High-temperature high-pressure (HTHP) working conditions of oil-well cement paste alter the expansive mechanism of MgO expansive agent, thus significantly accelerating the expansion speed and increasing the ultimate expansion. The free expansion rate of shrinkage-compensating pastes with MgO expansive agent at different contents was investigated. The microstructural characteristics, including morphology and porosity, were evaluated to reveal the expansive mechanism of MgO expansive agent in HTHP environment. It is indicated that the shrinkage of oil-well cement paste can be compensated by admixing MgO expansive agent at proper contents and reactivity. At 150℃ and 20 MPa, S-type (220 s) MgO with a relatively low reactivity is recommended and its dosage is less than 4%. At HTHP, the migration of Mg²⁺ from MgO particles is accelerated. Besides the formation of expanding mircocracks, Mg(OH)₂ crystals also are interlocked with tobermorite crystals as a hydration product of cement, thus enlarging their gap and causing an extraordinary expansion, compared to the expansive binder cured at normal temperature and pressure.

In order to achieve particle segmentation and extract morphological information from the images of brick-containing recycled coarse aggregate bulk in the stacked state, an image-based method was proposed. Based on the differences in hue and distance between particles of the recycled concrete and crushed brick aggregate, the model was incorporated into the threshold segmentation and distance transformation, thus optimizing the watershed segmentation algorithm. It is indicated that the optimized segmentation algorithm could be used to effectively extract the contours of individual aggregate particles and calculate their morphological information, such as aggregate gradation, proportion of each aggregate, flakiness ratio and sphericity.

BSE-Raman was used to investigate in-situ Raman spectra of hydration products and their spatial distribution profiles. It was found that the in-situ structural information of the silicon-oxygen tetrahedral Q¹, Q² and Q³ chains of C-S-H could be readily obtained, while the hydration products could be accurately identified. All the hydration products with different morphologies exhibited multiple characteristic peaks of chemical bonds of the hydration products. Therefore, it was concluded that the cement hydration products were mixed at nanometer scale. The pom-shaped hydration product had a skeleton consisting of C-S-H and ettringite. Due to its ability of quick and accurate in-situ identification of different phases, BSE-Raman has broad prospects in the microstructural characterization of cement-based materials.

Considering tunnel engineering conditions, we proposed a blockage model of tunnel drainage pipe in the non-karst area based on the calculation of calcium carbonate fouling rate. The model was used to calculate blockage time of the drainage pipe for existing and under construction tunnels. For existing tunnel, the calculation result of the model is close to the measured results, thus verifing the validity of the model. For under construction tunnel, in order to prevent the drainage pipe from being blocked within 20 years, Ca²⁺ leaching rate of primary support concrete subjected to 6 mol/L NH₄Cl solution eroding for 14 d should not exceed 0.032%.

In view of the large deviation of the effect of organic steel corrosion inhibitor in simulated pore solution and concrete, a dry-wet cycles test apparatus for accelerated evaluation of organic steel corrosion inhibitor was developed based on the concrete simulated pore solution and the controllable temperature and humidity. Four test systems were used to examine acceleration effects and test deviations of steel corrosion. The deviation of the test results of the organic steel corrosion inhibitors is significantly minimized via accurately controlling the test environment parameters and optimizing the acceleration systems. The results of the steel bars in the simulated pore solution and the concrete have a stronger correlation. Also, organic steel corrosion inhibitor, such as imidazoline, carboxylic acid amine and amino alcohol, effectively inhibits the rapid corrosion of steel bars under the condition of dry-wet cycles in chloride salt environment.

