Assessment of Durability and Resilient Strain Deformation of Expansive Subgrade Treated With Nano-Slag-Based Geopolymer

Nano-geopolymer binders (NGB) of 5%, 10%, 15%, and 20% concentration were used to stabilize expansive subgrade soil against strain deformation, as well as to improve its durability. The composites were subjected to a series of zero-swelling, wetting (W-D) cycles, and dynamic resilient modulus tests to determine the subgrade resilient strength against 100,000 applied repetitive loads (ARL). The results revealed that the resilient moduli of the stabilized and unstabilized subgrade soils exhibited strain-hardening responses at low cyclic stress levels. Therefore, the rate of plastic strain deformation became microscopically negligible, and the tested subgrade was considered stable at this stage. Conversely, at high cyclic stresses, the nano-geopolymer-stabilized subgrade continued to exhibit strain hardening between 80,000 and 100,000 ARL. The unstabilized subgrades exhibited strain softening at an ARL of 20,000 owing to the poor adhesion between the NGB and soil particles, leading to excessive strain deformation. The results revealed that the W-D resistance of the treated subgrade was up to 96% compared to the unstabilized subgrade, which lost over 30% of its particle mass after the 6-number cycle. This study indicates that the NGB-treated subgrade possesses the potential to sustain medium-to-high ARL loads owing to improved stiffness through polymerization reactions.


Introduction
The ever-increasing socioeconomic development and the declining availability of appropriate raw materials for pavement construction are challenges faced by civil engineers in the construction industry.Because of these challenges, substandard materials such as expansive subgrades are often modified to meet the required mechanical suitability for pavement construction.Expansive soil is a major substandard geomaterial with a low bearing capacity and excessive settlement, particularly in pavement structures.Generally, it is characterized by poor resistance to resilient strain deformation and swelling stress.This is because expansive subgrades constitute largely swelling clay minerals, that is, smectite, bentonite, montmorillonite, beidellite, vermiculite, attapulgite, nontronite, and chlorite, which are capable of causing massive volumetric movements upon the slightest moisture infiltration.However, these expansive clay minerals trigger surface rutting and fatigue cracking in pavement structures, which seriously affect their performance and lifespan of pavement structures [1][2][3][4].
Expansive subgrades require treatment to improve their mechanical properties during pavement construction.However, to satisfy the desired suitability for pavement performance, it is recommended to first understand the causes and mechanisms of subgrade swelling that cause pavement distress and 1332 (2024) 012001 IOP Publishing doi:10.1088/1755-1315/1332/1/012001 2 fatigue [5].The swelling subgrade typically contains clay minerals that attract and absorb water.This occurs when water is introduced into the pavement, and the water molecules are then pulled into the voids of the subgrade soils.The subgrade continues to absorb water until the voids are forced further apart, leading to an increase in the soil pore pressure [6].
Ikechukwu et al. [7] studied the effects of the swelling stress on the shear-strength failure resistance of an expansive subgrade.They concluded that pavement structures were significantly affected by the swelling stress from the subgrade, particularly in low-volume roads.Upon moisture absorption, the clay minerals trigger excessive swelling stress, which exceeds the pavement asphalt surcharge load.Therefore, the failure of the overlaid pavement layer to confine the swelling stress generated by the subgrade leads to cracks and fatigue in the pavement structure and sometimes triggers complete failure.
In addition to the swelling stress behavior of the expansive subgrade, cyclic stress is also a major cause of pavement defects, resulting in significant failure of the pavement structure.Cyclic stresses are generated by traffic loading, which, in turn, contribute to poor resistance against permanent strain deformation on the expansive subgrade [8].The behavior of expansive subgrades subjected to cyclic stress is depicted in the form of resilient and permanent strain deformation, which is characterized by a general response according to the shakedown theory [9].The expansive subgrade is known for its weak resistance to cyclic stress and permanent strain deformation owing to the clay content.Thus, stabilization of expansive subgrade has become a common practice within the field of pavement-geotechnics engineering.Chemical stabilization is considered the most effective and sustainable technique for improving the mechanical performance of subgrade soils for pavement construction [10].The application of chemical stabilizers changes the gradation and microstructure of subgrade soil, thus promoting cation exchange, which triggers the flocculation and agglomeration of soil particles [11].However, few studies have been published on expansive subgrade soil-geopolymer mixtures against swelling, cyclic stress, and permanent deformation over a large number of cycles [12].
In this study, the swelling behavior, durability, and permanent strain deformation of treated expansive soil were evaluated for the effective utilization of a subgrade stabilized with a nanogeopolymer binder (NGB).NGB was developed by synthesizing a moderate aqueous 10 M potassium hydroxide (KOH) solution with nano-slag.The swollen subgrade was treated with 5%, 10%, 15%, and 20% of NGB.Zero-swelling tests, saturation-desaturation cyclic tests, and dynamic resilient modulus tests were performed on various fabricated specimens to evaluate the resistance of the treated expansive subgrade against swelling and permanent strain deformation as well as its resistance against wet-dry durability cycles.

