Resourceful utilisation of tailings waste remains a hotspot in global research. While silica-aluminate-rich copper tailings can serve as raw materials for geopolymer preparation, their high Si/Al ratio significantly limits the geopolymerization degree. This study investigates the feasibility of developing copper tailings-based geopolymers for road base applications, using copper tailings as the primary raw material supplemented with 30 % soft soil, 15 % fly ash, and 5 % cement. The effect of NaOH content on the strength characteristics of copper tailings-based geopolymers was explored by the unconfined compressive strength test and triaxial test. The mineral composition and microstructure of copper tailings-based geopolymers specimens were characterised based on the microscopic technique. The results show that: (1) With the increase of NaOH content, the unconfined compressive strength of the copper tailings base polymer increases and then decreases, and reach the maximum value when the NaOH content is 1 %. Compared with the sample without NaOH, the addition of 1 % NaOH increased the unconfined compressive strength by 47 % at the early stage and 69 % at 28d curing age. (2) An optimal NaOH content significantly improves the shear performance of the copper tailings-based polymer, primarily by enhancing its cohesion. Triaxial test results demonstrate that 1 % NaOH addition increases cohesion by 73 % at 28d curing age. (3) The NaOH promotes the formation of geopolymer gel, refines the pore structure, and increases sample density, thereby enhancing strength. Overall, the research results can provide a reference for the application of copper tailings solid waste in roadbed materials.

Cementation, even in small amounts, tends to alter the mechanical properties of soil significantly. Ordinary Portland Cement (OPC) is a widely used binding admixture, but there has been an increasing need for replacement owing to its carbon footprint. One such alternative is Calcium Sulfoaluminate cement (CSA), which has higher initial strength gain and lower carbon footprint than OPC. Since existing strength prediction models available from literature were developed for conventional cement types such as OPC and Portland Blast Furnace Cement (PBFC), those are not applicable for predicting the strength evolution of soil treated by other types of cements (e.g., underpredicting the initial strength of CSA treated sand). It is because the prediction models available are generally either soil-specific or cement-specific. This paper proposes a unified strength prediction model that works irrespective of cement and/or soil types by introducing a slope parameter that controls time-dependent strength gain. The proposed model is validated by data collected from literature on various soils and cement types. The three-parameter model demonstrates strong applicability for predicting the strength evolution over a wide range of water-to-cement ratios.

Fine-grained soils containing diatom microfossils exhibit unique geotechnical behavior due to their biological origins, but their strength properties controlled jointly by diatom content (DC) and stress history remain to be revealed. In this study, reconstituted diatomaceous soil was prepared by mixing pure diatom and kaolin powders in different proportions. These mixtures were subjected to undrained consolidated triaxial shear tests performed using the Stress History and Normalized Soil Engineering Properties (SHANSEP) procedures, revealing how the DC and stress history affect the soil strength. Adding diatoms improved the mixture strength, and a critical DC of approximately 20% was determined, beyond which the normalized undrained strength of the soil was considerably higher than that of common clay without diatoms. Also, a DC higher than 20% associates with the dilatancy of the studied soil with high OCR. Improving the strength of diatomaceous soil by adding diatoms differs essentially from the case of common clay because the plasticity index of the former remains almost unchanged. New formulas incorporating DC and OCR are proposed for predicting the strength of diatomaceous soil, and data for several well-studied soils confirm their validity. This study improves the understanding of fine-grained soils with biological origins and provides important data regarding the mechanical behavior of diatomaceous soil.

Sustainable foam lightweight soil (FLS) with the introduction of solid waste-based binders and dredged mud has shown high engineering and environmental value in expressway reconstruction and extension projects. Accelerated testing through high-temperature curing is considered a crucial method for early-stage assessment of sustainable FLS construction quality. This study aims to explore the curing temperature effect on the strength development of the FLS with different mix proportions and the applicability of accelerated curing method. Strength tests were first conducted on kaolin clay-based FLS with three wet densities and three water contents under different curing temperatures (T), and the strength of the dredged mud-based FLS was also tested to broaden the applicability. Results indicate that higher T and increased wet density significantly enhance the strength of clay-based FLS at any curing age, while higher water content reduces it. The wet density and water content of the proposed FLS recommended in this study considering the strength and lightweight requirements are 800 kg/m3 and 100%, respectively. Moreover, the effectiveness of the accelerated aging method for clay-based FLS is demonstrated by the fact that no dramatic strength loss occurs due to foam expansion and collapse at elevated T of up to 50 degrees C. On this basis, a strength prediction model based on the concept of activation energy is proposed for both kaolin clay-based and dredged mud-based FLS considering the temperature effect. Changes in wet density have a minimal impact on model parameters, but variations in soil type and water content require updating these parameters to ensure prediction accuracy. Finally, an early quality control method is introduced for applying the sustainable FLS in field projects.

