This study examines the effects of ultrafine recycled powder (URP) obtained from construction and demolition waste on the hydration kinetics, setting behaviour, and chemical shrinkage of Portland cement pastes. The presence of ultrafine particles in the recycled powder provides more sites for nucleation, thereby promoting the hydration process and accelerating the rate of nucleation. As a result, the setting time is reduced while chemical shrinkage is increased. Incorporating URP improves the early-age mechanical properties. When 7.5% URP is added, the highest compressive strength and flexural strength of cement mortar at a curing age of 3 d are 23.0 MPa and 3.7 MPa, respectively. The secondary hydration between the hydration product and reactive silica from URP contributes to gel formation and enhances mechanical property development. This research provides theoretical insights into utilizing recycled powder in cement-based materials and enhances our understanding of its impact on hydration kinetics.

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 meet the material requirements for repairing cracked components serving in cold temperatures, the differences in the setting hardening characteristics and physical and mechanical properties of fly ash-modified magnesium phosphate cement (MPC) repair mortar cured at 0 °C and 20 °C were comparatively studied in this paper. The results show that under the same fly ash content, the compressive strength of MPC repair mortar cured at 0 °C is lower than that cured at 20 °C, and the setting time, fluidity, flexural strength, interfacial bonding strength, strength retention rate, and linear shrinkage rate are not significantly deteriorated. The above properties of MPC repair mortar vary with the increase in the fly ash content but still, remain appropriate when the fly ash content is not higher than 15 wt%. The microstructure of MPC is obviously refined when the content of fly ash is 10 wt%. Therefore, the MPC repair mortar cured at 0 °C and 20 °C has good physical and mechanical properties. In particular, the flexural strength and interfacial bonding strength of the specimens cured at 0 °C are higher than that cured at 20 °C at all curing ages, and at the curing age of 3 days, those are 7.9 MPa and 5.4 MPa, respectively.

In this study, push-out tests were conducted on 20 specimens to explore the bond–slip performance of geopolymer concrete-filled steel tubes. The investigation focused on the effects of various design parameters such as length–diameter ratio, diameter–thickness ratio, concrete strength, and internal structural measures of the steel tube on the bond–slip performance. Analysis of the test phenomena, load–slip curves, and strain distribution curves of each specimen revealed insights into the shear strength calculation methods for welded stud structure and ring rib structure specimens. The results indicated a slight buckling deformation at the loading end of the steel tube in the structural specimen, while no significant deformation was observed in the non-structural specimen. The strain distribution along the height direction of the steel tube exhibited a triangular pattern, with the strain increasing gradually. Improvements in the interfacial bonding performance were noted with reductions in length–diameter ratio and diameter–thickness ratio of the steel tube, as well as increases in concrete strength. When the steel tube wall thickness t increases from 3.5 mm to 4.5 mm, the peak load of GC30-1 increases from 382.13 kN to 419.59 kN, an increase of 9.81%. After improving the concrete strength of GC30-1 and GC30-3 specimens, the peak load increases from 382.13 kN and 274.54 kN to 436.46 kN and 306.12 kN, respectively, an increase of 14.2% and 11.5%. Furthermore, the welding structure of the steel tube significantly enhanced the shear bearing capacity of the interface. The ratio of load calculation value to test value fell within the range of 0.917 to 1.098, indicating good agreement between the calculated and experimental values. These research results can provide reference for engineering applications of geopolymer concrete.

High-strength bolted connections are fundamental load-bearing components in critical engineering infrastructures such as wind turbines, bridges, and heavy machinery. Under complex service environments involving dynamic loading, vibration, corrosion, and temperature variations, bolts are prone to interacting failure mechanisms, including fatigue fracture, corrosion-assisted cracking, hydrogen embrittlement, and progressive preload loss, which pose significant challenges for reliable condition monitoring and early fault diagnosis. This review provides a structured synthesis of recent advances in bolt health monitoring and intelligent fault diagnosis. A unified framework is established to link multi-physics failure mechanisms with multi-modal sensing technologies and data-driven diagnostic methods. Key sensing approaches—such as piezoelectric impedance techniques, ultrasonic phased array inspection, and computer vision-based monitoring—are critically reviewed in terms of their physical principles, diagnostic capabilities, and limitations. Furthermore, the transition from traditional model-based and signal-processing-driven methods to machine learning- and deep learning-based approaches is examined, with emphasis on multi-modal data fusion, real-time monitoring, and lifecycle-oriented health management enabled by IoT and digital twin technologies. Finally, key challenges and future research directions toward robust and scalable intelligent bolt health management systems are outlined. This review’s primary contribution lies in establishing a novel, integrated framework that links failure physics to sensing and diagnosis, thereby providing a structured roadmap for transitioning from isolated component monitoring to lifecycle-oriented, intelligent health management systems for critical bolted connections.

In this study, steel fibers were used to improve the mechanical properties of high-strength self-compacting concrete (HSSCC), and its effect on the fracture mechanical properties was investigated by a three-point bending test with notched beams. Coupled with the digital image correlation (DIC) technique, the fracture process of steel-fiber-reinforced HSSCC was analyzed to elucidate the reinforcing and fracture-resisting mechanisms of steel fibers. The results indicate that the compressive strength and flexural strength of HSSCC cured for 28 days exhibited an initial decrease and then an enhancement as the volume fraction (V_f) of steel fibers increased, whereas the flexural-to-compressive ratio linearly increased. All of them reached their maximum of 110.5 MPa, 11.8 MPa, and 1/9 at 1.2 vol% steel fibers, respectively. Steel fibers significantly improved the peak load (F_P), peak opening displacement (CMOD_P), fracture toughness (K_IC), and fracture energy (G_F) of HSSCC. Compared with HSSCC without steel fibers (HSSCC-0), the F_P, K_IC, CMOD_P, and G_F of HSSCC with 1.2 vol% (HSSCC-1.2) increased by 23.5%, 45.4%, 11.1 times, and 20.1 times, respectively. The horizontal displacement and horizontal strain of steel-fiber-reinforced HSSCC both increased significantly with an increasing V_f. HSSCC-0 experienced unstable fracture without the occurrence of a fracture process zone during the whole fracture damage, whereas the fracture process zone formed at the notched beam tip of HSSCC-1.2 at its initial loading stage and further extended upward in the beams of high-strength self-compacting concrete with a 0.6% volume fraction of steel fibers and HSSCC-1.2 as the load approaches and reaches the peak.