The proper design of protective structures may start from improving the characteristics of soils. In order to obtain reasonable safety criteria, several research studies have recently been dedicated to enhancing complex civil engineering structural systems with the use of nanotechnology. Thus, the following paper investigates the effect of nanospheres, including nanosilica (nano-SiO2) and nano zinc oxide (nano-ZnO), on lime-stabilized high-plasticity clay soil. For this purpose, unconfined compressive strength (UCS) and California bearing ratio (CBR) tests were performed on samples. The results showed that the use of the selected nanospheres greatly increased the UCS of the samples compared to untreated soil. The UCS value of samples containing 6% lime and 1.5% nano-ZnO after 28 days of treatment increased by 5-fold compared to the UCS of untreated samples. In addition, the samples containing 6% lime and 2% nano-SiO2, with similar curing conditions, experienced a 5.3-fold increase in their UCS value compared to the untreated samples. These compounds were considered as the optimal amounts and showed the highest mechanical strength in both UCS and CBR tests. The same trend was achieved in the CBR test, in which the CBR value for the optimal mixtures containing nano-ZnO and nano-SiO2 was 14.8 and 16.6 times higher than that of high-plasticity clay soil, respectively. Finally, the results obtained from scanning electron microscopy (SEM) analysis revealed that the nanospheres caused a dense and compact matrix to form in the soil, which led to the enhancement of the mechanical strength of the treated samples.
When designing protective structures, high-plasticity clay soils, which are widely scattered throughout the world, are very problematic. This is mainly due to the fact that they are highly sensitive after being exposed to moisture. The presence of these soils in construction projects should be specifically addressed and treated if necessary due to their undesirable behaviour such as swelling, shrinkage, dispersion, low mechanical strength, and high level of settlement [
In the past few decades, many studies have been conducted on the chemical stabilization of soils using these traditional materials [
Examples of recent soil stabilization research.
Reference | Soil type | Stabilizer type | Curing time (days) | Tests |
---|---|---|---|---|
Yi et al. [ |
Soft high-plasticity clay | Lime, GGBS | 7, 28, 90 | UCS, MIP |
Ghorbani et al. [ |
Sulfate silty sand | Lime, micro-SiO2 | 7, 28 | UCS, CBR |
Choobbasti et al. [ |
Sandy soil | Cement, nano-SiO2 | 7 | UCS |
Bahmani et al. [ |
High-plasticity clay soil | Cement, nano-SiO2 | 7, 14, 28 | UCS |
Ghasabkolaei et al. [ |
High-plasticity clay soil | Cement, nano-SiO2 | 7, 14, 28 | UCS, CBR |
Yoobanpot et al. [ |
Soft high-plasticity clay | Cement, fly ash residue | 3, 7, 28, 90 | UCS |
Alnahhal et al. [ |
Sand | OPC, CKD, nano-CKD | 7, 28, 56 | UCS |
García et al. [ |
Soft high-plasticity clay | Nano-SiO2 | — | UCS |
Sharma et al. [ |
High-plasticity clayey sand | Lime, cement | 1, 3, 7, 14, 21, 28 | UCS, shear strength |
Abbasi et al. [ |
Dispersive high-plasticity clayey soils | Nano-high-plasticity clay | 1, 3, 7 | Pinhole |
Choobbasti et al. [ |
Sand | Cement, nano-SiO2 | 7 | UCS, triaxial |
In order to obtain reasonable safety criteria, several research studies have recently been dedicated to enhancing complex civil engineering structural systems using nanotechnology. Nanodimension stabilizers are highly effective in soil stabilization from both physical and chemical viewpoints. Nanospheres have a particularly high specific surface area and are therefore more involved in chemical reactions [
Nano-SiO2 and nano-ZnO are two types of additives that have very good properties in combination with soil. In recent years, these nanospheres have attracted great research interest because of their high pozzolanic activity in cement-based systems. According to Mostafa et al. [
The high-plasticity clay soil in the present study does not have a sufficient mechanical strength and causes severe damage to a construction built on it due to the weak structure of soil particles. Therefore, finding a reliable and practical technique was the main goal of this research. In this study, the effect of nano-SiO2 and nano-ZnO on the mechanical strength parameters of high-plasticity clay soil stabilized with lime was investigated and their microstructural changes were carefully considered. For this purpose, unconfined compressive strength (UCS) and California bearing ratio (CBR) tests were performed on samples. In addition, scanning electron microscopy (SEM) analysis was applied to observe the microstructural properties.
