of soil, SF, and PP fiber.
The treated specimens were grouped into three categories to investigate the effect of different types of curing methods. The prepared specimens have been covered with the latex membrane for the curing process. The specimens have been kept in the environmental chamber at the 20 ± 2 °C and 100 ± 5% humidity for the MC curing. The specimens have been warped in the wet gunny bag for the GC and for SC the specimens have been submerged in the water tank at room temperature.
Chemical and microstructural analysis. The microstructural components of the PP fiber-reinforced SF stabilized expansive soil was studied using SEM, ATR-FTIR, XRD, and EDX analysis. The selected specimens after the UCS and STS test were taken for the SEM and EDX analysis. The freeze-cut-dying method was used to preparing the soil specimen for the microstructural analysis. This method can preserve the original soil microstructure and minimize flaws such as erroneous orientation and false void, which occur in the SEM image 67 .   Table 3. Silica fume properties considered.
The soil specimens have been crushed and coated with (Cu) copper material to reduce the charging effect on the samples during SEM analysis. To remove the water and organic contamination, soil specimen has been degreased and dried in vacuum. The required amount of the sample has been pasted on the carbon tape and then surface coated by Cu material 68 .
Since the PP fiber is used as the reinforcement material and does not have any influence on the chemical structure of the stabilized soil, hence the ATR-FTIR and XRD analysis have been carried out only on SF stabilized specimens with different curing method. The KBr pellet technique has been adopted for the FTIR. IR Spectral characterization of the pellet was performed from 400 cm −1 to 4000 cm −1 using ATR-FTIR spectrometer (PerkinElmer) equipped with a potassium bromide beam splitter. The XRD test has been performed on the peat sample of size less than 75 microns. The SF content highly influences the chemical alternations in the expansive soil. Electrical conductivity, pH, and Calcite content test have been conducted to determine the chemical alternation in expansive soil due to addition of SF.

Mechanical property analysis.
A total of 408 specimens were prepared using the aforementioned method.
The detailed mix design plan with the test section, and several specimens tested are tabulated in Table 4. To understand the durability characteristics of the untreated and treated BC soil, the specimen was kept in the freezing-thawing chamber. After completion of each curing cycle i.e., 7, 14, 28 days of PP reinforced SF stabilized expansive soil specimens. The freezing-thawing test has been conducted in the closed system. Before F-T analysis, the treated and untreated specimens were warped with transparent plastic to reduces the atmospheric interaction. The F-T cycle has been set at freezing temperature −20 °C for the first 12 hrs and then thawing temperature +20 °C to another 12 hrs 69 . The F-T test has been carried out up to 10 cycles, considering the expansive soil, which shows stable deformation and strength characteristics after an adequate F-T cycle 70 . After every two F-T cycles i.e., 0, 2, 4, 6, 8, 10, the one specimen has been taken out from each sample type and tested under UPV, UCS, STS experiments The reproducing of the experimental results is highly desirable; to understand the repeatability of test results all the experiments were carried out thrice on treated and untreated BC soil. The findings of repeatability indicate 9.8% as the highest standard deviation. All experimental results are presented on average for each experiment. The detailed experimental methodologies for stabilizing the BC soil shown in Fig. 2.

Results and Discussion
The effect of different curing method on PP fiber-reinforced and SF stabilized expansive under the freezing-thawing cycle have been evaluated. The experimental program has been conducted on 408 treated and untreated expansive soil specimens. The test results with varying curing methods and curing period have been compiled in the following section with detailed discussion.
Effect of the curing method on UCS value of treated and untreated BC soil. The effect of the curing method on the unconfined compressive strength of the PP fiber-reinforced and SF stabilized expansive soil is shown in Fig. 3. The results of the GC, MC, and SC with 7, 14, and 28 days has been presented. The improvement in the unconfined compressive strength can be significantly observed. A similar failure pattern has been observed in all three curing period. However, the strength values have been increased exponentially. It has been observed that with the increment in the SF content, the compressive strength increased. However, with the addition to the higher concentration of the PP fiber content, the compressive strength has been reduced. The increase in www.nature.com/scientificreports www.nature.com/scientificreports/ the compressive strength with the inclusion of SF can be attributed to the formation of Calcium silicate hydrate (C-S-H). The electrical conductivity, calcite content, and pH value test have been conducted to understand the chemical reaction of the SF with expansive soil, and test results were presented in Fig. 3(d). The value of EC is increased continuously by increasing the percentages of SF, which confirms that the chemical reaction takes place  www.nature.com/scientificreports www.nature.com/scientificreports/ with the inclusion of SF with BC soil and water. It is well established that the rise in atmospheric temperature will improve the hydration of cement 71,72 . The EC values increase significantly with the addition of SF content. The pH values have also been increased with increasing the SF content, which signifies the changes in the nature of BC soil-SF suspension and confirms the chemical alteration in the treated specimen.
