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2 2024

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Investigation on the impact of low atmospheric pressure on bubble stability in cement‑based materials

Lan Xu-li, Zeng Xiao-hui, Zhu Hua-sheng, Teng Guo-cui, Long Guan-cheng, Tang Zhuo, Xie You-jun

Archives of Civil and Mechanical Engineering

2024

Vol. 24, no. 1

art. no. e33, 2024

With more infrastructures being built in high-altitude regions, the impact of low atmospheric pressure on the stability of air bubbles in cementitious materials has aroused extensive attention. However, its influence is still inconclusive since it is difficult to isolate the factor of air pressure and study it. To solve this problem, the study investigated the bubble dynamic evolution in cement paste at standard atmospheric pressure (P = 0.1 MPa) and low atmospheric pressure (LAP = 0.8P, 0.6P, 0.4P, 0.2P) and revealed the mechanism of bubble instability under low atmospheric pressure. We established the buble dynamic evolution equation in cement paste, computed the real-time bubble radius, resultant force, and critical time at different air pressure by MATLAB software. Results show that the maximum resultant force decreases by 50.0–57.7% when air pressure drops from P to 0.2P; both critical time ( t ) and radius increment (ΔR) increase with the reduction of air pressure and the increase of initial radius. It indicates that when at 0.2P, the resultant force which inhibits bubble expansion is reduced, the time required to prevent bubble expansion increases by 94–137%, and ΔR grows 122–140% larger than that at P, and the bubble is more prone to instability and rupture than that at P. The key to improving bubble stability in low atmospheric pressure is to reduce the initial radius of bubbles and increase the strength of the bubble film. This work firstly discovered the impact of low atmospheric pressure on bubble dynamic evolution in cement paste and proposed a new perspective to explore bubble stability at low atmospheric pressure.

Effect of nano‑SiO2/CaSO4 whisker–silica fume on the strength, drying shrinkage, and capillary water absorption of cement mortar

Wang Han, Zeng Xiaohui, Liu Zhiru, Luan Shuoxing, Tang Zhuo, Long Guangcheng, Tan Qingquan, Wang Haixu, Liu Suihu, Ren Xin, Ma Gang, Lan Xuli

Archives of Civil and Mechanical Engineering

2024

Vol. 24, no. 1

art. no. e13, 2024

To compensate for the defects of silica fume (SF), which aggravates drying shrinkage, and increase the strength and durability of cement mortar, it was modified using 1 wt% nanosilica (NS) or calcium sulfate whisker (CSW) compounded with 2 wt% SF. The strength, volume stability, and durability of the cement mortar were characterized using the compressive strength, drying shrinkage, and capillary water absorption. The physical composition and microstructure of the samples are discussed in detail based on thermogravimetry, mercury intrusion porosimetry, and scanning electron microscopy measurements. The combination of SF and NS increased the generation of C–S–H gels and reduced the total porosity, thus increasing the early strength of the cement mortar and decreasing the capillary water absorption. Notably, on combining SF and CSW, additional calcium aluminate hydrates (AFts) were generated, and the mesoporosity (10–50 nm) was reduced. The fibrous AFt phase increased the later strength of the cement mortar, and the presence of additional AFt increased the solid phase volume, which compensated for the drying shrinkage.