This study investigates the microhardness and geometric degradation mechanisms of interfacial transition zones (ITZs) in recycled aggregate concrete (RAC) exposed to saline soil attack, focusing on the influence of supplementary cementitious materials (SCMs). Ten RAC mixtures incorporating fly ash (FA), granulated blast furnace slag (GBFS), silica fume (SF), and metakaolin (MK) at 10 %, 15 %, and 20 % replacement ratios were subjected to 180 dry-wet cycles in a 7.5 %MgSO4-7.5 %Na2SO4-5 %NaCl solution. Key results reveal that ITZ's microhardness and geometric degradation decreases with exposure depth but intensifies with prolonged dry-wet cycles. The FAGBFS synergistically enhances ITZ microhardness while minimizing geometric deterioration, with ITZ's width and porosity reduced to 67.6-69.0 mu m and 25.83 %, respectively. In contrast, FA-SF and FA-MK exacerbate microhardness degradation, increasing porosity and amplifying microcrack coalescence. FA-GBFS mitigates the diffusion-leaching of aggressive/original ions and suppresses the formation of corrosion products, thereby inhibiting the initiation and propagation of microcracks. In contrast, FA-SF and FA-MK promote the formation of ettringite/gypsum and crystallization bloedite/glauberite, which facilitates the formation of trunk-limb-twig cracks.

In the reinforcement of micro-cracks and soil, cement grouting often suffers from poor injectability due to particle size limitations. While ultra-fine cement produced through physical grinding can address this issue, it significantly increases cost and energy consumption. Moreover, ultra-fine cement is prone to aging when exposed to moisture and CO2 in the air. To address these issues, this study proposes a new approach for in-situ particle size reduction of cement slurry through the mild corrosion of acetic acid. The refining effect of acetic acid on cement particles was investigated, along with its impact on mechanical properties and hydration products. The results show that acetic acid accelerates cement dissolution, promoting early-stage strength development and microstructure formation. The addition of 1.2 wt% acetic acid reduced the D90 particle size of the slurry by 36.4 %. Acetic acid also enhances the release of Ca2+ from clinker, increasing the precipitation of Ca(OH)2, CaCO3, and calcium silicate hydrate (C-S-H) at early stages, which serves as the primary source of early strength. Additionally, it raises the Ca/Si ratio of the early-formed C-S-H gel. However, excessive acetic acid can inhibit the further development of strength at later stages. The research demonstrates that premixed acetic acid activation is an effective approach for enhancing the performance of cementitious grouting materials, with promising potential to reduce energy consumption associated with physical cement grinding.

The large-scale development of urban underground spaces has resulted in hundreds of millions of cubic meters of accumulated shield soil dreg waste, occupying huge amounts of land resources and potentially causing groundwater pollution and soil salinization. In this study, shield soil dreg waste is recycled and activated to substitute cement in ultra-high performance concrete, aiming to promote solid waste management and sustainable construction. The slump, mechanical performance, and autogenous shrinkage of the concrete are investigated through macro-scale tests, and the underlying mechanism is revealed via micro-scale experiments. The incorporation of calcined shield soil dreg reduces flowability and leads to a 10.2 % deterioration in compressive strength of the ultra-high performance concrete while mitigating autogenous shrinkage. The primary reason is due to the low CaO content of shield soil dreg, which limits the formation of calcium silicate hydrate, and its high SiO2/Al2O3 content slows hydration kinetics. The environmental and economic benefits of the concrete are determined via life cycle analysis. Recycling shield soil dreg waste into concrete results in about 35 % reduction in carbon emission and 22 % reduction in energy consumption. According to multi-criteria assessment, the overall performance of the concrete considering economic cost, environmental benefit, as well as physical and mechanical properties increases compared to the pristine concrete, achieving well-balanced economic feasibility, environmental sustainability, and engineering performance. The findings of this study provide an effective approach for recycling shield soil dreg and preparing low-carbon concrete, thus promoting solid waste management and sustainable construction.