The increment in the pH value increases the shear strength parameter of the soil specimen 73 . The higher specific surface area of the soil matrix makes it an effective filler. This reduced the volume of soil voids and induced the homogeneous production of CSH / CASH gel, i.e., sound microstructure 74,75 . The significant improvement in the CCt is also observed with the addition of the SF. Increment in the CCt can be attributed to the formation of the C-S-H gel 20 . Among all curing methods, SC curing shows the highest improvement in UCS values; however, the effect of the GC curing method is lowest. It can be ascertained that both the method i.e., SC and MC show promising improvement. Calcium silicate hydrate (C-S-H) gel is produced when SF content associated with the water while undergoing the pozzolanic reaction, as shown in Eq. 1. The chemical reaction of SF and calcium makes more brittle and steeper treated specimens. The brittle behaviors of SF stabilized soil exponentially increase the UCS value.
The maximum UCS has been obtained as 2862.13 kPa for a sample ID BC + 8%SF + 0.50%PP at 7 days curing as shown in Fig. 3(a). A similar trend has been observed for 14 days and 28 days, as shown in Fig. 3(b,c). The maximum UCS has been observed as 3627.54 kPa, and 5003.51 kPa for the 14 days and 28 days, respectively, for the sample ID BC + 8%SF + 0.50%PP. However, it can also be observed that at 7 days curing period, the maximum UCS for GC, MC, SC are obtained as 2161.20 kPa, 2565.43kPa, and 2862.13 kPa, respectively. This result deduces the effect of curing on the same sample. From the result, it can be ascertained that the effect of the submerged curing is higher than the moisture-curing and gunny bag curing. www.nature.com/scientificreports www.nature.com/scientificreports/ Effect of curing method on chemical alteration of treated and untreated BC soil. The study of the alternation in the microstructural property of the coupled soil-PP-SF is presented in the study. The expansive soil has been chemically stabilized by SF content with 2%, 4%, 6%, and 8%; hence it alters the chemical bonds of the BC soil. The PP fiber is used as the reinforced material and has no major influence on the chemical structure of BC soil due to inert hydraulic conductive behavior 57 . To assess the chemical alteration formed due to the presence of the SF content, ATR-FTIR analysis has been carried out. The FTIR spectra for the 7, 14, 28 days GC, MC, and SC cured specimens are shown in Fig. 4. The spectra of each concentration of SF and method of curing has been compared with the BC soil. The range of 500 cm −1 to 1200 cm −1 presented the presence of minerals, 1200 cm −1 to 3000 cm −1 organic matters, and 3500 cm −1 to 4000 cm −1 clay minerals 76 .
The characteristic bands of 3093 cm −1 -3716 cm −1 represent the vibration (stretching) of the hydroxyl groups (OH) of illite and kaolinite 77 . The peak of the hydroxyl group (OH) of illite and kaolinite has been observed similarly in all the sample ID. A similar trend in the specimens has been observed for peak presented at the 3434 cm −1 that attributed to the OH of water. It is observed that with the addition of the SF the 3093 cm −1 -3716 cm −1 band seems to be increased, the maximum stretch for the 8% SF content during submerged curing as shown in Fig. 4(d). The broad band at 3455 cm −1 signifies the presence of hydrating material (C-S-H & C-A-S-H) 78 .