The high-plasticity clay soil that was used in this study was collected from a depth of 1 m at an excavation site within the University of Guilan (5th kilometer of the Rasht-Tehran road). Based on the UCS value obtained from the studied clay (174.55 kPa), it was stated that the soil of this region exhibits very low strength and consequently would not withstand the loads imposed upon it. Moreover, the poor particle-size distribution shown in Figure
Particle-size distribution of the studied high-plasticity clay soil (obtained according to ASTM D2487-11 [
Physical properties and Atterberg limits of the studied high-plasticity clay soil.
Parameter | Value |
---|---|
|
2.7 |
Liquid limit (LL) (%) | 62.5 |
Plastic limit (PL) (%) | 30.11 |
Plasticity index (PI) (%) | 32.39 |
Maximum dry density (MDD) (kg/m3) | 1470 |
Optimum moisture content (OMC) (%) | 23 |
Unconfined compressive strength (UCS) (kPa) | 174.55 |
Unsoaked California bearing ratio (CBR) (%) | 4.9 |
Chemical compositions of the studied high-plasticity clay soil (obtained from X-ray fluorescence analysis).
Formula | Content (%) |
---|---|
SiO2 | 53.9 |
Al2O3 | 16.4 |
CaO | 3.14 |
Fe2O3 | 8.8 |
MgO | 2.0 |
K2O | 5.2 |
Na2O | 0.57 |
P2O5 | 0.1 |
TiO2 | 0.79 |
Other particles | 0.2 |
L.O.I | 8.9 |
The hydrated lime used in this study was obtained from Qom Limestone Factory and contained about 51% quick lime (CaO) with particles finer than sieve No. 60 (0.250 mm). Table
Chemical compositions of hydrated lime (provided by the manufacturer).
Formula | Content (%) |
---|---|
K2O | 4 |
SO3 | 0.8 |
MgO | 2.65 |
CaO | 51.64 |
Fe2O3 | 0.13 |
Al2O3 | 0.24 |
SiO2 | 1.36 |
L.O.I | 39.18 |
In this study, lime was replaced by 1, 1.5, and 2% of nanospheres including nano-SiO2 and nano-ZnO with average sizes of 20–30 and 30–50 nm and surface areas of 220 m2/g and 50 m2/g, respectively. In this research, regardless of the specifications given by the nanomaterial manufacturer, the specific surface area of the nanospheres was measured by nitrogen adsorption at 77 K by using the Brunauer–Emmett–Teller (BET) method. The nanospheres were obtained from the Iranian Pishgaman Nanomaterial Company. In this study, particle-size distribution was calculated for each nanosphere sample from the SEM images by using ImageJ software. Then, the number of pixels occupied by the number of particles was counted. It should be noted that ImageJ software has been used for postprocessing and particle analysis by many researchers. Figure
SEM microstructure and particle size of nanospheres: (a) nano-SiO2 and (b) nano-ZnO.
Selected properties of studied nanospheres.
Name of the property | Nano-SiO2 | Nano-ZnO |
---|---|---|
Color | White | White |
Average particle sizes (nm) | 20–30 | 30–50 |
pH | 7 | 6 |
Specific surface area (SSA) (m2/kg) | 220000 | 50000 |
Purity (%) | 98.31 | 99.14 |
The results of previous studies have shown that the characteristics of soil improve to a certain extent, with the use of additives, and that higher amounts of them can have adverse effects on soil strength. In order to obtain the most favourable mixture of lime-nanospheres, the ratio of cementitious materials should be strongly considered due to the fact that the replacement of larger amounts of lime-nanospheres can lead to a poor mixture with lower strength. Therefore, the optimum amount of nanospheres as a substitute for lime can only be determined by trial and error. By and large, based on the results of previous studies, the optimum amount of lime in clay soil stabilization has been reported to be between 4 and 8 [
Mixture proportions of used materials.
