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Autoclaved aerated concrete (AAC) is widely used for its lightweight, high thermal insulation, and superior machinability. However, its loose and porous microstructure can cause a significant decrease in compressive strength after carbonation and the mechanism of the deterioration remains uncertain. This study delved into the alteration trends of AAC's compressive strength and dry density within carbonation environments. Additionally, a variety of testing techniques were employed to elucidate the microscopic mechanisms. The results showed that when the calcium/silicon ratio (Ca/Si ratio) stood at 0.75, AAC exhibited its highest degree of resistance against carbonation. Microscopic analysis revealed a progression in hydration product characteristics as the Ca/Si ratio increased. The transformation journey encompassed a shift from the initial structure to a needle-like tobermorite configuration and, subsequently, to a flaky structure. The study offers invaluable insights that furnish guidance for enhancing the durability of AAC and broaden the horizons of its potential applications. [ABSTRACT FROM AUTHOR]

• The effect of seawater and sea sand at different stages of preparation on the performance of AWC mortar was systematically studied. • The adverse effect of seawater on the properties of AWC mortar at the molding stage was found for the first time. • The mechanism of the influence of seawater on AWC properties at the molding stage was revealed. Seawater and sea sand are widely used in marine concrete to overcome the shortage of freshwater and river sand. However, the effect of the various salts in seawater and sea sand on the performance of anti-washout underwater concrete (AWC) is unclear. In this study, the influence of seawater and sea sand, as well as the aggressive ions in seawater, on the anti-washout ability, mechanical performance, and microscopic structure of AWC mortar are studied systematically. The AWC mortar molded in seawater has a lower compressive strength even though its indicators of anti-washout ability perform better than that in freshwater. This is because Mg2+ in seawater chemically binds with OH– in cement to form Mg(OH) 2 film absorbed by anti-washout admixture(AWA), which inhibits the release of substances in AWC while also introducing more defects in AWC. This study lays the foundation for promoting the application of seawater and sea sand AWC in marine engineering. [ABSTRACT FROM AUTHOR]

The anti-washout concrete (AWC) made with seawater and sea sand has a limited marine infrastructure application due to its unfavorable properties. Our study proposes an effective performance enhancement method for AWC using silica fume. A comprehensive examination was carried out to investigate the effects of silica fume on AWC's flowability, anti-washout ability, compressive strength, microstructure, permeability, and abrasion resistance, with the AWC prepared with seawater and sea sand, and molded in seawater. It was discovered that 10 % silica fume can most effectively improve the performance of AWC. This enhancement is due to the pore structure refinement realized with C-S-H gel generated from pozzolanic reaction. Exceeding 10 % silica fume content can have a detrimental effect on the slump-flow of mixtures, while only providing marginal improvements in impermeability and compressive strength. The study lays a fundamental groundwork for the wide use of sea-based resources and the application of AWC in marine engineering. • The science behind modification of AWC mortar molded in seawater using silica fume was revealed. • The effects of silica fume on the performance of AWC molded in seawater were thoroughly explored. • The optimal content of silica fume to improve performance of AWC molded in seawater was identified. [ABSTRACT FROM AUTHOR]

This study investigates the effects of lime-saturated water and water–CO 2 cyclic recuring on strength recovery of thermally damaged high-performance concretes (HPC). The HPC samples were subjected to elevated temperatures up to 1000 °C in 200 °C increments and underwent recuring. Phase assemblage and distribution, microstructure evolution, and pore structure of the HPC samples were identified. According to the results, recovered compressive strength of the HPC samples with low silica content can surpass their original strength after 600 and 800 °C exposure and recuring. In contrast, HPC with high silica content is unfavorable for strength recovery at temperatures above 800 °C because the low-calcium phases formed have low reactivity. After 1000 °C exposure, only water–CO 2 cyclic recuring coalesces the disintegrated microstructure and recovers the compressive strength. Strength recovery primarily depends on healing the microcracks and large pores rather than the coarsened cement paste. [ABSTRACT FROM AUTHOR]