This paper studies the chloride diffusivity of concrete subjected to long-term exposure in a simulated marine environment with sustained compressive loading for up to 224 days. A series of concrete prisms were loaded under five different compressive stress levels (0, 0.2, 0.3, 0.5, and 0.7fc) and were subjected to different numbers (4, 8, 12, and 16) of wet/dry seawater exposure cycles. At the end of each exposure period, the concrete specimens were analyzed to determine their chloride penetration profile, the apparent chloride diffusion coefficient Da of the concrete, and the surface chloride content Cs. It was found that the Da and Cs values are both time and stress dependent and that these two dependencies have complex interactions. The value of Da decreases with time due to the hydration of the cement matrix, whereas the value of Cs increases over time. The effects of compressive stress on the values of Da and Cs strongly depend upon the applied stress level, which has a threshold value at app...

The research reported herein studied the permeability of concrete containing recycled-concrete aggregate (RA), superfine phosphorous slag (PHS), and ground granulated blast-furnace slag (GGBS) with and without stress. Test results showed that the chloride diffusion coefficient of RA concrete (RAC) without external loads decreased with time, and the permeability of RAC is much lower than that of the reference concrete due to the on-going hydration and the pozzolanic reaction provided by the PHS and GGBS additives in the RAC mixture. The permeability of chloride under flexural load is much more sensitive than that under compressive load due to the differences in porosity and cracking pattern. At low compressive stress levels, the permeability of chloride decreased by the closing of pores and microcracks within RAC specimens. However, in a relatively short time the chloride diffusion coefficient and the chloride content increased rapidly with the increase of compressive stress when it exceeded a threshold stress level of approximate 35% of the ultimate compressive strength. Under flexural stress, the chloride transport capability increased with the increase of stress level and time. At high compressive and flexural stress levels, creep had a significant effect on the permeability of chloride in the RAC specimens due to the damage from the nucleation and propagation of microcracks over time. It is apparent that mortar cracking has more of a significant effect on the chloride transport in concrete than cracking in the interfacial transition zone (ITZ).

The construction of lunar bases represents a fundamental challenge for deep space exploration, lunar research, and the exploitation of lunar resources. In-situ resource utilization (ISRU) technology constitutes a pivotal tool for constructing lunar bases. Using lunar regolith to create geopolymers as construction materials offers multiple advantages as an ISRU technique. This paper discusses the principle of geopolymer for lunar regolith, focusing on the reaction principle of geopolymer. It also analyzes the applicability of geopolymer under the effects of the lunar surface environment and the differences between the highland and mare lunar regolith. This paper summarizes the characteristics of existing lunar regolith simulants and the research on the mechanical properties of lunar regolith geopolymers using lunar regolith simulants. Highland lunar regolith samples contain approximately 36% amorphous substances, the content of silicon is approximately 28%, and the ratios of Si/Al and Si/Ca are approximately 1.5 and 2.6, respectively. They are more suitable as precursor materials for geopolymers than mare samples. The compressive strength of lunar regolith geopolymer is mainly in the range of 18~30 MPa. Sodium silicate is the most commonly utilized activator for lunar regolith geopolymers; alkalinity in the range of 7% to 10% and modulus in the range of 0.8 to 2.0 are suitable. A vacuum environment and multiple temperature cycles reduce the mechanical properties of geopolymers by 8% to 70%. Future research should be concentrated on the precision control of the lunar regolith’s chemical properties and the alkali activation efficacy of geopolymers in the lunar environment.

Chloride transport in structural concrete has been found to be both stress- and time-dependent. Several models have been developed to take into account the time dependency of the chloride diffusivity of concrete. However, less is understood about the stress dependency and the interaction between the two dependencies. Experiments were conducted to investigate chloride transport in concrete subjected to different tensile stress levels and different periods of wet–dry cycling exposure (56, 112, 168 and 224 d). Two key parameters were investigated – surface chloride content (C_s) and the apparent chloride diffusion coefficient (D_a). It was found that regardless of the tensile stress level, the values of D_a and C_s respectively decreased and increased with time due to the continuous hydration of cement and the periodic surface wetting with chloride solution. However, both the decreasing and increasing rates were found to be dependent on the tensile stress levels. Both D_a and C_s increased with the tensile stress level due to the opening of pores and the development of microcracks in concrete. Practical models were developed to reflect the above time and stress dependency of D_a and C_s, as well as their interactions. The models were validated through comparisons of the experimental and predicted chloride profiles of all the tested specimens.