Materials
The expansive soil used in this study was collected from a subway construction site in Pietermaritzburg, Kwazhou Natal Province, South Africa.According to ASTM D1140 (2017) protocol, the particle size analysis and Atterberg limit test results classified the subgrade soil as well-graded with over 75% fines passing through a micro sieve size of 75 µm.The subgrade soil was quantified as highly plastic clay soil with a liquid limit value of 68%, corresponding to a soil designation of (CH).
The slag used in this study was collected from Sphathe Engineering and Mining Company in South Africa.These materials were classified according to the ASTM C6 [13] standard through X-ray fluorescence (XRF).Nanosized slag was developed using a top-down method after planetary ball milling for 6 h with an interval of 60 min between the resting periods.Subsequently, the surface area of the slag increased from 0.228 to 28.40 m2 /gm and the particle sizes of the nano-slag were evaluated within the range of 5-8 nm using high-resolution transmission electron microscopy.
Hydroxide (KOH) containing K + alkaline cations was chosen as the alkali activator in this study because of its efficiency in the alkaline activation process.The Limeco Chemical Company supplied the reagents in pellet form, which were diluted in distilled water to achieve a target concentration of 10 M.An alkaline solution of 10 M was chosen because it has been proven to be effective for alkaline activation [5].Potassium hydroxide was supplied in pellet form with a purity of more than 90%.The pellet was diluted with 1 L of distilled water and stirred for approximately 20 min to ensure complete dissolution.The solution was then left to cool for approximately 24 h before use to maintain a concentration of 10 M.
The chemical compositions of the expansive subgrade soil and nano-geopolymer binder developed using slag were analyzed using XRF, and the results are presented in table 1.To determine the total negative charge of the expansive soil, cation exchange capacity (CEC) tests were performed following Indian Standard 2720 [14] using an ammonium chloride solution of 485±3 ppm content equivalent to NH 4 + concentration.The results showed that the expansive soil contained fine particles greater than >75%.This indicated that the expansive soil had a positive surface charge for cation exchange, with a corresponding CEC value of 48.37 meq/100 g.

Experimental procedures
The aim of this study is to conduct a series of laboratory experiments.A zero-swelling stress test was conducted according to the IS 2720 Part 41 protocol [15].A particle soil size of 0.075 mm was used for this test because it represented more than 70% of the soil's fine and clay minerals responsible for swelling activities in the subgrade.Additionally, wetting and drying cycles were performed according to ASTM D559 protocols [16].AASHTO T307 [17] dynamic resilient modulus tests were performed using a loading sequence of the actual field cyclic stress on the subgrade.First, the specimens were preconditioned by applying 1000 cycles followed by 100 cycles for each sequence.The strain cyclic loading was loaded at a constant increment of 0.1 and a frequency of 1.0 Hz that is how it was automated.Cyclic loading at a constant strain (0.1 Hz) and frequency (1.0 Hz) was applied, coupled with confining pressures of 20, 50, and 100 kPa, and the test data (resilient modulus, cyclic stress, and stress-strain hysteresis loops) were processed through data acquisition software that runs on a personal computer with a Windows operating system.The average permanent strain was determined as the average strain of the first cycle, according to the recommendations by Vucetic and Mortezaie [18].

