Ran Hai

Geopolymer concrete (GPC), here defined primarily as concrete based on low-calcium alkali-activated aluminosilicate binders, exhibits durability responses that depend strongly on precursor chemistry, gel assemblage, pore structure and exposure conditions; high-calcium or slag-rich systems are therefore discussed separately as related alkali-activated material (AAM) systems. This critical review synthesizes representative peer-reviewed evidence on freeze–thaw cycling, drying and autogenous shrinkage, elevated-temperature/fire exposure, chloride, sulfate and acid attack, carbonation, reinforcement corrosion and coupled multi-hazard deterioration. The evidence indicates that freeze–thaw damage is governed by pore-water phase change, cracking and transport-pathway formation, while chemical attack involves ion ingress, dealkalization, dealumination/decalcification and elemental leaching. Direct coupled evidence is strongest for freeze–thaw combined with chloride/salt or sulfate exposure; shrinkage–transport interactions are mainly correlation-supported, whereas carbonation–chloride–freeze–thaw-related corrosion remains largely a mechanism-based inference. Future work should prioritize standardized sequential multi-hazard protocols, chemistry-specific parameter databases and field-calibrated transport/corrosion models to support reliable service-life prediction and engineering design.

To reveal the meso-mechanism of the axial compression failure of recycled aggregates stabilized by multi-source solid waste geopolymers and to overcome the limitation that traditional laboratory tests cannot observe the meso-state of the material, a three-dimensional numerical model of this material was established using PFC3D discrete element software to simulate an unconfined compressive strength test. The meso-contact parameters for the specific mix proportion were calibrated using the back-analysis method. The stress–strain curves obtained from the simulation and laboratory tests were compared and analyzed. The meso-rules of axial compression deformation and failure of the material were analyzed from the perspectives of crack evolution, particle movement, and force chain transmission. The results show that the descending branch and failure mode of the stress–strain curves from the discrete element simulation are highly consistent with those from laboratory tests, whereas slight differences exist in the ascending branch owing to model limitations and material randomness. The through-cracks of the specimen failure are mainly distributed in the middle and lower parts. The cracks initiate from the middle of both sides and propagate longitudinally. The increase in cementation bond breakage reached 349.5% during the rapid crack development stage. After failure, the particles in the middle and lower parts of the specimen flowed around, and the particles in the upper part moved downward slowly. The force chains were uniformly distributed in the elastic stage. However, a large number of force chains broke, and the high-force chains were concentrated in the middle and lower parts in the crack development stage. This study verifies the feasibility of the discrete element method in simulating the meso-mechanical behavior of this material and provides a meso-theoretical basis for its mix ratio optimization and engineering applications.

Geopolymers possess good mechanical properties and durability, and their partial replacement of traditional Portland cement is noteworthy for promoting the development of low-carbon building materials. To clarify the influence mechanism of the mechanical properties of slag-fly ash-based geopolymer mortar, this paper investigated the hydration heat, composition, and morphology of hydration products with various contents and moduli of waterglass. The results showed that the compressive strength of geopolymer mortar increased with increasing waterglass content, and first rose and then fell as the waterglass modulus raised, while its flexural strength increased and then decreased with the growth in both. The compressive and flexural strength of geopolymer mortar with 1.2-modulus waterglass at 20 wt% cured for 28 days were 88.4 MPa and 9.0 MPa, respectively. The hydration temperature and cumulative hydration heat of geopolymer mortar was elevated with the increase in waterglass content, and declined with the rising waterglass modulus. The hydration products of the geopolymer consisted of dense amorphous and flocculent structures wrapped around each other. The microstructure of the geopolymer cured for 3 days was loose when the content of 1.4-modulus waterglass was 5 wt%. The relative areas of the flocculation in the geopolymer cured for 28 days increased while the waterglass modulus was greater than 1.4, forming an interface with the dense amorphous structure generated during the early hydration stage, leading to a decrease in its mechanical properties. Therefore, it is recommended for slag-fly ash geopolymer mortar that the waterglass modulus is between 1.2 and 1.4 and its content is no less than 10 wt% to ensure suitable mechanical properties. This study also provided a referenceable time period for the pouring and operation of the engineering application of slag-fly ash-based geopolymer repair mortar.

