This study investigated the impact of optimum dosages of nano-calcium carbonate (nano-CaCO3) and nanosilica on the engineering behavior of black cotton soil. The desired percentage of nano-addition, 2%, for both nanomaterials, was determined by analyzing the plasticity-compaction characteristics and the relative strength index values of treated samples. The study unveiled that the entire clay microstructure was transformed into a nanocrystalline matrix after treatment. The deviatoric strength enhancement with confining pressure and curing period was significant after treating the soil with either nano-CaCO3 or nanosilica. The nanosilica treatment was found to be more effective in improving the California bearing ratio (CBR) strength of black cotton soil samples compared with nano-CaCO3 stabilization. The addition of nanomaterials induced the formation of nanocrystalline hydrate gels and silica gel, resulting in an increased resistance to volumetric deformation under compressive stresses. The hydraulic conductivity of nano-treated samples dropped due to the highly tortuous networks between pores in the nano-crystalline structure. The experimental results were substantiated by analyzing the microstructure of nano-treated soils using X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared (FTIR) techniques.

The waste tire rubber may be incorporated with the cement soil to improve its frost resistance. However, it remains a significant challenge to optimise the rubber content between its mechanical strength and durability under freeze-thaw conditions. In this study, the macroscopic mechanical properties of ordinary cement soil and rubber-cement soil (with particle sizes of 30 and 60 mesh) were explored under different freeze-thaw cycles (0, 3, 6, 9, 15) by taking the wave propagation and unconfined compressive strength (UCS) tests. Subsequently, a series of scanning electron microscope (SEM) and X-ray diffraction (XRD) tests were conducted to analyse the microstructure of the specimens, further clarifying the freeze-thaw damage mechanisms in rubber-cement soil. The results show that freeze-thaw cycles cause irreversible internal damage to the cement soil, leading to continuous reductions in both wave velocity and UCS. After 15 freeze-thaw cycles, the wave velocity loss rates are 95%, 72.2%, and 89.7% for ordinary cement soil, cement soil mixed with 30-mesh and 60-mesh rubber particles, respectively. The corresponding UCS loss rates are 95.4%, 82.7%, and 89.2%, respectively. The above results suggest that 30-mesh rubber-cement soil exhibits superior frost resistance. From a microstructural perspective, the rubber particles delay and inhibit the propagation of frost heaving cracks, forming a denser spatial structure for calcium silicate hydrates (C-S-H) gel, thereby improving the freeze-thaw resistance. By integrating macroscopic mechanical testing and microstructural analysis, this study reveals the mechanical properties and damage mechanism of rubber-cement soil under freeze-thaw conditions, providing valuable insights for its engineering applications.

To study the failure mechanism of paleosol landslides, taking the paleosol of a landslide body in Yan'an as an example, scanning electron microscopy (SEM), nuclear magnetic resonance (NMR), and conventional triaxial tests were used to obtain particle composition, microstructure scanning results, T2 spectral distribution, and stress-strain curves under different freeze-thaw cycles. The results showed that with the increase of freeze-thaw cycles, the number of micropores in paleosol increased to 70.5% and stabilized, while the number of micropores decreased to 18.4%, mesopores decreased to 7.5%, macropores decreased to 3.6%, and eventually stabilized. The fractal dimension of pore shape distribution in paleosol increases along a convex curve to 1.42. The T2 spectrum presents three stepped small peaks, with the peak spectral area of relaxation time ranging from 0.01 ms to 3.16 ms being the largest, indicating that small pores dominate. As the number of freeze-thaw cycles increases, the peak area of smaller relaxation times expands, indicating that freeze-thaw cycles have destroyed the structure of paleosols and generated a large number of tiny pores. Under conditions of higher confining pressure and lower moisture content, when there are fewer freeze-thaw cycles, the strain corresponds to higher stress. The freeze-thaw cycle makes the stress-strain curve of paleosol harder, indicating that the original structure is damaged and the new structure appears as disordered particles.

