This study systematically investigated the pore structure response of kaolin and illite/smectite mixed-layer rich clay in a reconstituted state to one-dimensional (1D) compression by first performing oedometer tests on saturated clay slurries, followed by characterising their pore structure using multi-scale characterisation techniques, with the primary objective of advancing the current understanding of the microstructural mechanisms underlying the macroscopic deformation of such clays. Under 1D loading, the volume reduction observed at the macro level essentially represented the macroscopic manifestation of changes in inter-aggregate porosity at the pore scale. It was the inter-particle pores that were compressed, despite the interlayer pores remaining stable. Two distinct pore collapse mechanisms were identified: kaolin exhibited a progressive collapse of particular larger pore population in an ordered manner, whereas illite/smectite mixed-layer rich clay demonstrated overall compression of inter-aggregate pores. Accordingly, mathematical relationships between the porosity and compressibility parameters for these two soils were proposed, with the two exhibiting opposite trends arising from their distinct microstructural features. Approaching from the unique perspective of pore structure, quantitative analysis of pore orientation and morphology on the vertical and horizontal planes demonstrated some progressively increasing anisotropy during compression. These findings provide important insights into porescale mechanisms governing clay compression behaviour and enrich the limited microporosity database in soil mechanics.

Biogrouting has been proposed for improving mechanical properties of soils and rocks, whose performance greatly depends on the location of biocement at pore-scale. To enhance the performance of biogrouting, many strategies were proposed, including the addition of assistants, controlling curing moisture degree, and flocculation of bacteria. Clay is one such assistant which has been proven to be effective, with an assumption of increasing active biocement, i.e. those located between soil particles. In this work, we employed microfluidics to directly observe whether clay minerals can certainly control the location of precipitates and how they function. First of all, the capacity of bentonite and kaolin for adsorbing bacteria were investigated. Then, the location of CaCO3 crystals with and without clay minerals were visually observed using microfluidics. Pore-filling ratios and CaCO3 ratios, which are closely related to permeability and strength of biocemented soils, were quantitatively analyzed from collected images. Finally, the effects of bentonite and kaolin and their dosages on the location of biocement were comprehensively discussed. The results demonstrated that the performance of bentonite and kaolin on adsorbing bacteria and regulating biocement location is distinct due to differences in the morphologies of clays. These findings can help us to improve biogrouting performance on soil stabilization and propose new strategies in various practical applications, such as CO2 sequestration, heavy metal remediation, and oil recovery enhancement, all with a foundational understanding of their mechanisms.

The main problem in expansive soil treatment with steel slag (SS) is the relatively slow hydration reaction that occurs during the initial period. To circumvent this, SS-treated expansive soil activated by metakaolin (MK) under an alkaline environment was investigated in this study. Based on a series of tests on the engineering properties of the treated soil, it can be reported that SS could enhance the strength and compressibility of expansive soil, with strength increasing by approximately 108 % for SS contents exceeding 10 % compared to 3 % lime-treated soil, and the compression index reducing by 20 %. Further addition of MK plays a dual role, enhancing strength for higher SS content while excessive MK leads to strength reduction due to insufficient pozzolanic reactions and hydration product transformation. Expansive and shrinkage behaviors are notably improved, with a 5 % increase in SS content reducing the free swelling ratio by 0.66 %-5.9 %, and the combination of 15 % SS and 6 % MK achieving a nearly 300 % reduction in the linear shrinkage ratio. Microstructural analysis confirms the formation of hydration gels, densification of the soil structure, and reduced macropores, validating the enhanced mechanical and shrinkage resistance properties of the SS-MK-treated expansive soil. Additionally, to develop predictive models for mechanical and the content of hardening agents (SS and MK), the experimental data are processed utilizing a backpropagation neural network (BPNN). The results of BPNN modeling predict the mechanical properties perfectly, and the correlation coefficient (R) approaches up to 0.98.

