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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]