Geopolymer is a green and environmentally friendly new type of gel material generated from the reaction of activators with silica-alumina raw materials, possessing extensive research and application value. Fly ash can be used as reaction material, enabling the industrial solid waste to be recycled and reused. In this paper, unconfined compressive strength test, X-ray diffraction analysis, Fourier transform infrared light, 29Si nuclear magnetic resonance, scanning electron microscope and energy disperse spectroscopy, and physisorption experiment were carried out to study the effects of the type and dosage on the mechanical property and microstructure of fly ashbased geopolymer activated by calcium hydroxide, calcium sulfate and their compound. The results indicated that calcium hydroxide and calcium sulfate can promote the dissolution of fly ash crystals, providing adequate Ca2+ for the formation of gel products. The products of calcium hydroxide and calcium sulfate activation are hydrated calcium aluminate, hydrated calcium sulfate, and ettringite, respectively. The effect of the compound activator is superior to that of single activators. With the increase of the proportion of calcium hydroxide, the reorganization and polymerization of gel products were accelerated so that the integrity and continuity of the microstructure of geopolymer were improved, and then the strength increased. When the ratio of calcium hydroxide to calcium sulfate was 3:1 and the total dosage was 20 %, the unconfined compressive strength reached the maximum value. According to this study, it was investigated that type and dosage of calcium activators had evident influence on the geopolymerization of fly ash. And the reutilization and resource utilization of fly ash waste can be achieved, which can provide an engineering theoretical basis for using fly ash-based geopolymer to alter the physical and chemical properties of special soils and achieve solidification effect.

This paper reports the second part of the keynote lecture, whose part I has been already published in this journal, presenting extensive experimental research on the investigation of clay microstructure and its evolution upon loading. Whether the first part focused on the micro to macro behaviour of different reconstituted clays, this part instead concerns the microscale features of the corresponding natural clays, their changes under different loading paths and the ensuing constitutive modelling implications. The experimental investigation is carried out according to the methodology outlined in the part I-paper, hence micro-scale analyses are presented on natural clays subjected to macro-scale mechanical testing, with the purpose to provide experimental evidence of the processes at the micro-scale which underlie the clay response at the macroscale. As for the reconstituted clays in the part I-paper, original results on stiff Pappadai and Lucera clay, this time in their natural state, are compared to literature results on clays of different classes, either soft or stiff. The results presented in this paper, together with those discussed in the part I, allow for a conceptual modelling of the microstructure evolution under compression of natural versus reconstituted multi-mineral clays, providing microstructural insights into the macro-behaviour described by constitutive laws and advising their mathematical formalization in the framework of either continuum mechanics or micro-mechanics.

This keynote lecture discusses the results of a long lasting experimental research, devoted to the investigation of clay microstructure and its evolution upon loading. Micro-scale analyses, involving scanning electron microscopy, image processing, mercury intrusion porosimetry and swelling paths to test the clay bonding, are presented on clays subjected to different loading paths, with the purpose of providing experimental evidence of the processes at the micro-scale which underlie the clay response at the macro-scale. Data from the literature on clays of different classes, either soft or stiff, are compared to original results on two stiff clays, Pappadai and Lucera clay, both in their natural state and after reconstitution in the laboratory. The results presented herein allow building a conceptual model of the evolution of clay microstructure upon different loading paths, providing microstructural insights into the macro-behaviour described by constitutive laws and advising their mathematical formalization in the framework of either continuum mechanics or micro-mechanics. For editorial purposes, the research results are presented in two parts. The first part, presented in this paper, concerns the results for reconstituted clays, whereas a second part, concerning the corresponding natural clays, is discussed in a second companion paper.

Silt is generally stabilized with industrial waste for subgrade filler material. However, problems such as high cost, poor water stability, and easy shrinkage hinder the use of industrial waste-stabilizing materials. Experiments are conducted to compare the effect of nano-silica (NS) and nano-MgO (NM) on the unconfined compressive strength (UCS) of silt. It is undertaken by stabilized silt with NM and NS in four different concentration grades of NM(S)-3, NM(S)-4, NM(S)-5, NM(S)-6 ( 0.3%, 0.4%, 0.5%, 0.6% by weight of silt). XRD, FT-IR, and SEM tests are carried out to discern the latent mechanisms so that the mineral composition and pore structure within the soil matrix can be explained. The results demonstrate that the increase in different grades of nano-MgO and nano-silica stabilized silt plays an important role in improving the mechanical properties of the silt. Nano-silica is more conducive to strength and water resistance enhancement due to the formation of ettringite and C-S-H gel. Nevertheless, a small amount of nano-MgO can better improve the water stability of stabilized silt in the early stage. Optimal results are obtained for silt treated with NS-5 (2% cement, 2% GGBS, 1% FA, 0.5% nano-silica) and NM-5 (2% cement, 2% GGBS, 1% FA, 0.5% nano-MgO). XRD, FT-IR, and SEM analysis of the samples show, respectively, that the amorphous (C-S-H) structure and the soil particles embedded in the cementitious matrix comprise the strength of the silt. Nano-MgO is mainly involved in carbonation and pozzolanic reactions. The cement hydration reaction and ettringite formation, which unite smaller particles to produce larger particles, are enhanced by the addition of SiO2. In summary, this paper recommends the use of nano-MgO to improve the early strength of stabilized silt, and the use of nano-silica to improve the long-term strength.