Magnesium-based cements, which combine MgO and nesquehonite (MgCO3 & centerdot;3H2O), are promising lowcarbon binders which sequester CO2 and gain strength through the formation of hydrous carbonatecontaining brucite (HCB). This study analyses the evolution of solid and liquid phases in MgOnesquehonite suspensions (0 to 60% nesquehonite by mass, w/s = 10) over a year using XRD, TGA, FTIR, Raman spectroscopy and liquid analysis (pH, ionic composition), and compares them with previously studied paste systems (w/s = 1.25). The results show that HCB is the main hydrate after 28 days, while artinite forms in addition in blends with more than 30% nesquehonite after 91 days, consuming CO2 from HCB. Calculated saturation indices and the evolution of dissolved Mg and inorganic carbon confirm the dissolution of nesquehonite and the subsequent formation of HCB and artinite. Compared with paste systems, suspensions show faster phase evolution and artinite formation due to the higher water availability and continuous mixing. The findings confirm the metastability of HCB and provide data for the thermodynamic modelling of durable MgO-based binders.
Deciphering the fracture mechanisms of concrete at the mesoscale is pivotal for predicting the structural integrity of quasi-brittle materials. This study develops a high-fidelity 3D discrete element method (DEM) framework, characterized by real-shaped, crushable aggregates and a sophisticated contact constitutive model designed to address the inherent limitations of traditional DEM, such as insufficient compression-to-tension ratios and the lack of softening effects. Implemented on the open-source platform MUSEN with GPU-accelerated computing, the model enables large-scale simulations of wedge splitting tests (WST) with unprecedented efficiency. Validation against laboratory experiments confirms the model's capability to capture the macroscopic mechanical response and the intricate F-CMOD relationship. Through the evolution of internal principal stress fields, the study clarifies how boundary-induced end effects and mesoscopic heterogeneity govern crack initiation and propagation. Furthermore, a parametric analysis of aggregate and interfacial transition zone (ITZ) strengths reveals a fundamental transition from transgranular to intergranular failure modes. The results quantify the crack-bridging and toughening mechanisms provided by aggregates, providing a robust computational tool for the multiscale design and safety assessment of concrete structures.
Single-layer electromagnetic wave-absorbing ceramsite concrete exhibits poor reflection loss and insufficient effective absorption bandwidth in the 8 to 18 GHz frequency range. To address this, expanded perlite (EP) and polypropylene fiber (PF) were used as wave-transparent agents in the matching layer, while carbon fiber (CF) and basalt fiber (BF) were used as wave-absorbing agents in the absorption layer, to prepare double-layer structured fiber-reinforced electromagnetic wave-absorbing ceramsite concrete. The arch method testing system was used to analyze its electromagnetic wave absorption properties in the 8 to 18 GHz frequency range. The experimental results indicate that the double-layer structured specimens exhibit enhanced microwave absorption properties compared to their single-layer counterpart, with improved low-frequency electromagnetic wave reflection loss and broadened effective absorption bandwidth. The microwave absorption properties vary with the changes in fiber types and volume fractions in the matching and absorption layers: when EP is incorporated into the matching layer, the absorption layer with CF achieves optimal microwave absorption parameters at 0.6% CF volume fraction, while with BF, the properties improve as the BF volume fraction increases; when PF is incorporated into the matching layer, higher PF volume fraction than CF or BF in the absorption layer yields lower minimum reflection loss and wider effective absorption bandwidth.
Precast concrete T-beams often develop early-age cracking during storage due to large daily temperature differences, threatening the long-term durability of bridges. To investigate the factors leading to cracking, this study establishes a coupled thermal-mechanical model that accounts for heat of hydration, thermal exchange, solar radiation, concrete shrinkage, and creep. Based on this model, the effects of curing method and formwork removal timing on the cracking risk of T-beams were analyzed. The results indicate that the use of steel formwork + PU insulation board and full shading measures can effectively mitigate the impact of temperature differences and radiation on thermal stresses generated in precast concrete T-beam. The simulated stress patterns closely match field observations, confirming the model's reliability. This study provides valuable guidance for cracking prevention and optimized curing strategies for precast concrete T-beams in regions with large daily temperature differences.
The use of precast concrete permanent formwork systems in reinforced concrete (RC) construction can deliver meaningful benefits only when their design is optimized by balancing structural, economic, and environmental objectives. This study proposes a multi-objective optimization framework for RC beams with permanent formwork, simultaneously considering flexural performance, material cost, and CO2 emissions. A case study is conducted in which key geometric and material parameters are treated as variables and the results are compared with those of reference RC beams using reusable steel formwork. Parametric and sensitivity analyses reveal trade-offs among cracking resistance, flexural capacity, cost, and emissions, with formwork thickness, concrete strength, and reinforcement ratio identified as dominant factors. The multi-objective optimized solutions achieve up to 250% higher cracking moment and over 30% increases in yield and ultimate capacities while maintaining comparable or lower material cost and CO2 emissions than reference beams.