Effects of nano-geopolymer binder on swelling stress
The effect of the geopolymer binder on the swelling stress of the expansive subgrade soil is presented in figure 1.The tested specimens were fabricated at optimum water content using 5%, 10%, 15%, and 20% of geopolymer binder to the combined mass of the subgrade material.The expansive subgrade soil exhibited a swelling stress of 880 kPa, confirming that the subgrade was highly expansive.With the addition of 5% NGB to the soil, the swelling stress decreased from 880 kPa to 645 kPa.Furthermore, the swelling stress decreased from 645 kPa to 380 kPa with 10% addition of NGB.However, a significant decrease in swelling stress was observed with 15% and 20% inclusion of the geopolymer binder.Thus, the swell stress decreases from 880 kPa to 140 and 90 kPa, respectively.The decreasing trend in the swelling stress was due to the effects of the geopolymer binder.The geopolymer binder triggered a polymerization reaction with the reactive clay minerals in the clay owing to the dissolution of Si and Al in the alkaline activator.The free Si and Al ions available in the soil and nano-slag activator were mobilized by this dissolution process of polymerization reactions, forming tertiary compounds that drastically decreased the swelling stress.The results obtained in this study are supported by a report published by Abdullah et al. [19].The formation of new chemical compounds by polymerization contributes to the decreased swelling.The moisture within the soil voids was replaced and confined by the nano-geopolymer binder, increasing the strength and stiffness of the treated soil.The capacity of the geopolymer binder to restrict the swelling stress depends on the geopolymer reaction between the soil clay minerals and the nano-geopolymer binder, which reduces the volumetric change within the subgrade.

Figure 1.
Variation in nano-geopolymer contents with swelling stress.

Effects of wetting-drying cycles on stabilized subgrade
The effect of the variation in the wetting-drying cycles on the resilient strength of the geopolymertreated subgrade is shown in figure 2. All treated specimens were dried for 28 days in a curing chamber, after which they were exposed to facilitated weathering conditions of the wetting-drying cycle processes according to the methods suggested in ASTM D559 [16].However, the specimens were fully submerged for 5 h (wetting) followed by a minimum of 48 h of oven drying at 40 °C until water content reached the previous water content value before immersion.This process was repeated for 12 cycles and subsequently subjected to resilient modulus testing to simulate more severe weathering conditions.
The test results revealed that the unstabilized expansive subgrade failed after the 6 th cycle of wetting-drying conditions 30% of the soil mass of the specimens was lost.Therefore, the unstabilized expansive subgrade exhibited a resilient modulus value of 15MPa after the completion of the wetting-drying process.Failure was triggered by the absorption of excessive moisture, percentage of fine content, and loss of the soil mass of the specimens.Furthermore, the geopolymer-treated specimens survived all 12 cycles of wetting-drying conditions.Thus, the effects of the wetting-drying process triggered a slight increase in the resilient strength of the geopolymer-treated specimens compared to the strength of the non-treated specimens.The geopolymer-treated specimens survived the wetting-drying cycle because of the formation of secondary chemicals after the geopolymerization reaction.The survival of the treated specimens can also be attributed to the formation of intergranular matrices among the soil particles.Thus, this dominantly contributes to the increase in the resilient modulus values.Moreover, when dehydrated, a higher nano-geopolymer content was accompanied by denser and more extensive particles, resulting in a thicker film layer against moisture upon submergence of the treated specimens.The results obtained in this study are in line with those reported by (Stoltz et al. [20] and Cuisinier et al. [21], who revealed that the durability and strength of soil treated with geopolymer binders depend on the percentage and completion of the geopolymer reaction between the clay minerals and the hydraulic stabilizer.

