In this study, effects of metakaolin and lime on the microstructural characteristics, unconfined compressive strength (UCS), shrinkage, suction, and shear resistance of laterite were investigated. Soil samples treated with 5 wt% of lime (LaL) or 4 wt% metakaolin and 5 wt% of lime (LaLM) were prepared. Samples with an optimal water content of 32% were compacted and cured for 180 days, followed by saturation and dehydration until the desirable water content of the samples was attained. Then, the UCS, shrinkage, and suction and shear resistance of the samples at a normal stress of 200 kPa were determined. In addition, scanning electron microscopy imaging as well as mercury intrusion porosimetry tests were performed to examine the microstructural changes. Results indicate that the shrinkage of treated soil samples is significantly improved in comparison with that of the untreated soil samples. Lime effectively improves the UCS and shearing resistance of laterite. Moreover, metakaolin is composed of amorphous silicon and aluminium oxides and shared edge-face structures on the microscopic scale; hence, it can considerably capture calcium ions from a lime solution, generating cementitious hydrates in the interaggregates of laterite. Results also revealed that the combination of 5 wt% of lime and 4 wt% of metakaolin can improve the UCS and shearing resistance, but the linear shrinkage is particularly restrained, significantly decreasing by 4 times compared with that of the lime-treated soil sample and by 8 times compared with that of the untreated soil sample. The study results demonstrate that metakaolin and lime can be effectively used to improve laterite in lieu of the conventional lime treatment for mitigating geotechnical engineering disasters.
Laterite is the weathered product on parent rocks in hot and wet tropical climates. It contains not only silicon and aluminium oxides but also cementitious materials such as free iron oxide; hence, it exhibits a red colour [
The water sensitivity of laterite is characterised by miring and water loss shrinkage [
Metakaolin exhibits high pozzolanic activity. When kaolin is heated to 500°C–700°C, an amorphous material is produced. This material is 50–55% of SiO2 and 40–45% of Al2O3 by weight, and it is extremely reactive in an alkaline calcium-rich solution. In [
Laterite was obtained from Guilin, Guangxi Autonomous Region (China). By X-ray fluorescence, its chemical composition was determined to predominantly comprise silicon aluminium oxide (74 wt%) and iron oxide (19 wt%) (Table
Major components of laterite.
Composition | Al2O3 | SiO2 | Fe2O3 | TiO2 | K2O | Others |
---|---|---|---|---|---|---|
Mass content (%) | 40.6 | 33.3 | 19.1 | 2.4 | 2.1 | 2.5 |
Basic properties of laterite.
Proportion | Liquid limit (%) | Plastic limit (%) | Free swell (%) | Optimum moisture content (%) | Maximum dry density |
---|---|---|---|---|---|
2.71 | 56.5 | 35.4 | 16.9 | 30.2 | 1.48 g/cm3 |
Compaction curves of laterite.
Lime was purchased from Suqian, Jiangsu Province (China), and its chemical composition was predominantly CaO (98.34 wt%), with a small amount of Al2O3 (0.57 wt%). Its specific gravity was 2.32, and its initial water content was 3.1%. Metakaolin was purchased from Royal Dutch Shell plc, and its chemical composition was predominantly Al2O3 (46.2 wt%) and SiO2 (49.5 wt%). Its specific gravity was 2.63, and its initial water content was 1.5%. The particle distribution characteristics of laterite, lime, and metakaolin were investigated using a Microtrac S3500 particle size analyser (Figure
Particle size distribution curves of testing materials.
To investigate the effects of metakaolin and lime on the shrinkage of laterite, six kinds of specimens were prepared (Table
Testing plan.
No. | Test name | Sample category | Initial state of sample |
---|---|---|---|
1 | Shrinkage test | La | |
2 | Unconfined compression strength and suction test | Same as above | |
3 | Direct shear test | Same as above | |
4 | Pore analysis test | LaL |
Notice:
The relationship between the dry density (
For comparison of test results, all compacted samples have the same initial void ratio. To be more specific, the initial dry density of the laterite specimens was 1.43 g/cm3, while those of the laterite-lime specimens and laterite-lime-metakaolin specimens were 1.422 g/cm3 and 1.420 g/cm3, respectively.