Siliceous components with different crystalline states (i.e., crystalline silica and amorphous silica fume) were used to adjust the content of free calcium oxide (f-CaO) in regenerated binding materials (RBMs). The effect of siliceous components on carbonation reactivity, mechanical properties, and microstructural characteristics of the final products was investigated. The results indicate that the matrix stability of the RBMs prepared with two siliceous components is significantly enhanced after carbonation. The compressive and flexural strengths of RBMs prepared with silica are 73.1 MPa and 21.0 MPa, respectively, and calcium carbonate crystals formed through the carbonation reaction are mainly vaterite and calcite. The compressive and flexural strengths increase to 106.3 MPa and 25.3 MPa at increment rates of 45.52% and 20.55%, respectively, when silica fume is used to prepare RBMs. In this case, calcium carbonate crystals are mainly vaterite, with a small content of calcite and aragonite. The improvement in the mechanical properties of RBMs with silica fume is due to the high carbonation reactivity, high degree of carbonation and dense structure.

The dispersion and orientation of steel fibers in the matrix have a significant impact on the mechanical properties and durability of steel fiber concrete (SFC). Some factors affecting the dispersion and orientation of fibers in SFC were summarized in terms of mixing proportion, rheology of the mixtures, construction technology and element shape. The results showed that the distribution characteristics of the fibers were affected synergistically by various factors before the mixtures were formed and hardened. Moreover, the distribution of steel fibers was optimized accordingly under different working conditions. There were three types of poor dispersion of steel fibers. Besides, three approaches were used to improve the orientation of steel fibers based on the construction technologies and the element shapes. In addition, some problems in steel fiber distribution characteristics and possible future research direction were also discussed.

The properties of cement concrete at low atmospheric pressures (LAP) were reviewed, including the foaming performance of air-entraining agent solution, air content and pore structure of concrete, concrete strength and concrete shrinkage. The effective power of the concrete mixer at LAP was reduced, so that the mixing quality of fresh concrete was adversely affected. The effect of LAP on the air content and pore structure of concrete was dependent on the type of the air-entraining agent. LAP has little effect on the air content and pore structure of concrete in the presence of a good air-entraining agent and the maintaining of mixing quality control. Moreover, LAP can detrimentally affect the mechanical and shrinkage properties of concrete. However, the mechanical properties of the well-cured concrete cannot be deteriorated at LAP. In addition, the problems mentioned in the literatures and the corresponding solutions were also discussed.

As an important construction material of offshore reef infrastructures, coral concrete becomes one of recent popular research aspects. Compared to aggregates derived from terrigenous rocks, coral aggregates are unique because of their mineral composition, pore structure and particle morphology. Coral aggregate concretes are different from those made of ordinary aggregates in terms of mixture proportion design, static/dynamic mechanical properties and durability, while they are similar to lightweight aggregate and recycled aggregate concretes. Meanwhile, the properties of coral concretes are also affected by the type and volume fraction of fibers. In addition, the development of coral concretes is gradually extended from a material level to a structural level. Appropriate modification is of great significance to extend the service life of the structures in offshore environment. In this review, the problems encountered in these modification processes and the corresponding potential solutions were discussed.

The X-ray CT in-situ tensile test of 2.5D-SiC_f/SiC composites was conducted to reveal the damage evolution and failure mechanism of the ceramic matrix composites. The CT data of the materials during the loading process and failure fracture were obtained. A deep learning-based image segmentation method was used to propose a three-dimensional visualization model of the internal damage of the composites, and the number, volume, spatial distribution and spatial angle of the microcracks generated due to the stretching were analyzed. At high loads, the tensile microcrack volumes are 0-5.3×10^-4 mm³, the fracture microcrack volumes are 0-2.2×10^-4 mm³, and the proportion of the microcracks with spatial angles of about 0° and 45° is relatively high. Meanwhile, the microcracks are closely correlated to the pore distribution. According to the results by fracture damage analysis, the tensile damage process of 2.5D-SiC_f/SiC composites includes matrix cracking, fiber fracture, fiber pull-out and interface debonding. The fiber pull-out lengths are in the range of 16.02-250.32 μm, and the average length is 88.26 μm. It is indicated that the image segmentation method based on deep learning could offer an effective way to reveal the mechanism of material damages and evaluate the degree of the damages.