Abstract: Concrete buildings exposed to the air for a long time will be carbonized by CO₂ gas in the air. The mechanical properties of concrete materials will change under carbonation, which will affect the safety of concrete structures. Based on the test, this paper studies the internal pore structure and tensile strength of plain concrete and basalt fiber concrete under carbonation by using rapid carbonation. The two kinds of concrete were carbonized rapidly at 0 d, 3 d, 7 d, 14 d and 28 d. The carbonation depth of the specimens at different carbonation ages was measured. The pore structure changes of two kinds of concrete at different carbonation ages were measured by nuclear magnetic resonance (NMR) device, and the static load splitting tensile test was carried out on the specimens by electro-hydraulic press. The results show that the carbonation depth of plain concrete and fiber concrete increases with the increase of carbonation age. The carbonation depth and rate of basalt fiber concrete are always lower than that of plain concrete. The addition of basalt fiber reduces the carbonation rate of concrete. With the increase of carbonation age, the peak value of T₂ spectrum of the specimen decreases, and the spectrum shifts to the left. Carbonation reduces the number of pores in the concrete material, reduces the pore diameter, and then reduces the porosity. There is a good exponential correlation between carbonation age and porosity, and the porosity of plain concrete is always higher than that of basalt fiber concrete. The proportion of micropores and mesopores in the specimen is more than 85.89%. The longer the carbonation age, the larger the proportion of micropores and mesopores in the specimen. Carbonation generates carbonate crystals that fill the cracks, reducing the proportion of macropores and cracks in the specimen. With the increase of carbonation age, the tensile strength of the two kinds of concrete increases first and then decreases. The early carbonation will reduce the porosity of the specimen, enhance the whole specimen, and its tensile strength will increase. The expansion force generated by the late carbonation will damage the specimen, and the tensile strength of the specimen will decrease. The strength decline of basalt fiber concrete is significantly lower than that of plain concrete. The addition of fiber enhances the carbonation resistance of concrete materials.

Key words: basalt fiber concrete, carbonization rate, pore structure, T₂ spectrum, tensile strength, composites