The average salinity of seawater is 3.5%, with a significant presence of corrosive ions, primarily Cl- and SO42-. In contrast to cement engineering in terrestrial natural environments, cement-reinforced structures exposed to corrosive marine environments not only endure ion erosion but also undergo periodic desiccation due to tidal variations in seawater. The coupling of these effects results in a reduction in the mechanical properties of cemented soil, inevitably leading to the degradation of cemented foundations, posing a serious threat to their safety and normal functionality. Investigating the improvement of the mechanical properties of cemented soil in corrosive coastal environments is a crucial engineering challenge in current coastal construction projects. To address this engineering challenge, this study proposes the use of Nano-SiO2 to enhance the mechanical characteristics of cemented soil, aiming to improve the strength and durability of cement-reinforced structures. Simulating the main corrosive ions in seawater by using different concentrations of SO42- ions, the study subjected cemented soil samples to dry-wet cycles to simulate the desiccation caused by tidal changes in seawater. Unconfined compressive strength tests were conducted on cemented soil and nano-cemented soil samples under coupled conditions, revealing that the incorporation of Nano-SiO2 increased the strength of cemented soil and slowed down the corrosion rate. With an ion concentration of 12.3 g/L, after 60 dry and wet cycles, the compressive strength of nano-cemented soil increased by 90% compared to conventional cemented soil, with a mass loss only half that of conventional cemented soil. XRD, SEM, and NMR tests on various cemented soil samples indicated that the addition of Nano-SiO2 filled small pores, suppressed pore development, and interacted with cement hydration products, forming a gel-like structure that improved the compactness of cemented soil. This, in turn, mitigated ion corrosion and the degradation of cemented soil under dry-wet cycles.

Downward transport of stratospheric air into the troposphere (identified as stratospheric intrusions) could potentially modify the radiation budget and chemical of the Earth's surface atmosphere. As the highest and largest plateau on earth, the Tibetan Plateau including the Himalayas couples to global climate, and has attracted widespread attention due to rapid warming and cryospheric shrinking. Previous studies recognized strong stratospheric intrusions in the Himalayas but are poorly understood due to limited direct evidences and the complexity of the meteorological dynamics of the third pole. Cosmogenic S-35 is a radioactive isotope predominately produced in the lower stratosphere and has been demonstrated as a sensitive chemical tracer to detect stratospherically sourced air mass in the planetary boundary layer. Here, we report 6-month (April-September 2018) observation of S-35 in atmospheric sulfate aerosols ((SO42-)-S-35) collected from a remote site in the Himalayas to reveal the stratospheric intrusion phenomenon as well as its potential impacts in this region. Throughout the sampling campaign, the (SO42-)-S-35 concentrations show an average of 1,070 +/- 980 atoms/m(3). In springtime, the average is 1,620 +/- 730 atoms/m(3), significantly higher than the global existing data measured so far. The significant enrichments of (SO42-)-S-35 measured in this study verified the hypothesis that the Himalayas is a global hot spot of stratospheric intrusions, especially during the springtime as a consequence of its unique geology and atmospheric couplings. In combined with the ancillary evidences, e.g., oxygen-17 anomaly in sulfate and modeling results, we found that the stratospheric intrusions have a profound impact on the surface ozone concentrations over the study region, and potentially have the ability to constrain how the mechanisms of sulfate oxidation are affected by a change in plateau atmospheric properties and conditions. This study provides new observational constraints on stratospheric intrusions in the Himalayas, which would further provide additional information for a deeper understanding on the environment and climatic changes over the Tibetan Plateau.