To address scour hazards surrounding offshore foundations, a new method employing novel alkali-activated cementitious grout (AACG) has been proposed for improvement of seabed soil. Ground granulated blastfurnace slag (GGBFS) was replaced by fly ash (FA), steel slag (SS) or FA + SS to prepare precursors, the replacement amounts were 10 %, 20 %, 30 % and 40 %. Fresh-state and mechanical properties, minerals and microstructures were investigated. A novel scour simulation test device was developed to simulate engineering conditions of scour and remediation. Flow-soil coupled scour resistance tests were conducted, shear tests and SEM measurements of solidified soil were carried out. The results showed that the optimal ratio of GGBFS:FA:SS was 6:2:2 for AACG. The optimized AACG has better fluidity and lower brittleness, and its 28 d unconfined compressive strength (UCS) achieves 13.5 MPa. For AACG solidified soil, the maximum scour depth was reduced by 33.3 % and the maximum sediment transport amount was decreased by 53.2 %, which were compared to those of cement - sodium silicate (C-S) double slurry. Moreover, the increase degrees of internal friction angle, cohesion and critical shear stress were 700 %, 7.9 % and 786 %, respectively. The scour resistance of AACG solidified soil was superior. The inherent relationship between UCS and critical shear stress was discussed. UCS can be used to rapidly assess the scour resistance of consolidated soil. This study introduced an eco-friendly AACG as an innovative stabilizer for soil reinforcement around offshore structural foundations, offering significant application and environmental values for scour control.

If building loads cannot be transferred into the soil, ground improvements are often used, which require the addition of cement with considerable emissions of CO2. Thermo-mechanically processed crushed concrete fines can partially replace the required cement. This article deals with comprehensive laboratory tests to improve the soil mechanical properties of a typical sand as a building ground and demonstrates the applicability of thermo-mechanically processed concrete fines for the substitution of 25 wt.-% to 50 wt.-% of cement for practical construction purposes. Processing temperatures of 400 degrees C and 600 degrees C proved to be particularly effective, with greater reductions in strength and stiffness occurring outside this temperature range.

This study investigates the impact of cementation on the mechanical behavior of sands with various cement content (CSR) in drained triaxial compression, employing both Acoustic Emission (AE) and Environmental Scanning Electron Microscopy (ESEM) measurements. The experimental findings, encompassing quantitative statistics of stress-strain relations, microstructure variations, and AE characteristics, demonstrate that: the addition of CSR from 1% to 20% leads to an exponential rise in peak strength and stiffness, marking a transition from ductile to brittle mechanical failure, which is pinpointed between CSR levels of 5% to 10%. AE characteristics unveil an upward-opening parabola of normalized AE hits with CSR, a clear transition zone identification, and three distinct types of AE rate evolutions corresponding to failure patterns of ductile bulging, shear banding, and brittle fracturing, respectively. It suggests an intimate correlation with the intrinsic differences in micro-mechanical behaviors and AE propagation properties of cemented sands with varying CSRs. Notably, the bulging and shear banding processes are divided by AE into three stages, whereas fracturing is characterized into five stages. Two precursory AE anomalies associated with incipient failure and complex failure modes are observed, emphasizing the advantage of using AE to reflect the internal micro-mechanical behavior of cemented sands over conventional stress-strain manifestations.

This study explores the influence of the water-cement ratio and fiber content in engineered cementitious composite (ECC) on the mechanical characteristics of foamed lightweight soil (FLS) through experimental analysis. Two types of cementitious materials-ECC and ordinary Portland cement (OPC)-were utilized to create FLS specimens under identical parameters to examine their mechanical performance. Results indicate that ECC-FLS exhibits superior toughness, plasticity, and ductility compared to OPC-FLS, validating the potential of ECC as a high-performance material for FLS. To assess the influence of the ECC water-cement ratio, specimens were constructed with varying ratios at 0.2, 0.25, and 0.3, while maintaining other parameters as constant. The experimental results indicate that as the water-cement ratio of ECC increases, the flexural strength, compressive strength, flexural toughness, and compressive elastic modulus of the lightweight ECC-FLS gradually increase, exhibiting a better mechanical performance. Moreover, this study investigates the effect of basalt fiber content in ECC on the mechanical properties of FLS. While keeping other parameters constant, the volume content of basalt fibers varied at 0.1%, 0.3%, and 0.5%, respectively. The experimental results demonstrate that within the range of 0 to 0.5%, the mechanical properties of FLS improved with increasing fiber content. The fibers in ECC effectively enhanced the strength of FLS. In conclusion, the adoption of ECC and appropriate fiber content can significantly optimize the mechanical performance of FLS, endowing it with broader application prospects in engineering practices. ECC-FLS, characterized by excellent ductility and crack resistance, demonstrates versatile engineering applications. It is particularly suitable for soft soil foundations or regions prone to frequent geological activities, where it enhances the seismic resilience of subgrade structures. This material also serves as an ideal construction solution for underground utility tunnels, as well as for the repair and reconstruction of pavement and bridge decks. Notably, ECC-FLS enables the resource utilization of industrial solid wastes such as fly ash and slag, thereby contributing to carbon emission reduction and the realization of a circular economy. These attributes collectively position HDFLS as a sustainable and high-performance construction material with significant potential for promoting environmentally friendly infrastructure development.