The broad IR bond obtained due to the interaction of free dissolved Ca species from SF and forming C-S-H gel. This will also offer a nucleation site for geopolymer gel growth and additional binding material 79 . However, minimal changes have been observed in the gunny bag curing, and it shows the less formation of C-S-H gel, it also conforms with the low calcite content observation in Fig. 3(d). The FTIR spectra show the stretched peak at 874 cm −1 and 1468 cm −1 , ascertaining the presence of carbonate content (C-O bond). With the increase in SF content, the intensity of the band has been decreased, depicting reduction of carbonate group due to the presence of SF content 80 . The splitting in the carbonation band also observed near 1468 cm −1 to 1430 cm −1 due to partial carbonation of C-S-H gel in the air atmosphere 81 . The carbonation band can be attributed due to the formation of calcium carbonates 82   www.nature.com/scientificreports www.nature.com/scientificreports/ Effect of curing method on the mineralogy of treated and untreated expansive soil. The 7,14, and 28-days crystalline phases for SF stabilized BC soil with GC, MC, and SC curing determined by X-ray diffraction (XRD) analysis, are presented in Fig. 5. The XRD spectra indicate the presence of hydration product i.e., Calcium Silicate Hydrates (C-S-H), Calcium Silicate Aluminates Hydrations (CASH), and composed of portlandite (CH). The analysis of the XRD pattern is shown in Fig. 5 and was obtained using the Pananalytical X′pert Highscore Plus software. X-ray diffraction analysis data shows the peak of quartz as a distinct peak.
XRD analysis depicts the formation of Tobermorite, a mineral formed due to the pozzolanic reaction of the silica fume. The calcium hydrate peak is around 74°, which is the result of the hydration of cement. Calcium hydroxide help in increasing the pH of the soil-SF matrix, which further enhances the pozzolanic reaction. The peak at 60° shows the presence of the quartz; with the increase in the stabilization agent SF and curing period. The decrease in the peak can be caused due to the consumption of the quartz mineral in the pozzolanic reaction. As a result of the formation of the tobermorite mineral formed 86 .The intensity of tobermorite mineral peak the SC cured specimens with 28 days is observed highest. With this, it is ascertained that the formation of hydrated gel during SC curing takes place at higher concentrations. A similar trend has been observed for the MC curing; however, a minimal effect has been seen in GC curing.
Microstructural coupling effect of PP fiber and SF content on BC soil. The coupling effect of PP fiber-reinforced and SF stabilized BC soil on unconfined compressive strength and split tensile strength has its explanation in a microstructural mechanism that can be captured through FE-SEM analysis. Figure 6 presents the microstructural arrangement of the BC Soil-PP fiber and SF. Figure 6(a) shows the microstructure of the BC soil at 20 um, and it can be observed that the various cavities are present in the sample. The SEM images presented in Fig. 6(c) shows irregular wavy edges of the particles and the flaky structure of montmorillonite 87 . Figure 6(b) shows the flat-lying plates typical of kaolinite, whereas Fig. 6(f) depicts expanded, flared, "cornflake" or "oak leaf " texture of Na-montmorillonite 88 . The Fig. 6(h) shows the image formation of the sample ID BC+ 8%SF; it can be observed very dense soil matrix between the soil particle even at the 20 um. This shows the formation of the CSH gel filled the cavity present in the BC soil and formed a dense matrix. This result also supported by the spectra obtained for the FTIR and XRD in Figs. 4 and 5 respectively. Figure 6(e) shows the interesting result of the formation of C-S-H gel on the clay mineral and covering the soil particle. The gel formed on the clay particle also reduces the over consolidation behavior of expansive soil 89 . Figure 6(i) gives the microstructure of the PP fiber, and the various cavities are present in the PP fiber layered structure, and it can cause a failure plane during the compression loading. With the formation of C-S-H gel, the cavity of PP fiber filled with the produced gel and stiffed PP microstructure formed, as shown in Fig. 6(j,k). After STS tests the fiber break, as shown in Fig. 6(m), however, it can be observed that fiber reinforcement helps to bind the clay structure. Figure 6(l) shows the presence of CSH gel makes PP fiber stiffer; as a result, it gives higher tensile strength. The formation of C-S-H gel with the 8% SF content with three different curing methods i.e., GC, MC, and SC curing, has been showing in Fig. 6(n-p). The formation of C-S-H gel is higher for the SC curing method; however, very stiff and homogeneous formation has been observed due to MC curing. MC curing maintains the temperature and humidity at the constant level and formation takes place in homogeneous nature. Figure 6(k) also shows the interaction of PP and Soil, the connection seems very strong and filled with the gel. This behavior can be attributed due to soil-PP fiber interaction with C-S-H gel and as a result, strong bond observed. Figure 7 shows the EDX analysis of the selected SEM images for 2%, 4%, 6%, and 8% SF content. The intensity of calcium peak is dominant in all sections, which is usually expected due to the development of a hydration reaction. However, in all the cases, the intensity of the silica peak likely represents the existence of a pozzolanic effect 90 . This is mostly followed by the peaks of calcium and alumina. The 8% SF content shows the highest intensity of silica peak that offers more favorable UCS reponse 90 . The elements identified by the EDX test were confirmed with the results obtained for the FTIR and XRD as shown in Figs. 4 and 5. In general, major crystalline phases that occurred in the CSH gel matrix of SF stabilized BC soil specimens are responsible for the strength. The UCS performance of the specimens is well connected with the pattern of spectra obtained in the EDX. In general, for an increasing UCS value of SF stabilized BC soil, higher amounts of Si, and a sufficient amount of Ca and Al become appropriate conditions. The formation of the aluminosilicate gel or calcium silicate hydrate gel is produced due to the pozzolanic reaction of SF stabilized BC soil specimens, which provides dense structure 91 .