Frost-induced deterioration remains a critical factor limiting the durability of reinforced concrete (RC) bridge deck slabs in cold regions. Repeated freeze-thaw (FT) cycles lead to the formation of horizontally layered cracks in RC bridge deck slabs, eventually leading to the fall-off of concrete and the cave-in of pavement. To elucidate the mechanism responsible for such damage, a two-stage experimental investigation was conducted using a thermoelectric cooler (TEC)-based system on concrete cylindrical specimens. In the first stage, specimens were subjected to accelerated FT cycles to simulate frost deterioration possible in RC bridge deck slabs. The extent of deterioration was quantified through ultrasound velocity measurements. Physical properties exhibited progressive degradation with an increase in the number of FT cycles. For example, after 25 FT cycles, the oven-dry and saturated weights decreased by 3.2% and 2.3%, respectively. Additionally, X-ray computed tomography (CT) analysis was employed to analyze the crack network and cracking volume. In the second stage, steady thermal conditions were imposed, with one face of the specimen undergoing freezing and the opposite face thawing in the presence of water. Horizontal cracking associated with ice lens growth was observed in all specimens that had been pre-damaged by FT cycling. The results imply a possibility of ice lens formation in inducing horizontal cracking in frost-damaged RC bridge deck slabs.
In this study, bischofite and dolomite were used as raw materials to investigate a low-cost production process for active Magnesium Oxide (MgO) through the chlorination roasting method, with a focus on optimizing the technics parameter. The performance differences of the prepared MgO were analyzed by X-ray diffraction (XRD), BET and other test methods. Subsequently, Magnesium Oxychloride Cement (MOC) was prepared, and the effects of the synthesized MgO on the performance of MOC were evaluated through tests including compressive strength, hydration heat, XRD, scanning electron microscopy (SEM), scanning electron microscopy (MIP) and hydration heat. Modifications of MOC were carried out using single-seed crystals addition, combined seed crystals addition, and various mineral admixtures (steel slag, blast furnace slag, and fly ash). The results indicate that MgO prepared by calcining bischofite and dolomite at 850 degrees C for 3 h achieved activity level nearly reaching 100%, making it suitable for MOC production, with a 28-day compressive strength of 125.5 MPa. Moreover, the prepared MgO exhibited good compatibility with mineral admixtures. When MOC was modified using seed crystals and fly ash, the softening coefficient after 30 days of water immersion reached 0.912.
Existing functionally graded concrete structures are prone to weak interfacial zones due to sequential casting. To address this, this study presents a construction method integrating liquid nitrogen-frozen concrete with normal concrete. The core objective is to investigate the influencing factors and laws of interfacial bonding, and to clarify the interfacial bonding mechanism of the composite system. In this approach, fresh mortar is rapidly frozen into solid blocks via liquid nitrogen to halt cement hydration, and then co-cast with subsequent fresh mortar for molding. The splitting tensile strength, compressive strength and microstructural characteristics of composite specimens under different temperatures were tested. Techniques including scanning electron microscopy, microhardness testing and temperature monitoring were employed to reveal the interfacial hydration mechanism. The results indicate that with appropriate temperature control, the interfacial bonding strength of the pre-frozen composite mortar is comparable to that of monolithic cast mortar. These findings are expected to alleviate the problem of weak interfacial bonding in functionally graded concrete, realize the integral forming of graded structures, and can be applied to practical engineering scenarios such as anti-scouring of functionally graded bridge piers and anti-cracking of concrete graded structures in high-altitude regions in the future.
Stimuli-responsive polymers (SRPs) provide a promising path towards self-repair of cracks in cementitious materials. This review critically synthesizes SRP-based self-healing in Ordinary Portland Cement (OPC) and alkali-activated/geopolymer systems, based on polymers with moisture, pH, and thermo-responsive properties, including superabsorbent polymers (SAPs), hydrogels, shape memory polymers, and microcapsules. Literature that was published from 2000 to 2025 was screened from the major databases of information. More than one hundred and four relevant studies were chosen for comparative assessment. Quantitative performance indicators such as crack-closure efficiency, mechanical recovery, permeability reduction, and swelling behavior are used to investigate the reported healing performance and to understand the importance of differences in pore-solution chemistry of OPC and geopolymers on polymer activation and durability. The synthesis shows that the current evidence is mostly OPC-centric, while geopolymer applications are still limited in number and have not long-term tested for durability, standardized testing methods are unavailable, and the field-scale validation of these materials is lacking. Key research priorities are identified to facilitate durable polymer design and support more rapid translation of SRP-enabled self-healing systems to sustainable cementitious infrastructure.
