Industrial solid waste-modified cement grouts provide a promising pathway for reducing cement consumption and promoting waste utilization, yet comparative evidence remains limited on how different wastes influence mechanical performance and non-destructive indicators under the same loading framework. This study investigates cement-based grouted bodies modified with fly ash (FA), S95 slag powder (SP), or coal gangue powder (CG) across water-cement ratios (W/C = 0.5-0.9) and replacement levels (0-40%). Slurry viscosity and grouted-body formation ratio were measured, and specimens were tested under monotonic uniaxial compression with synchronized monitoring of compressive strength, electrical resistivity, acoustic emission (AE), and wave velocity. Increasing W/C consistently reduced formation ratio, strength, and wave velocity. At a fixed W/C, SP generally improved strength at moderate replacement (20-30%), FA maintained near-reference strength mainly at 10-20%, whereas CG predominantly decreased strength, particularly at ≥30%. Loading-induced electro-mechanical responses were waste-dependent: resistivity at peak load (ρmax) showed the strongest association with strength for FA and SP systems, while cumulative AE ring count (CRC) was more informative for CG. An eco-efficiency proxy further indicated diminishing strength return per unit replacement with increasing replacement level, with SP exhibiting comparatively higher efficiency at low-to-moderate replacement.

Severe electrochemical corrosion and leaching degradation of underground structures are induced by coupled stray current and groundwater in rail transit systems like subways. The microstructural evolution of fly ash geopolymer (FAG) during accelerated leaching was systematically investigated by simulating a coupled direct current (DC) stray current and soft water environment. Pore solution alkalinity and electrolytic cell OH- concentration were utilized as evaluation indicators, combined with X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP). Results indicate that substantial OH- leaching occurs under the coupled stray-current and soft-water exposure, leading to reduced pore-solution alkalinity and changes consistent with the degradation of the N-A-S-H binding network. Consequently, porosity, most probable pore diameter, and the quantity of harmful pores are increased. However, no obvious changes in the major diffraction peaks associated with quartz and mullite were detected within the resolution of the qualitative XRD analysis. Furthermore, a linearly decreasing trend over time is observed for the coupled leaching rate. Mix proportion analysis demonstrates that FAG leaching resistance is improved by reducing the water-to-binder ratio; specifically, superior gel phase content and pore structure are maintained at a ratio of 0.30. Additionally, cumulative OH- leaching is effectively reduced by decreasing the sodium silicate modulus, with optimal resistance exhibited between 1.0 and 1.2. Concurrently, pore solution alkalinity before and after leaching is significantly elevated by increasing the alkali dosage. This exerts a pronounced inhibitory effect on OH- leaching, thereby substantially enhancing the overall leaching resistance.

The investigation of frost durability in hydraulic concrete is of critical importance. In this study, microencapsulated phase change materials (mPCMs) with varying phase transition temperatures (5 degrees C and - 5 degrees C) and dosages (0 %, 5 %, and 10 %) were incorporated into hydraulic concrete to develop modified mPCM - concrete systems with distinct temperature gradient configurations. Experiments involving temperature regulation, rapid freeze - thaw cycles, and nondestructive ICT (Industrial Computed Tomography) testing were conducted. The results indicated that mPCMs effectively delayed internal temperature fluctuations, mitigated void structure degradation, and suppressed thermal cracking, albeit with a reduction in concrete strength. Increasing the dosage of mPCMs (5 degrees C) enhanced frost resistance, whereas mPCMs (- 5 degrees C) exhibited the opposite trend. Composite phase change concrete demonstrated optimal frost resistance. A dataset comprising 144 samples was established through ICT testing. To optimize predictive performance, the sparrow search algorithm (SSA) was employed to tune hyperparameters of the XGBoost model, while recursive feature elimination with cross - validation (RFECV) was applied to eliminate redundant features. Interpretability analysis via SHapley Additive exPlanations (SHAP) identified mass and maximum equivalent diameter as dominant factors influencing compressive strength. Compared to deep neural network (DNN) and random forest (RF) models, the DNN demonstrated superior prediction accuracy, enabling reliable estimation of compressive strength for mPCM - modified concrete. The findings of this study are expected to advance the integration of nondestructive testing technologies with material design, offering novel insights for addressing frost damage in hydraulic concrete structures.

Stray-current and soft-water leaching can induce severe corrosion in reinforced concrete structures and buried metal pipelines within subway environments. The effects of water-to-binder ratio (W/C), modulus of sodium silicate (Ms), and alkali content (AC) on the mechanical properties of fly-ash-based geopolymer (FAG) at various curing ages were investigated. The influence of curing temperature and high-temperature curing duration on the development of mechanical performance were examined, and the optimal curing regime was determined. Furthermore, based on the mix design of FAG resistant to coupled erosion from stray-current and soft-water, the effects of stray-current intensity and erosion duration on the coupled erosion behavior were analyzed. The results indicated that FAG exhibited slow strength development under ambient conditions. However, thermal curing at 80 °C for 24 h markedly improved early-age strength. The compressive strength of FAG exhibited an increase followed by a decrease with increasing W/B, Ms, and AC, with optimal ranges identified as 0.28–0.34, 1.0–1.6, and 4–7%, respectively. Soft-water alone caused limited leaching, while the presence of stray-current significantly accelerated degradation, with corrosion rates increasing by 4.1 and 7.2 times under 20 V and 40 V, respectively. The coupled corrosion effect was found to weaken over time and with increasing current intensity. Under coupled leaching conditions, compressive strength loss of FAG was primarily influenced by AC, with lesser contributions from W/B and Ms. The optimal mix proportion for corrosion resistance was determined to be W/B of 0.30, Ms of 1.2, and AC of 6%, under which the compressive strength after corrosion achieved the highest value, thereby significantly improving the durability of FAG in harsh environments such as stray-current zones in subways.