Test no. | Lime (%) | Nano-SiO2 (%) | Nano-ZnO (%) |
---|---|---|---|
1 | 3 | 0 | 0 |
2 | 6 | 0 | 0 |
3 | 9 | 0 | 0 |
4 | 6 | 1 | 0 |
5 | 6 | 1.5 | 0 |
6 | 6 | 2 | 0 |
7 | 6 | 0 | 1 |
8 | 6 | 0 | 1.5 |
9 | 6 | 0 | 2 |
Each test was performed three times.
To carry out the unconfined compressive strength test according to ASTM D2166-91 [
Unconfined compressive strength test: (a) mould, (b) prepared samples, and (c) sample under loading.
To carry out the unsoaked California bearing ratio (CBR) test according to ASTM D1883-16 [
California bearing ratio test: (a) mould, (b) samples after 7 days of curing, and (c) California bearing ratio test apparatus.
Figure
Effect of lime on the UCS of soil after a curing time of up to 28 days.
Figures
Effect of nano-ZnO on the UCS of lime-stabilized soil after a curing time of up to 28 days.
Effect of nano-SiO2 on the UCS of lime-stabilized soil after a curing time of up to 28 days.
In general, the increase of UCS by adding different compounds can be attributed to short- and long-term reactions. Pozzolanic reactions lead to the formation of calcium silicate hydrate (CSH) and calcium aluminate hydrate (CAH) gels that fill the voids and thus increase the UCS of the samples [
In this study, the CBR test was performed after 7 days of treatment in order to more accurately evaluate the mechanical strength behaviour in all the samples. Figure
Effect of lime on the CBR of soil after 7 days of treatment.
For samples containing nano-ZnO, the CBR value increased with an increasing amount of nanospheres, as shown in Figure
Effect of nano-ZnO on the CBR of lime-stabilized soil after 7 days of treatment.
Effect of nano-SiO2 on the CBR of lime-stabilized soil after 7 days of treatment.
In this study, the SEM technique was applied to monitor structural changes and to better understand the interactions between the soil and additives. For this purpose, in addition to the untreated soil sample, two samples treated with additives that provided the highest mechanical strength (6% lime + 1.5% nano-ZnO and 6% lime + 2% nano-SiO2) were analyzed. The SEM images of these three samples are shown in Figure
SEM micrographs: (a) the high-plasticity clay soil, (b) 6% lime + 1.5 nano-ZnO, and (c) 6% lime + 2% nano-SiO2.
The purpose of this study was to evaluate the effect of nanoscale materials, including nano-ZnO and nano-SiO2, on lime-stabilized high-plasticity clay soils. For this purpose, a series of UCS and CBR tests were performed on samples, and later, based on SEM images, soil microstructure changes before and after treatment were investigated. Nanospheres have a much higher surface area because of their very small size, and they therefore participate in the reactions and accelerate the formation of cementitious products. Moreover, these materials are placed between lime and soil particles and fill voids, which increases the mechanical strength of the samples. Based on the tests performed on various compounds, the following results were obtained: The addition of nano-ZnO and lime to soil increased UCS in such a way that the optimum nano-ZnO value in a mixture with 6% lime was reported to be 1.5%, which led to a 5-fold increment after 28 days of curing compared to that of untreated soil. The addition of nano-SiO2 to lime-stabilized soil resulted in an increased mechanical strength, meaning that a UCS value of about 1000 kPa was measured in the sample containing 6% lime + 2% nano-SiO2 after 28 days of curing, which is equivalent to about 5.3 times that of the untreated soil. In all the samples, an increased curing time resulted in an increased UCS, which can be attributed to the completion of long-term pozzolanic reactions and the formation of calcium silicate hydrate and calcium hydrate aluminate gels. Based on the results obtained from the CBR test, it was found that the performance of nano-SiO2 was slightly better than that of nano-ZnO. The CBR value was reported to be about 81.1% for the sample containing nano-SiO2, while the highest CBR value for the nano-ZnO samples was about 72.6%. This increased performance was significant compared to untreated soil (4.9%).
No data were used to support this study.
The authors declare that they have no conflicts of interest.