To study the influence of ceramsite on the mechanical behavior and deformation characteristics of the Yellow River sediment‐based adobe materials, the uniaxial compression particle model was constructed by discrete element software based on the indoor uniaxial compression test, and then the stress–strain characteristics, energy, and crack evolution laws of the Yellow River sediment‐based adobe materials were systematically studied under uniaxial compression load after adding ceramsite. The results show that (1) The addition of ceramsite has a detrimental effect on the mechanical properties, peak stress, and peak strain of Yellow River sediment‐based adobe materials. However, it enhances their thermal insulation performance and ductility. Specifically, at a ceramsite content of 15 wt%, the thermal conductivity decreases by 55.0%, while the peak stress and peak strain decrease by 24.6% and 20.9%, respectively. Meanwhile, the ductility improves by 14.6%. (2) The changing trend of the specimen’s strain energy and relative toughness is similar to that of the peak stress. The total strain energy development is divided into three branches, and the energy release rate of 3 and 6 wt% specimens is the slowest. (3) Based on the DEM, the stress–strain curve of the Yellow River sediment‐based adobe materials under the change of ceramsite is basically consistent with the actual test results. During the axial compression test, the fracture contact and force chain are mainly concentrated around the ceramsite particles. The interfacial transition zone between the ceramsite and the matrix is first destroyed, and micro‐cracks appear.

To reduce the amount of ordinary concrete and then reduce carbon dioxide emission, improving the engineering application range of self-compacting fly ash concrete (FASCC), this study explored the bond–slip traits between FASCC and a steel tube. Six samples were created, and bond–slip push-out tests were performed with varying concrete strength grades and steel tube internal setups. Digital image correlation (DIC) technology was applied to track the surface strain of four samples throughout the experiment. The results show that the outer surface of the steel tube stays mostly undistorted after the concrete is pushed out. Prior to reaching peak load, the load–slip curves of each specimen exhibit a primarily linear load–displacement relationship. Post-peak, the curves diverge into two distinct patterns, namely a sudden drop and a gradual decline. As the strength grade of the inner concrete increases, the interfacial bond between the steel tube and FASCC improves. Additionally, under the same conditions, the internal structure of the steel tube significantly enhances bonding strength. The FA40-Z specimen shows a maximum load that is 25.6% and 53.7% higher than the FA40-G and FA40-C specimens, respectively. The strain evolution patterns of steel tubes within FASCC and regular self-compacting concrete demonstrate similar characteristics. These observations provide valuable insights for the application of FASCC in engineering projects.

High hydration heat and poor water resistance are the main factors restricting the application of magnesium ammonium phosphate cement (MAPC). To alleviate the problem, fly ash was used to partially replace dead‐burned MgO and NH₄H₂PO₄ in this paper. The effect of fly ash content on MAPC properties, such as setting time, fluidity, mechanical properties, and water resistance, was investigated. The micromorphology of hydration products and the influence mechanism of fly ash on the macrocharacteristics and hydration process of MAPC were also discussed. The results showed the mechanical properties of fly ash‐modified MAPC decreased with the increase of the fly ash content, but their increments at later hydration were greater than the control MAPC. Meanwhile, fly ash could improve the water‐resistance significantly and reduce the total hydration heat. The fly ash refined the struvite crystal and increased the compactness of MAPC, although no obvious hydration products of fly ash were observed. So, when the content of fly ash is 30 wt%, the MAPC has appropriate mechanical properties, while its water resistance is significantly improved, and its hydration heat is reduced compared with the control group.

Herein, the toughening mechanism and effects of 3-(aminopropyl)triethoxysilane (3-APTES) intercalation in calcium–silicate–hydrate (CSH) structures were investigated through molecular dynamics simulations. CSH established a model using 11 Å-tobermorite to simulate the tensile properties, toughness, adsorption energy, average orientation displacement and radial distribution function of 3-APTES intercalation at different Ca/Si ratios under conditions of a CVFF force field, an NVT system, and 298 K temperature. Simulation results demonstrate that 3-APTES alters the fracture process of CSH and effectively enhances its tensile properties and toughness. The presence of 3-APTES molecules increases the energy required to destroy CSH, thereby increasing the adsorption energy of CSH crystals. Furthermore, 3-APTES molecules effectively increase the atom density within the CSH structure. As the Ca/Si ratio increases, Ca–O bond formation is enhanced, with noticeable aggregation occurring because of modification by 3-APTES within the CSH structure. This study found that 3-APTES organic compounds can effectively improve the tensile, toughness, adsorption and other properties of the CSH structure, and further improve the microstructure of CSH.