Reducing the radon emission rate on the surface of self-compacting concrete prepared from industrial solid waste is crucial to lowering the risk of human lung cancer. This study prepared four types of self-compacting concrete with a composite cementitious system of silica fume and molybdenum tailings. The effects of various curing temperatures (0 degrees C, 20 degrees C, 40 degrees C, and 60 degrees C) and amounts of molybdenum tailings substitution on the pore structure, microstructure, compressive strength, and radon emission characteristics of self-compacting concrete were studied. Additionally, using Low-Field Nuclear Magnetic Resonance (LF-NMR), a segmented fractal analysis of the pore structure of self-compacting concrete within various pore size ranges was carried out. The findings suggest that raising the curing temperature and using a suitable quantity of molybdenum tailings enhance self-compacting concrete's compressive strength and the microstructure's density, while decreasing the porosity and radon emission rate. The variation in the micro-pore structure resulting from the aggregation of C-S-H gels strongly correlates with the radon emission rate. This association is evident through decreased porosity and increased fractal dimensions D-1 and D-2. This results in a denser microstructure of self-compacting concrete, weakening the connectivity of microcracks and pore throats, thereby reducing the transport pathways for free radon and lowering the radon emission rate.

To study the effect of nano-SiO2 on the frost resistance of phase change concrete, and to further explore the deterioration of its internal pore structure under the effect of freeze-thaw cycles, X-ray tomography was applied to the rapid freeze-thaw test for the study of frost resistance, and the pore structure of the modified concrete was reconstructed during the freeze-thaw cycles. By image processing and registration techniques, the change pattern of mesoscopic pore inside the phase change concrete modified by nano-SiO2 and the development and ductility characteristics of the pore structure in the interface transition zone were analyzed. The results showed that the admixture of nano-SiO2 and phase change materials could effectively improve the frost resistance of concrete. Frost resistance life was improved by at least 43 % and up to 71 %. Additionally, nanomaterials could constrain the change of pore structure inside the composite modified concrete and densify the interface transition zone. The ITZ porosity of ordinary concrete increased by 10.58 times after 150 freeze-thaw cycles, while the secondary modified phase change concrete with 10mPCMs/1.5NS only increased by 1.2 times.

A pioneering framework for uncertainty quantification in seismic analysis of concrete dams rooted in the stochastic scaled boundary finite element method (SSBFEM) is proposed. For seismic analysis of dams, octree decomposition is used for mesh generation. SSBFEM is developed to predict the structural responses with randomly distributed material properties. The script in this paper can integrate octree mesh and SSBFEM into ABAQUS user element (UEL). Implementing octree SSBFEM on commercial software provides accurate and convenient full order models for uncertainty quantification. Monte Carlo simulation (MCs) is deployed to calculate the statistical characteristics of structural responses under random variables. Additionally, singular value decomposition (SVD) and radial basis function (RBF) are leveraged to refine traditional MCs. The solution space of MCs is decomposed into lower-order subspaces. Any structural responses can be rapidly evaluated from linear combinations of subspaces. Particularly, the method has been successfully applied to seismic analysis of concrete dams with different random material properties. Finally, several illustrative examples are provided to validate the accuracy and efficacy of the algorithm.

Spontaneous combustion coal gangue (SCCG) is considered to be an aluminosilicate-based solid waste containing various toxic ions. The alkali-activation method for this material can not only fully use its potential hydration activity but also solidify the hazardous components to some extent. Through introducing additional Pb2+, the solidification behavior of heavy metal Pb2+ for an SCCG-based geopolymer was studied in the present paper. The solidification efficiencies were evaluated by Pb2+ leaching rates under neutral and acidic conditions, while its mechanism was explained by the methods of XRD, TG, FT-IR, SEM, and MIP. The results show that the Pb2+ solidification efficiency increases along with the curing age, and acidic rather than neutral conditions lead to a more intensive solidification capacity. Judging by the permissive maximum value of 5 mg/L, the Pb2+ original concentrations under neutral and acidic circumstances should be lower at 2.0 wt.% and 3.0 wt.%, respectively. The Pb2+ absorption is dominated by the physical process, due to the formation of no new hydration products. However, the Pb2+ addition would interrupt the reconstruction of the Si-Al network structure, slowing the accumulation of N-A-S-H gel and the densifying of the matrix. When the Pb2+ concentration grows, the sizes of hydration productions shrink continuously, more defects appear in the microstructure of the geopolymer, and the pore structure deteriorates rapidly, all of which accelerate the diffusion of toxic ions to the external condition.