Detailed analysis of a natural artinite sample by XRD, TGA, FTIR, Raman and solid state 13C CP MAS NMR confirmed a high purity of the sample as well as a crystal structure containing a single carbonate site. The solubility of artinite was experimentally investigated under varying saturation conditions. Undersaturation experiments at 7 to 25 degrees C, up to 1 year, showed that artinite remained stable in water, and that its solubility was higher than previously estimated in literature. In oversaturation experiments, initially dypingite formed at 7 to 20 degrees C, which slowly transformed into artinite and hydromagnesite. At 25 degrees C, no dypingite formation was observed, while initially formed artinite transformed to hydromagnesite within a year. Solubility products for artinite, dypingite, and hydromagnesite at 7, 15, 20 and 25 degrees C were derived from experimental and literature data. The thermodynamic data allowed calculating the impact of CO2 pressure and temperature on the stability of magnesium carbonate and hydroxide phases and confirmed that hydromagnesite is more stable than dypingite, artinite, and nesquehonite under near-ambient conditions. Increased CO2 partial pressure stabilizes nesquehonite while artinite becomes less favored. The revised artinite solubility product indicates that artinite is thermodynamically less stable than hydromagnesite, consistent with observations in MgOhydromagnesite cements.
Existing mix design methods for underwater anti-washout self-compacting concrete lack reliability due to unclear underwater flow behaviors. This study investigates the workability evolution of this concrete in shallow-water environments. The effects of water-to-binder ratio, superplasticizer, and underwater protective agent dosage on flowability and segregation were examined through systematic experiments. Results indicate that underwater slump flow is significantly lower and flow time markedly longer than in air, while increasing the protective agent dosage improves anti-dispersion stability. Water depth variations within the shallow range showed negligible influence. To interpret these mechanisms, the traditional rheological threshold theory was modified by introducing a mortar film retention coefficient to account for dispersion inhibition and an effective density difference to incorporate buoyancy effects. Based on these improvements, a rheological threshold model and an enhanced mix design method were established. Experimental validation confirms that the proposed model accurately predicts qualified mix proportion regions, demonstrating improved design precision within the tested shallow-water environmental parameters compared to traditional methods. This study provides a systematic methodology for the rational mix design and quality control of underwater concrete.
Cracks in tunnel linings induce bearing capacity degradation, making the rapid prediction of the damage ratio critical for structural safety assessment. However, quantifying the intrinsic mechanical performance solely from apparent surface cracks remains a challenge. To bridge this gap, this study proposes a physics-informed deep learning framework that maps visual crack features directly to the bearing capacity damage ratio. First, a high-fidelity numerical simulation system is established using the PseudoCrack Method implemented on the multi-scale thermodynamic platform. This approach avoids the mesh dependency of traditional fracture mechanics and is rigorously validated against existing physical model tests of lining structures in terms of crack morphology and load-displacement responses. Subsequently, a standardized synthetic dataset is constructed by distilling topological features from in-service hydraulic tunnels and applying a color-coded width visualization strategy. By conducting comparative training across eight state-of-the-art deep learning architectures, the ConvNeXtV1 model is identified as the optimal regressor, achieving a coefficient of determination of 0.89 on the test set. The proposed method effectively acts as a real-time "digital surrogate" for time-consuming non-linear finite element analysis, providing a mechanism-based, efficient solution for structural health monitoring of tunnel infrastructure.
To ensure the stress characteristics and durability of high arch dams during the construction and operation process in severe cold regions, this study proposes an integrated multifunctional prefabricated insulated formwork (MPIF). The MPIF not only works with cast-in-place arch dams to share stress but also provides life-cycle thermal insulation. Three full-scale specimens were tested under monotonic and cyclic loading to investigate the out-of-plane mechanical behavior of MPIF. Results indicate that under vertical loading, cracks primarily occur at the mid-span of the bottom beam, thebeam-column interfaces and the center of the grid slab. At a load of 1000 kN, the out-of-plane deformation remains within the elastic range. Structural deformation varies slightly under different loading conditions. Under cyclic loading, out-of-plane deformation is considerably larger than under monotonic loading, and the bearing capacity decreases markedly. Numerical analyses were performed to evaluate the influence of boundary conditions, slab thickness, loading sequence and bonding strength between new and old concrete on the stress characteristics of the integrated formwork. These findings provide a foundation for the practical application of MPIF in high arch dams in severe cold regions.
