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The mechanical properties, phase evolution, microstructure development and (re)hydration properties of limestone calcined clay cement (LC3) paste and mortar were investigated to understand their thermal stability under moderate temperatures (100–400 °C). The residual compressive strengths of LC3 and Portland cement (PC) mortars kept constant or slightly increased from ambient to 300 °C, and sharply dropped at 400 °C. At 100 °C, monocarboaluminate (Mc) and ettringite (AFt) converted into hemicarboaluminate (Hc) and monosulfate (Ms), and caused partial dehydroxylation of C-(A)-S-H. As the temperature increased, at 200–300 °C, due to the decomposition of large fraction of hydration products, macroscopic cracks and microstructure damage occurred. At 400 °C, the mean chain length (MCL) of C-(A)-S-H gel decreased, accompanied by the shift of Si and Al-coordination number and the formation of active sites. Under water saturation conditions, the microstructure showed self-repairing performance. [ABSTRACT FROM AUTHOR]