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This article presents the results of an experimental study to reveal the effects of calcined clay (CC) on the combined internal chloride and external sulfate attack (IC-ESA) of cast-in-situ concrete. Specimens are immersed in distilled water (DW) and 10% Na2SO4 solution (S10). Compressive strength, flexural strength, weight and size change, and sulfate penetration depth of CC blend cement cast-in-situ concrete (with wt% 0%, 5%, 10%, 15%, and 20%) are investigated. Microstructure and mineral composition are analyzed by X-ray diffraction (XRD), SEM-EDS, and TG/DTG methods. Results indicate that internal chloride attack (ICA) and external sulfate attack (ESA) accelerate the early hydration process of concrete and inhibit later strength development and specimens exposed to IC-ESA degrade faster. CC improves concrete strength and enhances corrosion resistance. When the replacement ratio is 5%, the performance reaches optimal levels. CC inhibits sulfate-induced expansion by limiting the formation of expansive acicular ettringite and improving concrete strength. [ABSTRACT FROM AUTHOR]