Although significant theoretical and technological advancements have been made in the application of concrete in saline soil regions over the past two decades, newly constructed reinforced concrete structures in these areas still face severe issues of corrosion and degradation. This is due to the complex deterioration environment in saline soil regions, characterized by the combined effects of salt corrosion, dry-wet cycles, and freeze-thaw conditions. The reduced service life of concrete structures in this region is closely related to the diffusion and distribution patterns of high-concentration chloride salts and various corrosive ions within the concrete. These patterns affect the content, transformation, and microstructure of corrosion products, ultimately leading to a shorter service life compared to other environments. This paper simulates the saline-alkali soil environment using solutions of different concentrations of chloride sulfates and magnesium salts, studies the diffusion and distribution patterns of chloride ions and sulfate ions in concrete under this environment, and analyzes the mechanism of action in conjunction with changes in microstructure. The experimental system adopts a dry-wet cycle test that can represent the characteristics of the semi-arid continental climate in Western China. The results show that although the content of free chloride ions and total chloride ions entering the concrete in the saline-alkali soil simulation solution is the lowest, the binding capacity of chloride ions is significantly greater than that of sulfate ions and far exceeds that in other environments. Under the action of high-concentration chlorides alone, the content of chloride ions in concrete is the highest, and the binding capacity of chloride ions also increases with the concentration of chlorides. The content of free sulfate ions and total sulfate ions entering the concrete in the saline-alkali soil simulation solution and their binding capacity are higher than in the control solution. Due to the ability of sulfate ions to hinder the diffusion of chloride ions in concrete, magnesium ions play a hindering role in the early stage and an accelerating role in the later stage. This results in concrete corroded by the saline-alkali soil environment, which has a characteristic of low chloride ion content and high sulfate ion content. The ions in the saline-alkali soil solution that cause concrete damage are Cl-, SO42-, and Mg2+. These ions react with the concrete to form Friedel's salt, Aft and AFm phase calcium aluminate, gypsum, Mg-S-H, and Mg(OH)2, among other substances. These corrosion products significantly impact the microstructure of concrete, causing the microstructure of concrete to transition from dense to loose to cracked much earlier than in other environments.

The massive accumulation of waste PET plastic (WP) and coal gangue (CG) would induce a series of environmental problems such as causing soil and water pollution. For reducing the environmental pollution induced by these two wastes, this study attempts to utilize the combination of WP and CG into cement-based materials. Cement mortars incorporated with fine waste plastic (FWP) replacing part of sand and concrete blended with CG and coarse waste plastic (CWP) as part of coarse aggregate were prepared and their work-ability, mechanical strengths, dynamic elastic modulus (DEM), chloride ion permeability, hydration and microstructures were systematically investigated. In addition, metakaolin (MK) as a kind of active admixture was added into mortar or concrete and its effect of MK on the property of cement mortar or concrete was evaluated. The results show that the strengths of cement mortars containing various level of FWP decrease with increase of FWP and CG level. The mechanical strengths of concrete containing MK and 25-100 % CG and CWP are appropriate at different ages. Although the strengths of concrete blended with MK and wastes aggregate are lower than that of concrete without wastes, it is obviously higher than that of concrete only containing wastes but not MK. Its slump of fresh concrete significantly declines with CWP and CG contents growth. The coulomb electric flux and chloride migration coefficient of concrete at 28d generally increase with CG and CWP level, which indicates a declined tendency of resistance to chloride ion penetration. Its DEM for concrete measured with ultrasonic testing method slightly decrease with rise of CG and CWP content (25-100 %) and can give a basic prediction of strengths and chloride ion permeability. Hydration and microstructures tests including TG/DTA, MIP and SEM/EDS demonstrate that the pozzolanic reaction of MK can result in more gels generated and strengthen the ITZ between WP or CG and cement paste thus evidently improving its mechanical and durability of concrete when compared to the reference specimen without MK. Although the properties of concrete blended with CG and CWP as part of coarse aggregate are inferior to pure natural gravel contained concrete, its strengths and resistance to chloride ion permeability can achieve requirements of engineering structures.

Oceans and saline soil environments strongly demand for concrete with high compressive strength and high chlorine salt corrosion resistance. Herein, a new preparation process of geopolymer concrete with high compressive strength and chlorine salt corrosion resistance was established, and the process was optimized by adjusting the water/cement ratio, the proportion of Na2O and the amount of fly ash. Mechanical properties tests show that the compressive strength of geopolymer concrete increases with increase in Na2O proportion but decreases with increase in water/cement ratio and fly ash. The compressive strength of geopolymer concrete is as high as 96.20 MPa, when the water/cement ratio is 0.6, the proportion of Na2O is 0.12, and the amount of fly ash is 10%. This may be because the C-(A)-S-H gel makes the geopolymer concrete denser. At the same time, electrochemical impedance spectroscopy and Tafel results imply that the geopolymer concrete also has great chlorine salt corrosion resistance, the lowest weight loss rate of steel bar is only 0.06% after 240 h accelerated corrosion. The leaching tests indicate that at the same depth, the total and free chloride ions in geopolymer concrete are minimum and some chloride ions are combined. The high chlorine salt corrosion resistance could be attributed to the increase in C-(A)-S-H gel which refines the pore structure of concrete, improves concrete compactness and binds the chloride ions. This paper provides a new method for the high-quality utilization of solid waste and a potential clue for the preparation of high-performance concrete.

The issue of ocean corrosion in coastal areas, particularly in concrete structures partially buried in the soil has attracted wide attention has garnered attention due to the unique challenges it presents. This study investigates the enhanced chloride ions intrusion in concrete due to bidirectional unsaturated gradients between the parts buried in soil and those exposed to the air. A 90-day chloride salt erosion experiment was conducted on both fully buried and semi-buried concrete across five different soil environments to analyze the transport properties of chloride ions. Drilling powder sampling and the detection of free chloride ion content in the concrete specimens were performed at intervals of 30, 60, and 90 days. The results indicate that the chloride ions in semi-buried concrete exhibit bidirectional unsaturated migration toward the airexposed end, leading to significantly peak chloride ion concentrations at the air-exposed end compared to those subjected to wet-dry cycles in a pure salt solution. Specifically, after a 90-day cycle, the total chloride contents at the air-exposed end of semi-buried concrete in pure sandy soil, 50 % clay soil, and pure clay soil increased by 68.8 %, 43.1 %, and 16.4 %, respectively. Soils with lower permeability intensified the vertical unsaturation gradient within the concrete, accelerating chloride ion migration towards the exposed end and resulting in higher accumulation at elevated positions. The research work underscores the critical impact of bi-directional unsaturated transport on concrete durability in coastal environments and calls for a deeper understanding of its applications for corrosion prevention.