In this study, the soil samples were dry mixed, and then water was added to the mixture soils until the predicted water content was attained to achieve sample moisture conditions. Finally, the mixtures of metakaolin and/or lime-laterite with deionized water were compacted layer by layer in a sample compaction container until soil samples were obtained with dimensions of 61.8 mm (diameter) × 20 mm (height) and 50 mm (diameter) × 50 mm (height) with an initial water content of 32%. Pure laterite samples were prepared in the same manner as the control samples. All test samples exhibited the same initial void ratio to ensure that different samples exhibited the same initial pore volume.
In this experiment, after compaction, all specimens were sealed and cured for 180 days at 25°C, which was followed by a saturation process. Then, each test was conducted as follows: Shrinkage tests were conducted according to the Chinese standard JTG E40-2007 [ For the UCS tests and suction measurements, some specimens were placed in a chamber at a constant temperature (55°C), and then their mass was estimated to determine the real-time water content during dehydration. When the real-time water content approached the predetermined value, the samples were sealed and cured for 2 months in the chamber at 25°C to ensure homogeneity. Then, 10–20 g of each sample was used for suction measurement using a Decagon WP4C dewpoint potentiometer. For direct shear tests, some samples were dehydrated by the vapor equilibrium method (Table For pore analysis tests, after the specimens were freeze-dried for 1 day, a small block was used to obtain the pore size distribution with a Pore Master (PM 60GT) system. For scanning electron microscopy (SEM) imaging, to directly determine the reaction mechanism in the treated soil, mixtures of compacted lime-kaolin and lime-metakaolin were used. Each sample was carefully cut into a size of 15 mm × 15 mm × 15 mm to avoid disturbance and ensure porosity. Then, the cut sample was placed into liquid nitrogen, and the sample was dried by using a vacuum freeze-drying device. After drying, the sample was used for SEM analysis.
Saturated salt solution and corresponding suction values (
Solution | K2SO4 | KCL | NaCl | KI | NaBr | K2CO3 | MgCl2 | CH3COOK | LiCl |
---|---|---|---|---|---|---|---|---|---|
Suction (MPa) | 3.76 | 23.59 | 38.92 | 51.11 | 75.68 | 115.15 | 152.94 | 204.65 | 299.14 |
To further investigate the mechanism of metakaolin and lime modification of laterite, X-ray diffraction (XRD) patterns, thermogravimetric analysis (TGA) curves, and infrared (IR) spectra were recorded to explain the manner in which metakaolin aids lime in restraining the shrinkage of laterite.
After the formation of metakaolin from kaolin by calcination at 500°C–700°C, XRD patterns were recorded, and the diffraction spectrum of metakaolin lacked clear diffraction peaks similar to those of kaolin (Figure
XRD pattern of kaolin and metakaolin.
To understand the characteristics of metakaolin, it is crucial to determine its composition and structure. Kaolin comprises feldspar and mica particles with alumina-rich elements or acidic igneous rocks. First, it undergoes weathering and disintegration, and it is then deposited from Al(OH)3 and Si(OH)4 in solution. Its chemical formula is Si2Al2O5(OH)4, and its molecular structure is shown in Figure
Basic structure unit of kaolin [
Kaolin is transformed to metakaolin at high temperatures by dehumidification, which is expressed as follows:
Thus, four hydroxyl groups are transformed into two water molecules and two oxygen anions remain in the material, as shown in the following chemical reaction:
In summary, the hydrogen bond connected to the basic structural unit disappears during dehumidification, and the laminated surface structure of kaolin is destroyed. Figure
Thermogravimetric analysis curves of kaolin and metakaolin.
The results indicated that the weight of kaolin decreases by 11.2% at 400°C–750°C. The weight loss rate of metakaolin is less than 1.0%, indicating that dehydroxylation was completed during previous calcination. In addition, the weight loss of kaolin at 400°C–516°C is significantly greater than that at 516°C–750°C because the hydroxyl groups between the silicon tetrahedron and the aluminoxy octahedron cannot escape from the kaolin as rapidly as those present on the surface. Therefore, the hydroxyl groups require a slow migration process.