Effect of curing method on UCS and UPV of treated and untreated BC soil under F-T cycle.
The effect of the curing method on the unconfined compressive strength of the PP fiber-reinforced and SF stabilized expansive soil is shown in Fig. 8 under the freezing-thawing cycle. It has been observed that with increasing the SF content, the values of UCS increase exponentially with respect to the BC soil UCS value, as shown in Fig. 2. In order to assess the durability of soil-PP fiber and SF matrix F-T cycle tests have been carried out. As observed in Fig. 3(d), the increase of the UCS strength can be attributed to the formation of C-S-H/ C-A-H gel.
The formation of a hydrated gel fills the pore in the soil cavities, and it surrounded soil particles to form a relatively stable spatial network structure 70 . The dense matrix formed due to the formation of hydrated gel increase stiffness and strength of stabilized specimens 67 . The freezing-thawing action induced a swelling-shrinkage behavior in the treated and untreated specimens. During the thawing cycle, the volume of the specimens increased, and while undergoing the freezing, the volume shrinks. Due to this non-recurrence volume change, the density decrease and caused a tangible reduction in the strength 92 . The average loss in the UCS was observed 40%, 33.5% and 30.0% for the GC, MC, and SC, respectively. Figure 8(a) shows the effect of F-T cycles on GC cured specimens, and it is observed that at the lower concentration of SF, i.e., 2% and 4%, the UCS decreased exponentially. www.nature.com/scientificreports www.nature.com/scientificreports/ During the SC curing, since the water is infiltrated in the entire specimen and a pozzolanic reaction takes place in the entire specimen homogenously. The homogenous distribution of the C-S-H gel produced a definite soil-PP fiber matrix and gave higher strength. The SC cured specimens retain the maximum strength during F-T cycles. After the 8 th cycle of the F-T, almost similar results were observed, and minimal influence noticed on the specimen. The specimen with 8%SF and 0.50%PP shows the highest UCS value at the 10 th F-T cycle, although, it lost almost 51.23% strength. At the same time, the specimens with 4%SF and 0.50%PP lost only 27.21% strength at the 10 th F-T cycle.
From these results, it can be concluded that with the increase in the higher concentration of SF, a more hydrated gel formed in the specimens. The hydrated gel formed, broke due to swelling -shrinkage behavior and with higher concentration lose more strength 93 . However, no significant influence of PP fiber has been observed www.nature.com/scientificreports www.nature.com/scientificreports/ with increasing the F-T cycle. It reflects the inert behavior of the PP fiber with temperature variation and can be considered as the more durable material. The 7 days of cured specimens have lost more strength during F-T cycles, which depicts the lower concentration of hydrated gel formation. With the increasing, the curing period, the stabilized specimens show effective in durability.  www.nature.com/scientificreports www.nature.com/scientificreports/ The effect of the curing method on the ultrasonic pulse velocity test (UPV) of the PP fiber-reinforced and SF stabilized expansive soil is shown in Fig. 9 under the freezing-thawing cycle. The UPV of the specimens has been calculated after every alternate F-T cycle, i.e., 0,2,4,6,8,10 cycles before UCS and STS tests. The UPV value has decreased with the increase in the F-T cycles and process a similar trend, as observed in UCS results, as shown in Fig. 8. The UPV values have decreased due to the breaking of the hydrated CSH/CASH gel during the F-T cycle, and as a result, the treated specimens lose the strength 90 . The UPV value can be used to predict the durability of the SF stabilized BC soil reinforced with PP fiber. From the experimental results, it is concluded with the reduction in the UCS value, the value of UPV also decreased.