Due to the detrimental ecological impacts and the exorbitant expenses associated with the cement industry, researchers have sought to find natural, sustainable, low-carbon alternatives to Portland cement for weak soil stabilization. This research used geopolymer based on metakaolin (MK), a natural pozzolanic material with different activator concentrations (NaOH and Na2SiO3), to stabilize loose poorly graded sand soils. The research investigated the effect of different amounts of addition MK (5, 10, and 15 %) on the soil's mechanical properties. Furthermore, the effect of parameters such as the type and concentration of the alkaline solution and curing time (1, 3, and 7 days) on the unconfined compressive strength, failure strain, Young's modulus, California bearing ratio, and direct shear test were evaluated. This research also aims to measure the sub- grade reaction modulus (Ks) by developing and manufacturing a laboratory testing apparatus and steel mold to simulate the natural conditions of sandy subgrade soil obtained from performing nonrepetitive static plate load tests. Additionally, scanning electron microscopy images (SEM) and X-ray diffraction analysis (XRD) were also used to study the microstructural changes and the chemical composition of the stabilized soil samples. The results indicate that the soil samples that were stabilized with MK 10 % and NaOH had notably higher compressive strength (2936 kPa), indicating a denser and less porous structure (improved stiffness stabilized soil) in comparison to the soil samples stabilized with MK 10 % and Na2SiO3 which was (447 kPa). Ultimately, Microstructural analysis showed that, due to the addition of 10 % MK, stabilized soils have a denser and more homogeneous structure.

Soft clay soils inherently exhibit low mechanical strength, imposing significant challenges for various engineering applications. The present research explores various techniques and stabilizers to enhance soft clay's suitability for construction purposes. This study evaluates the mechanism of stabilizing kaolin using recycled macro-synthetic fibers (RMSF) for the first time. Samples were prepared with 5 % LKD, with 25 % replaced by VA, and varying RMSF amounts of 0, 0.5 %, 1 %, and 1.5 % in lengths ranging from 4 to 6 mm. The specimens were cured for 7, 28, and 56 days and exposed to 0, 1, 4, and 10 freeze-thaw (F-T) cycles. Laboratory investigations were conducted through standard compaction, Unconfined Compressive Strength (UCS), Indirect Tensile Strength (ITS), Scanning Electron Microscope (SEM), California Bearing Ratio (CBR), X-ray diffraction (XRD), and Fourier Transform Infrared Spectroscopy (FTIR) tests on the samples at various stages of stabilizer addition, both before and after F-T cycles. The optimal mixture was 5 % LKD, with 25 % VA replacement and 1 % RMSF, which led to a considerable 11-fold enhancement in ITS and a 14-fold improvement in UCS compared to the untreated sample. Additionally, the secant modulus (E50) and energy absorption capacity (Eu) of the sample with the optimal combination content increased in comparison to the stabilized sample without RMSF. The CBR of the optimal sample reached 81 %, allowing for an 87 % reduction in pavement thickness compared to the untreated sample. According to the findings of this research, the combination of LKD, VA, and RMSF increased the compressive and tensile strength properties, bearing capacity, and durability of kaolin, making it an appropriate option for use in various practical civil projects like road construction.

In cold regions' engineering applications, cement stabilized soils are susceptible to strength degradation under freeze-thaw (F-T) cycles, posing significant challenges to infrastructure durability. While metakaolin (MK) modification has shown potential in enhancing static mechanical properties, its dynamic response under simultaneous F-T cycling and impact loading remains poorly understood. This study investigates the dynamic mechanical behavior of cement-MK stabilized soil through split Hopkinson pressure bar (SHPB) tests under varying F-T cycles. The effects of strain rate and F-T cycles on the dynamic failure process and mechanical properties of cement-MK stabilized soil were investigated. Pore characteristics were analyzed using a nuclear magnetic resonance (NMR) system, providing an experimental basis for revealing the degradation mechanism of F-T cycles on the strength of cement-MK stabilized soil. Based on the Lemaitre's strain equivalence principle, a composite damage variable was derived to comprehensively characterize the coupled effects of F-T cycles and strain rate. A dynamic constitutive model is established based on damage mechanics theory and the Z-W-T model. The results indicate that under the effect of F-T cycles induce progressive porosity increase and aggravated specimen damage. At varying strain rates, the strength of cement-MK stabilized soil decreases with increasing F-T cycles, while the rate of strength reduction gradually diminishes. Under impact loading, both strain rate and the number of F-T cycles significantly reduce the average fragment size of fractured specimens. The modified Z-W-T model effectively predicts the stress-strain relationship of the cement-MK stabilized soil under impact loading.

