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Hydrophobic treatment for cementitious material is one of the effective methods to improve the durability of concrete structures. In this study, the pore system of cementitious materials modified by a new type of carboxylic acid hydrophobic agent is reconstructed by X-ray computed tomography technology. The changes in porosity, the connectivity degree in 2D / 3D space, the tortuosity and permeability of samples with hydrophobic agents are investigated. The experimental and simulation results show that the addition of a hydrophobic agent will tremendously increase the volume of a large pore but the solid phase observed in pore space can be confirmed to be the calcium carboxylate flaky crystals which modify the water transportation channels. In conclusion, the change of transportation channel from the surface can explain the hydrophobicity of cement pastes in the presence of a hydrophobic agent. This study provides a new perspective to reveal the hydrophobic mechanism of cement-based materials and guidance to design the new microstructure of cementitious materials [ABSTRACT FROM AUTHOR]

Carbonation resistance ability is one of the most important durability-related proprieties of cement-based materials. Through the carbonation depth experiment, isothermal conduction calorimetry, XRD, BET, and water vapor sorption, the effect of calcium nitrite (Ca(NO3)2) on the carbonation properties of cement-based materials is obtained. The result indicates that the addition of Ca(NO3)2 improves the carbonation resistance property of cement-based materials if the hydration of cement pastes and microstructure is modified earlier without affecting the late hydration process. In addition, the refined pores and higher tortuosity cut down the channels, thereby impeding the ingress of carbon dioxide gas into cementitious materials, as confirmed by BET and water vapor sorption. The Ca(NO3)2 exhibits high performance in improving the carbonation resistance and extending the life of strengthened concrete. [ABSTRACT FROM AUTHOR]

The low toughness under the tension of cement and concrete materials has been a long‐standing issue for decades and it has become increasingly urgent to address in modern society due to the growing demand for the development of high‐performance and sustainable constructions. Manipulating calcium silicate hydrate (C‐S‐H), the main hydration product of Portland cement, which determines the mechanical properties of cementitious materials, is an attractive method for improving their toughness following a bottom‐up approach. Inspired by the microstructure of nacre, a high energy absorption C‐S‐H‐based composite with a highly ordered structure is fabricated by a designed ternary building block, in which exfoliated montmorillonite provides a template for the nucleation and growth of C‐S‐H generating the "brick", and polyvinyl alcohol acts as a "mortar" binding all the building blocks together. With the hierarchical toughening strategy explored here, the obtained C‐S‐H composite achieves a remarkable energy absorption of 16.2 ± 2.6 MJ m−3, which surprisingly outperforms the ultra‐high toughness cementitious materials by a factor of 20–60 and is even higher than that of natural nacre and other nacre‐like composites. These findings not only provide valuable insights into enhancing the toughness of cementitious materials but also open possibilities for broadening potential applications of C‐S‐H. [ABSTRACT FROM AUTHOR]

To investigate the early mechanical strength and water penetration resistance of tunnel shotcrete under full-section construction method, a case study is conducted on an inclined shaft of the Zhengzhou-Wanzhou railway constructed by full-section method. Then, the adaptation between designed strength grade of shotcrete and full-section construction method is analyzed via concrete performance test, actual construction verification, and hydration microscopic analysis. The test results reveal the following: (1) The early age mechanical strength of shotcrete per hour develops faster than molded concrete. The eight-hour compressive strength of shotcrete increases from 3.2 to 7.0 MPa when increasing the designed strength grade from C25 to C40. (2) The water penetration resistance of shotcrete is positively and linearly related to 28-day compressive strength. (3) The magnitude and compactness of plate-like C-S-H in the C35 shotcrete perform better than C25 shotcrete, which can reduce the porosity of hydration microstructure induced by rod-like ettringite crystal, thus increasing 28-day compressive strength and enhancing water penetration resistance. (4) The eight-hour mechanical strength of primary support structure after applying C35 high-performance shotcrete reaches 6.0 MPa, effectively reducing the early and accumulated displacement deformation of primary support. [ABSTRACT FROM AUTHOR]