Fire resistance research of a new building material is the important part of public protection and necessary for its large scale use in construction project. Although cheap calcium carbonate whisker (CW) could improve the mechanical properties of cementitious composites under ambient temperature, there is no information about the influence of high temperature on CW reinforced cementitious composites. In order to bridge this gap, this research studied the strength and microstructure changes of CW and CW reinforced cement paste after exposed to evaluated temperatures. CW and Portland cement paste with 0%, 10%, 20% and 30% CW having water/cement ratio of 0.30 was exposed to the temperatures of 20, 200, 300, 400, 500, 600, 700, 800, 900 and 1000°C. 40 mm × 40 mm × 160 mm specimens were employed to determine the residual flexural and compressive strength of CW reinforced cement. Scanning electron microscope (SEM), matched energy dispersive spectrometer (EDS), thermal gravimetric analyzer (TGA) and X-ray powder diffractometer (XRD) were employed to detect the microstructure changes. CW could act as micro-fiber to increase the residual flexural and compressive strength of cement and the enhancement is better than carbon fiber and polypropylene fiber from room temperature to 600°C, which means that CW can be used to replace carbon fiber and polypropylene fiber as fire resistance additional material in cementitious composites. TGA presented that in the process of cement hydration and heating, CW may bring different hydration products phase: Ca(OH)2 and C–S–H (I). CaCO3 produced by cement hydration is not as stable as CW. The onset phase transformation temperature from aragonite CaCO3 to calcite is about 400°C, which can be proved by the XRD test and the morphologies under SEM. At the same time, internal autoclaving in cement paste produced new hydration products on the surface of the calcite CW. The bond between these new hydration products and calcite was stronger than the old hydration products and aragonite CW, proved by the EDS test. Hence, after expose to temperature range from 400°C to 600°C, CW demonstrates better reinforcing effect than that at lower temperatures. CW reinforced cementitious composites may possibly be employed as a protection material for fire easily damaged structures, due to its good residual mechanical properties after expose to evaluated temperature lower than 600°C and good economical efficiency.