Effects of resilient strain deformation on geopolymer stabilized subgrade
The variations in the resilient strain deformation with the geopolymer-stabilized subgrade for a repeated cycle number of 100,000 at confined pressures of 20, 50, and 100 kPa are shown in figures 3(a) and (b).
The test results revealed that the resilient strain increased as the number of loading cycles increased for the unstabilized subgrade soils, causing permanent strain deformation.The unstabilized subgrade soil recorded an average resilient strain of 0.3% whereas, the geopolymer stabilized subgrade soil recorded a resilient strain of 0.08%.The test results implied that the unstabilized subgrade soils exhibited a higher rate of strain deformation than the geopolymer-stabilized subgrade soil specimens.At the initial stage of the loading cycles between 0 and 1,000 cycles, the unstabilized subgrade specimens marginally sustained the loading intensity as the resilient strains remained unchanged.The unstabilized subgrade specimens failed to sustain the intensity of the loading cycles as the loading increased between 2,000 and 100,00 cycles, hence causing an exponential increase in resilient strain from 2.2% to 4.8%.For the geopolymer-stabilized specimens, the resistance against the resilient strain was more consistent at a low loading cycle intensity, such that no impact of the cyclic stress was observed.This implies that the geopolymer-stabilized specimens sustained a higher number of load cycles with a slight increase in resilient strain.Moreover, a resilient strain impact was observed on the specimens with 5% nano-geopolymer content compared with the subgrades stabilized with 10%, 15%, and 20% geopolymer contents.In addition, the 15%-and 20%-nano-geopolymer-treated specimens exhibited strong strain accumulation from the initial stage to the medium loading cycle intensity, beyond which a permanent strain was observed.However, the strain accumulation was mobilized owing to the effects of the stiffness, confined pressure, and geopolymerization reaction between the clay minerals and the nano-geopolymer.The behavior of the stabilized subgrade soil agreed with the results reported by Sandjak and Tiliouine [22] and Tang et al. [23].This behavior was also reported by Bian et al. [24].The confining pressure slightly increased the stiffness of the unstabilized subgrade soil.This results in a minor improvement in the resilient strain resistance of the untreated specimens.In addition, the confining pressures significantly enhanced the adhesion of the particle-geopolymer interface and improved the resilient strain resistance of the stabilized subgrade soil owing to the mechanical interlocking of the soil particles.Generally, nano-geopolymer inclusions enhance the resilient strain resistance of the subgrade from low to medium strain energy and improve the energy-absorbing properties of the composite, which increases its resistance against strain deformation at medium strain energy.

Figure 3a.
Variation in strain deformation versus number of loading cycles stabilized @ 20 and 50 kPa.

Figure 3b.
Strain deformation versus number of load cycles for stabilized subgrade @ 100 kPa.
However, the performance of the treated expansive subgrade evaluated in this study is a general indicator for assessing the durability against strain deformation.In general, nano-geopolymer binders enhance the strength resistance against permanent strain deformation and improve the energy-absorbing properties of the composite, allowing the composite to accumulate a higher number of load cycles.The resistance to strain deformation was enhanced by a gradual increase in the nano-geopolymer content; thus, this ultimately led to the accumulation and sustenance of loading cycles, causing resistance to the deformability of the resilient strain after strength development must have occurred through a polymerization reaction.

Conclusions
In this study, the durability performance and strength resistance against permanent strain deformation of an NGB-stabilized expansive subgrade were investigated.Based on the obtained results, the following conclusions were drawn: 1) The measured dynamic resilient modulus of the nano-geopolymer-stabilized subgrade soil increased under several deviatoric stresses; thus, a low resilient modulus was recorded when zero percent content NGB was applied to the subgrade.Furthermore, the deviatoric stress was noted to have little effect on the nano-geopolymer-treated subgrade, whereas the deviatoric stress induced negative effects on the unstabilized subgrade, indicating that the deviatoric stress is more pronounced in tension as the NGB-stabilized specimens provide the required resistance against tension.2) Under wetting-drying cycle conditions, the dynamic resilient modulus of the unstabilized subgrade decreased as the number of load cycles increased.In the early stages of the W-D cycles, the unstabilized subgrade became more vulnerable to desiccation and moisture absorption by creating more voids within the soil matrix.The W-D cycles of the nano-geopolymer-stabilized specimens were not affected because the nanoparticle size of the NGB filled the voids within the soil matrix.This mobilized the complete geo-polymerization reaction and the development of a thicker film layer at the surface of the specimens, thereby preventing the specimens from absorbing any moisture during the W-D processes.
3) The swelling stress of the geopolymer-treated subgrade decreased exponentially as the NGB content increased.Hence, the decrease in the swelling pressure is attributed to the interaction between the clay minerals and the geopolymer binder.However, the most evident scale effect, which cannot be ruled out, is the decrease in the liquid limit and plasticity index values, which mobilizes the noted decrease in swelling stress in the treated subgrade.4) The resilient strain deformation of the unstabilized subgrade depended on the number of load cycles.Hence, it decreased as the number of load cycles increased owing to the nonlinearity of the subgrade stress-strain framework.In contrast, the resilient strain deformation of the stabilized subgrade soils sustained the intensity of the loading cycles from low to medium strain energy.The sustained load cycle intensity decreased at high strain energies owing to the inability of the stabilized specimens to accommodate the damping caused by the loading cycles.This implies that the nano-geopolymer-stabilized subgrade developed an appreciable stiffness to sustain the intensity of the loading cycles.

5 Figure 2 .
Figure 2. Variation in cyclic W-D durability with resilient modulus.

Table 1 .
Chemical composition research materials.