Analysis on pore characteristics of hybrid basalt-polypropylene fiber-reinforced concrete based on nuclear magnetic resonance technology

The pore characteristics of hybrid basalt-polypropylene fiber-reinforced concrete (HBPRC) was tested by Nuclear Magnetic Resonance (NMR). The effects of basalt fiber (BF) and polypropylene fiber (PF) as well as their blends on the compressive strength, porosity, pore size distribution and tortuosity of HBPRC were analyzed comparatively. The fractal dimensions of the pore structure in four pore-size regions were quantified based on the T₂ spectra of NMR and pore structure fractal theory. The results show that with the addition of BF, the T₂ spectrum reflects that the appropriate amount of BF can reduce the porosity of concrete, and it is beneficial to reduce the percentage of large pore volume. But as the PF content increases, the T₂ spectral area increases and there is a tendency for the pores of the concrete to become larger. Incorporation of BF-PF hybrid fibers produces a positive synergistic effect on the pore characteristics of concrete. The synergistic effect is optimal when the dosage of both BF and PF is 0.05%, which improves the compressive strength by 3.52%, reduces the porosity by 1.47%, increases the volume percentage of gel pores by 8.76%, increases the tortuosity by 8.20%, and reduces the volume percentage of large pores by 5.30%, and optimizes the distribution of pore size, as compared with ordinary concrete. There are obvious fractal characteristics in the pore structure of HBPRC, and the fractal dimensions of the pore structure of HBPRC increases sequentially in the region of transition pores, capillary pores, and large pores. In addition, the larger the fractal dimension, the greater the compressive strength. It is concluded from the microscopic analysis that the bonding state and distribution of fibers in the concrete matrix are the main reasons affecting the pore fractal characteristics of HBPRC.