To study the effect of phase-change materials and nanomaterials as new composite materials on the mechanical properties and frost resistance of concrete, it is beneficial for low-carbon and environmentally friendly buildings to be achieved. The effects of freeze-thaw cycles and different preloading strains were considered. ICT scanning was performed, and the compression performance of composite-modified concrete (CMC) were tested. The internal pore structure and strength indicators of the CMC were analyzed during uniaxial compression. Meanwhile, by conducting uniaxial compression tests on CMC at different temperatures, changes in the mechanical properties during real-time freezing and thawing processes were obtained. The results demonstrated that the fractal dimension grows and the compressive performance of the CMC declines as the number of freeze-thaw cycles and preloading strain increases. The proportion of spheroids in the internal pore shape gradually decreases, whereas the proportions of rods, discs, and blades increases. CMC effectively delays the rise or fall of internal temperature during the freeze-thaw process and reduces its internal structural deterioration. Compared with the freeze-thaw effect, an increase in the preloading strain has a more notable impact on the compressive performance of CMC. A stress-strain constitutive model is proposed for a 10 % microencapsulated phase-change materials and a 1.5 % nanoSiO 2 composite-modified concrete (10 mPCMs/1.5 NS CMC), influenced by temperature, through regression analysis. Meanwhile, under low temperatures, the degradation of mechanical properties of 10 mPCMs/1.5 NS CMC and the influence of different preloading strains are considered, leading to the development of stress-strain constitutive models for freeze-thaw cycles and different preloading conditions.

Solid waste tailing materials from molybdenum mining in China occupy extensive land areas and threaten the health of nearby residents. To address the risk and the increasing shortage of natural construction materials, using those tailings as a construction material is a potential option for the construction industry. Modified concrete mixed with other tailings materials has been widely studied. The application of molybdenum tailings modified concrete to cold regions needs to fully consider its durability. Through the concrete cube compressive strength test and rapid freeze-thaw cycle test after different standard curing time, the strength formation process and frost resistance of molybdenum tailings modified concrete are explored. On this basis, the optimal content of Mo tailing material was determined. Results demonstrated that the compressive strength and frost resistance of Mo-tailing-modified concretes increased first and then decreased with the increase in Mo tailing content. The optimal modification effect was achieved when the Mo tailing content reached 20%. Under that circumstance, the porosity of Mo-tailing-modified concrete was at a minimum before and after the freeze–thaw cycles, thus increasing mechanical and antifrost properties significantly.

The damage of concrete by freeze–thaw has always been a key issue affecting the safe operation and service of projects in cold regions. In this paper, concrete modified by microcapsulated phase change materials (mPCMs) was evaluated for its frost resistance; the evolution of its internal pore structure and meso-deterioration during freeze–thaw cycles was investigated by industrial computerized tomography; additionally, NanoSiO2 was incorporated for secondary modification to reduce strength loss as the frost resistance of phase change concrete increased. The results show that the incorporation of mPCMs effectively inhibits heat diffusion and temperature change in concrete, conducive to frost resistance, and the freeze–thaw life of phase change concrete increases drastically compared to ordinary concrete. With increasing mPCMs, increment in porosity within concrete decreases and the overall pore distribution tends to be dominated by relatively large pores. When the content of mPCMs exceeds 10%, the pore structure is relatively stable during freeze–thaw. However, the incorporation of mPCMs also reduces the strength of concrete. Under the guarantee of certain strength, the optimal content of mPCMs is initially chosen for 10%; the optimal content of nanomaterials added for reinforcement is 1.5%, with a strength improvement of 11.68%.

Fly ash from the incineration of domestic waste contains heavy metals, which is harmful to the environment. To reduce and prevent their contamination, heavy metal ions need to be sequestered. In this study, the geopolymer prepared by fly ash, a kind of power plant waste, is used to cure the heavy metal Pb2+, and to investigate the effect of different concentrations of Pb2+ on the compressive strength of the solidified body at different ages; the curing effect is judged by the toxic leaching concentration of heavy metals; the resistance of the solidified body to immersion is evaluated by comparing the change in strength before and after leaching; the fly ash-based geopolymer solidified body is compared with the cement solidified body in terms of curing effectiveness; the properties of the geopolymer and its mechanism of curing heavy metals are explored by microscopic tests. The results show that the fly ash-based geopolymer solidified body has good resistance to immersion; the optimum curing concentration of Pb2+ in fly ash-based geopolymers is 2.0%; compared to pure geopolymers, the strength of the solidified body at 28 d decreases by only 13.0%, and the leaching concentration of Pb2+ is 4.73 mg·L−1, which meets the specification requirements; the curing effect of the fly ash-based geopolymer is better than the cement solidified body; the microscopic test results indicate that the curing of Pb2+ by the fly ash-based geopolymer is a combination of both chemical bonding and physical fixation.