Self-consolidating earth concrete (SCEC) addresses the long construction process of conventional earthen constructions and their structural limitations, while further efforts are needed to enhance its sustainability. This study explores the development of a kaolinite-based self-consolidating earth paste (SCEP) due to their blended powder system, incorporating raw and treated (calcined and ground-calcined) kaolinite under various activation techniques, such as water hydration, sodium hexametaphosphate (NaHMP), and sodium hydroxide (NaOH) activation. The synergistic effect of calcination and mechanosynthesis on rheological, mechanical, structural, and microstructural properties of SCEP were investigated. Mechanically treated kaolinite increased yield stress, plastic viscosity, storage modulus evolution, and build-up index, while delayed the strength development compared to the calcined kaolinite samples. Among the investigated activators, NaOH resulted in more promising structural build-up, storage modulus, and compressive strength development. These findings were elaborated with X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), calorimetry, thermogravimetric analysis (TGA), and scanning electron microscopy (SEM).

Stress-strain results from high-strain rate consolidated-undrained (CU) triaxial compression tests on partially saturated kaolin clay are presented. The work addresses the scarcity of high-strain rate data for cohesive soils and provides updated strain rate coefficients for kaolin clay. It covers strain rates from quasi-static (0.01%/s) to dynamic (800%/s) regimes. Kaolin clay specimens were prepared wet of optimum using static compaction at a constant water content of 32 +/- 1% and a degree of saturation of 96 +/- 2%. The specimens were then loaded into triaxial cells and consolidated under pressures ranging from 70 to 550 kPa for 24 h prior to testing. Tests were conducted using a modified hydraulic frame, and a methodology for correcting compression data to account for inertial effects observed during high-rate testing was adopted. The data revealed significant strengthening of clays with increased strain rates, especially at low confining pressures. Lightly confined clays (sigma 3 = 70 kPa) experienced a 165% strength increase, while highly confined clays (sigma 3 = 550 kPa) showed a 52% increase. Analysis using secant moduli revealed increased stiffening with loading rate. Posttest examination of specimens revealed a decrease of shear localization with increasing strain rate, indicating that a transition in failure mode contributes to the increased strengthening and stiffening of clays at high rates. The stress-strain data were used to calibrate the semilogarithmic and power law strain hardening models, yielding lambda and beta values that decreased linearly with increasing confining pressure. Equations relating lambda and beta to confining pressure were developed for practical applications, applicable to normally consolidated clays under confining pressures up to approximately 5 atmospheres.

This article experimentally evaluates the influence of shearing rate on the monotonic and cyclic response of isotropically-consolidated samples of Malaysian kaolin. On the one hand, a series of undrained monotonic triaxial tests were performed with varying shearing displacement rate. On the other hand, undrained cyclic triaxial tests were conducted considering different deviatoric stress amplitudes and loading frequencies. The well-known soil rate-dependency under monotonic loading was confirmed up to a displacement rate threshold. The experimental results under cyclic loading suggest that for the given loading frequency, the variation of the deviatoric stress amplitude remarkably influences the strains and pore water pressure accumulation rates. In addition, the results suggest that depending on the loading frequency different shapes of mobilized effective stress loops are obtained. Larger loading frequencies lead to banana-shaped effective stress loops, while smaller frequencies reproduce eight-shaped effective stress loops. Furthermore, higher loading frequencies result in a larger number of cycles required to reach failure conditions. The reasons for the observed differences in the behavior are thoroughly analyzed and discussed.

A series of geopolymers were synthesized by employing phosphoric acid (PA) as activator to activate low-calcium fly ash (FA) and metakaolin(MK), and geopolymer mortar was prepared using PA-activated FA-MK geopolymer and dredged soil. The PA-activated low-calcium FA geopolymer typically exhibited low compressive strength. Incorporating MK introduced reactive aluminum, which enhanced the compressive strength of the geopolymer. This strength improvement was further amplified as the M:F ratio (MK:FA ratio) increased. Under a certain M:F ratio, there existed an optimum H3PO4/Al2O3 molar ratio that maximized the compressive strength of geopolymer. A positive correlation was observed between the M:F ratio and the optimum H3PO4/Al2O3 molar ratio, with the latter exhibiting a gradual increase from 0.56 (M:F ratio = 0:1) to 0.64 (M:F ratio = 0.4:0.6) and ultimately 0.86 (M:F ratio= 1:0). The compressive and flexural strengths of the geopolymer mortar were significantly affected by the geopolymer/soil ratio and the PA concentration. When the actual PA concentration in geopolymer mortar approached the optimum PA concentration for the geopolymer paste, the mortar achieved its best mechanical properties. The stabilization of dredged soil using PA-activated geopolymer demonstrates significant sustainability benefits, while their cost-effectiveness and mechanical performance require further optimization. This research provides new approaches and data support for the reuse of low-calcium FA and dredged soil.