Effect of curing method on STS of treated and untreated BC soil under F-T Cycle.
The split tensile strength (STS) test has been carried out under F-T cycles, and the results are shown in Fig. 10. The inclusion of the SF content processes the hydrated gel in the treated specimens, and as a result, brittle shear failure is observed. The brittle shear failure can cause severe damages during the cyclic loading; hence the tensile strength plays a vital role in the performance of the stabilized BC soil. From Fig. 10(a), it is observed that the higher concentration of the PP fiber content i.e., 0.50%, and 1.00% induced higher tensile strength. The submerged curing specimens having higher tensile strength, i.e., 321.17 kPa, 407.07 kPa, and 561.47kPa for 7, 14, 28 days, respectively. During the F-T cycle, it was observed that initially specimen with 8%SF and 1.00%PP are having more STS value than the specimen with 6%SF and 0.5%PP. However, after the 8 th cycle of F-T, the STS value of 6%SF and 0.50%PP specimens were increased.
From the results, it can be concluded that the higher concentration of the PP requires more bond strength of the C-S-H gel, and once it broke during the F-T cycle, the value of the tensile strength decreased 94 . The curing www.nature.com/scientificreports www.nature.com/scientificreports/ method shows the same trend as indicated in the UCS and UPV, as presented in Figs. 8 and 9, respectively. After the 6 th F-T cycle, the untreated soil specimen was failed, and the result of STS values has been reported as zero.

Conclusion
The layered structure of the clay minerals and cations adsorbed for the charged equilibrium induced swelling-shrinkage in expansive soil. A higher nonrecurring swelling-shrinkage behavior of expansive soil culminate damages to the built infrastructure. This study presents the micro-physical investigation of polypropylene (PP) fiber-reinforced, and silica fume (SF) stabilized expansive soil (BC) subgrade. The coupling effect of soil, PP fiber, and SF has been explored under the freezing-thawing (F-T) cycle to assess the durability of treated BC Soil. The effect of different curing methods, i.e., GC, MC, and SC, to a period of 7, 14, and 28 days has been considered. All the experiments have been conducted at the laboratory; therefore, before implementing the proposed proportion of SF and PP fiber, the large scale field testing needs to be carried out. Based on the results and discussion presented, the following conclusions were made: • The SF possess similar property like cement and can be used as an alternative to BC soil stabilization. With the inclusion of SF content in the BC soil, hydrated gel CSH, and/or CASH formed, which fills the cavity and dense stabilized soil matrix. The filled densified soil structure has been observed in the SEM micrographs. • XRD analysis depicts the formation of Tobermorite, a mineral formed due to the pozzolanic reaction of the silica fume. The values of pH, calcite content, and EC are also increased, which attributed the chemical alteration in the treated soil. www.nature.com/scientificreports www.nature.com/scientificreports/ • The curing method and duration staggeringly influence the strength of SF treated BC soil; therefore, three different curing method, i.e., moisture-controlled curing (MC), gunny bag curing (GC), and water submerged curing (SC) to a period of 7, 14, and 28 days were considered. The SC and MC curing method shows an exponentially increase in the UCS and STS values. • Based on the F-T test, it can be concluded that the PP fiber having higher durability and can be used for controlling the brittle shear failure in chemically stabilized soil. Due to the high specific surface area of SF, it fills the cavities of the untreated BC soil and produced a dense soil and PP fiber a matrix due to the pozzolanic reaction as ascertained from the results of the microstructure. However, due to non-recurrence volume change during the F-T cycle, the STS and UTS values reduced. • Based on the mechanical properties of the treated BC soil specimen, the results of the MC and SC curing are higher and can be considered as the effective curing method. The maximum tensile and compressive strength has been observed for the with 8%SF and 0.50%PP; therefore, 8% SF and 0.50% PP combination matrix can be considered as the optimum proportion for the stabilization and furthers for the field application study.