Fault fracture zones, characterized by high weathering, low strength, and a high degree of fragmentation, are common adverse geological phenomena encountered in tunneling projects. This paper performed a series of large-scale triaxial compression tests on the cohesive soil-rock mixture (SRM) samples with dimensions of 500 mm x 1000 mm to investigate the influence of rock content P-BV (20, 40, and 60% by volume), rock orientation angle alpha, and confining pressure on their macro-mechanical properties. Furthermore, a triaxial numerical model, which takes into account P-BV and alpha, was constructed by means of PFC3D to investigate the evolution of the mechanical properties of the cohesive SRM. The results indicated that (1) the influence of the alpha is significant at high confining pressures. For the sample with an alpha of 0 degrees, shear failure was inhibited, and the rock blocks tended to break more easily, while the samples with an alpha of 30 degrees and 60 degrees exhibited fewer fragmentations. (2) P-BV significantly affected the shear behaviors of the cohesive SRM. The peak deviatoric stress of the sample with an alpha of 0 degrees was minimized at lower P-BV (60%). Based on these findings, an equation correlating shear strength and P-BV was proposed under consistent alpha and matrix strength conditions. This equation effectively predicts the shear strength of the cohesive SRM with different P-BV values.

Soil-rock mixtures in fault fracture zones are composed of rock blocks with high strength and fault mud with low strength. In this paper, in order to study the mechanical properties of the soil-rock mixture with non-cohesive matrix, a large-scale laboratory triaxial compression test with a specimen size of 500 mmx1000 mm is conducted, combined with numerical simulation analyses based on the two-dimensional particle flow software PFC2D. The macroscopic mechanical response and mesoscopic fracture mechanism of soil-rock mixtures with varying rock block proportions, block orientation angles and matrix strengths are studied. The results indicate the following: (1) When the proportion is less than 30%, the shear characteristics of the mixture are similar to those of its non-cohesive matrix. When the proportion is in the range of 30-70%, the internal friction angle and cohesion increase rapidly, and the softening characteristics of the mixture become more apparent. When the proportion exceeds 70%, the aforementioned effect slows. (2) The strength of the mixture is positively correlated with its matrix strength, and the influence of the matrix strength on the loading curve of the mixture is related to the block proportion. (3) When the block orientation angle is 0 degrees, the cohesion and internal friction angle are slightly greater than those at an angle of 90 degrees. Based on the above, for the soil-rock mixture with non-cohesive matrix, a strength prediction model based on the block proportion is given when the block orientation angle and matrix strength are consistent.

The large amount of sewage sludge ash (SSA) generated by incineration treatment of sewage sludge each year can seriously pollute the environment, and there is an urgent need to seek an effective resource utilization method for SSA treatment. SSA has the significant pozzolanic activity, and can be used as an auxiliary cementitious material. For that, SSA was adopted to modify lime soil in this work. The strength characteristics of SSA modified lime soil (SLS) were investigated by conducting unconfined compressive strength test and unconsolidated-undrained triaxial compression test, and X-ray diffraction (XRD) test and scanning electron microscopy (SEM) test were conducted to investigate the mineral composition, microstructure and porosity of SLS. It is observed that the unconfined compressive strength (UCS) increases first and then decreases with increasing SSA content in the range of 0-20 %, and reaches the maximum value while SSA content is 15 %, which increases by 25 % at the curing age of 28 days comparing to the specimen without SSA. Additionally, the triaxial strength and cohesive force both have the same change law as UCS with SSA increasing, and 15 % can be used as the optimal content of SSA to modify lime soil. The added SSA can promote the pozzolanic reaction, and more hydration products fill the specimen porosity so that the specimen strength can be improved. Furthermore, the porosity of SLS decreases first and then increases with SSA content increasing, and there is a linear relationship between the porosity and strength of SLS. Finally, a simple function is proposed, which can simulate the quantitative relationship between UCS and the triaxial strength with satisfying accuracy. Generally, the research results can be used as a reference for the application of SSA in soft soil foundation treatment.

Magnesium oxychloride cement (MOC) is an environment-friendly cement often used for stabilizing soft soils because of its exceptional mechanical properties. In this study, the influence of curing temperature on the strength development of MOC-solidified clay is explored, considering different MgO/MgCl2 molar ratios. Different tests were carried out to study the corresponding effects. The results show that the effect of curing temperature on the strength of MOC-solidified clay differs greatly from that of cement-solidified soil. Increasing the curing temperature leads to strength reduction, whereas decreasing the curing temperature increases the corresponding strength. Scanning electron microscope (SEM) and X-ray diffraction (XRD) analyses indicate that the variation in type and amount of hydration products of the solidified soil account for the strength development difference between MOC-solidified and cement-solidified soils. A model based on the experimental results is proposed to characterize the relationship between strength development and curing time. The strength influence factor (eta T) and the strength expedite factor (K) were introduced to demonstrate the relationship between strength development at a specific curing temperature as well as at room temperature.