The freeze-thaw damage of cementitious coarse grained fillings (CCGFs) significantly affects the firmness, stability, and durability of high-speed railway subgrades. It is favorable to employ geopolymer binders to improve the engineering performance of coarse grained fillings (CGFs), further ensure the safety of high-speed railway subgrades in cold regions due to their excellent mechanical and environmental-friendly performances. This study conducted a series of freeze-thaw and mechanical tests on geopolymer stabilized coarse grained fillings (GSCGFs). The influence of gradation, compaction degree, and freeze-thaw cycles on the integrity, strength, and stiffness of GSCGFs was investigated. The evolution law of their freeze-thaw damage was discussed quantitatively based on an improved damage factor. The results show that the mass loss rate of Group B GSCGFs with a fine-grained particle content of less than 15% was lower than that of Group A GSCGFs with a fine particle content between 15% and 30% overall. When other conditions remain unchanged, the mass loss rate of GSCGFs decreased with the increase of compaction degree but increased nonlinearly with the freeze-thaw cycles. The strength and stiffness of GSCGFs decrease nonlinearly with the freeze-thaw cycles and presented a first fast and then slow-down change trend, their stiffness evolution at different compaction degrees revealed a big difference due to the weakening bite effect and enhancing overhead structure among rock blocks. The strength reduction of Group A GSCGFs was less than that of Group B under the high compaction degree. The stiffness deterioration of Group A GSCGFs was about twice that of Group B. There seemed to be no absolute correlation that the strength of GSCGFs was positively correlated with their stiffness. By building an exponential relationship between the compressive strength of GSCGFs and the freeze-thaw cycles that followed the findings of previous several studies, an improved exponential damage evaluation model was proposed to represent the performance degradation of GSCGFs. The outcomes of this study can provide theoretical support for understanding the physical and mechanical behaviors of GSCGFs and applying them in engineering practices.

Ironmaking- steelmaking is a material and energy intensive process with a resource efficiency of only - 33 %. Resource efficiency enhancement requires recovering the wasted/unutilized material by-products and the energy associated with them in various forms. This review attempts to identify the material leakages and energy losses at each step of steelmaking (from iron ore mining) and explores approaches to plug the energy and material leakage; material efficiency brings in energy savings indirectly. Besides the material loss, accumulation of the byproducts (slime/tailings, steel slag, etc.), carbon emission, etc., cause environmental and ecological damage. The review discusses the prospects of slimes/tailings beneficiation through physical and physicochemical methods (often after some pretreatments). The manuscript also discusses the need to recover heat from molten slags (BF slag and BOF slag) to reduce the energy intensity. Further, it discusses the endeavors to overcome the latent hydraulic activity of granulated BF slag and ways to enhance the acceptability of BOF slag in different applications. A brief sum-up of global efforts towards net zero emission (in line with the Paris Declaration) through carbon recycling, low emission intensity processes, alternate fuels, etc., is included. Lastly, the authors list the challenges of the Indian iron & steel industry and the efforts from the government and steel industries towards achieving the projected crude steel production (300 million tons) without crossing the emission intensity thresholds (Paris Declaration). The endeavors strengthen the sustainability of the steel industry.

In order to reveal the intrinsic mechanism of the mechanical properties of lime-treated sandy soil from a microscopic perspective, triaxial tests were conducted to analyze the macroscopic mechanical characteristics of sandy soil with different lime contents (0%, 5%, 8%, and 12%). The changes in the microstructure of the lime-treated sandy soil were studied through scanning electron microscopy, energy-dispersive spectroscopy, and mercury intrusion tests, combined with fractal theory for quantitative characterization. The results indicate that the stress-strain curve of lime-treated sandy soil can be divided into four stages: linear elastic, non-linear, failure, and residual strength. With the increase in lime content, the peak stress and cohesion first increase and then decrease, while the internal friction angle first decreases and then increases, suggesting the presence of an optimal threshold for lime content between 5% and 12%. The failure mode transitions from diagonal shear failure to bulging failure, significantly enhancing stability; both the fitted Mohr-Coulomb and Drucker-Prager failure criteria effectively reflect the failure patterns of the specimens in principal stress space. The results based on the three fractal dimensions demonstrate that lime-treated sandy soil exhibits clear fractal characteristics, with the highest fractal dimension value at a lime content of 8%, corresponding to the highest overall strength. In addition, the fractal dimension shows a binomial relationship with pore characteristic parameters and shear strength parameters; it can effectively characterize the complexity of the microstructure and accurately predict changes in shear strength parameters.