After calcination, the distribution of the hydroxyl groups in kaolin also changed. Figure
Infrared spectra of (a) metakaolin and (b) kaolin.
At a frequency range of 3200–4000 cm−1, kaolin exhibited bands at 3680 and 3620 cm−1, corresponding to the elongation vibrations of the hydroxyl group, and the band at 991 cm−1 corresponds to the Si-O-Si elongation vibrations. The band at 908 cm−1 corresponds to the deformation vibrations of the Al-O-H hydroxyl groups on the alumina faces [17] (Figure
SEM images of (a) lime-kaolinite and (b) lime-metakaolin.
Clearly, metakaolin calcined from kaolin comprises silica and alumina, and kaolinite is the main mineral in laterite. The potential method for stabilizing laterite with metakaolin and lime was found to be efficient and environmentally friendly.
Figure
Shrinkage curves of treated and untreated laterite.
Figure
Suction as a function of water content.
Two series of samples were investigated: saturated and unsaturated. With the decrease in the water content, suction gradually increase, especially at a water content of less than 7.5%. With the increase in suction, the strength of the unsaturated soils increases. However, at a water content of less than 25%, the UCS of compacted laterite remained 2.0 MPa. Notably, at an extremely low water content, the UCS decrease. The saturated compacted laterite exhibits a similar variation, but its UCS is typically less than that of compacted laterite without dehydration at a water content of less than 25% (Figure
UCS as a function of water content. (a) Laterite. (b) Treated laterite.
Although the strengths of LaL and LaLM decrease during dehydration, the values still exceed 2.0 MPa under different moisture conditions in contrast to that of laterite. In contrast, the strengths of LaL and LaLM increase rather than consistently decrease, which is similar to that observed for completely dried laterite samples (Figure
Peaks of direct shear strength vs. moisture content.
The direct shear strengths of LaL and LaLM are always greater than that of compacted laterite, and the strength of compacted laterite initially increases and reaches a peak of 0.5 MPa at a water content of 25%, which is near the shrinkage limit (Figure
The ability of metakaolin to enhance the inhibitory effect of lime on the shrinkage of laterite can be attributed to its composition and structure. Figure
Metakaolin exhibits a substantial ability to catch calcium ions, changing the calcium-ion concentration in the treated soil (Figure
Calcium concentration as a function of curing time.
The molecular structure of metakaolin plays an important role in its ability to efficiently adsorb calcium ions from solutions. Konan et al. [
The comparison of the pore distributions of the two samples (LaL and LaLM, respectively) clearly reveals that if only lime is added to laterite, pores between 10
Pore size distributions of treated laterite. (a) LaL. (b) LaLM.
In this study, a series of tests were conducted on three samples (La, LaL, and LaLM, respectively) to determine the shrinkage, UCS, suction, direct shear strength, and pore size distribution. In addition, the shrinkage mechanism of laterite modified by lime and metakaolin was examined. The following conclusions can be drawn: After calcination, not only the hydroxyl groups disappeared but also the spatial structures of Si-O-Si and Si-O-Al (Mg) in kaolin are changed. Although the suction of compacted laterite increases during dehydration, microcracks are observed during this period, thereby impairing the strength of laterite. In addition, metakaolin is found to enhance the decrease shrinkage of laterite and increase the strength overall. The microscopic examination of metakaolin reveals a substantial amount of amorphous silicon and aluminium oxides and shares edge-surface contact structures, thereby enabling it to significantly capture calcium ions in the lime solution and subsequently generating cementitious hydrates in the interaggregates of laterite.
The authors declare that all the data presented in the manuscript were obtained from laboratory tests at China Three Gorges University in Yichang Hubei China. All the laboratory testing data were presented in the figures and tables in the manuscript. The data used in the study are available from the corresponding author upon request.
The authors declare no conflicts of interest.
This study was financially supported by the National Natural Science Foundation of China (51579137 and 51979150) and Youth Innovation Team Project of Hubei Province (T201803).