The variation of residual compressive strength of C30 calcareous aggregate concrete after elevated temperature was studied.Microscopic investigation on hardened cement paste(HCP) which had the same water to cement ratio and the same process methods of heating and cooling with the concrete, was conducted by thermogravimetric(TG) analysis, scanning electron microscopy(SEM) test and mercury intrusion porosimetry(MIP) test. The results indicate that the secondary hydration effect of HCP at medium and low temperatures(100300℃) has significant influence on the residual compressive strength of calcareous aggregate concrete at correspondent temperature interval.Meanwhile, the residual compressive strength of calcareous aggregate concrete has a good inverse correlation with the porosity of HCP after elevated temperature.