Study on the Influence of Air-Entraining Agent on Concrete Pore Structure Based on Nuclear Magnetic Resonance Technology

In this study, the effects of air-entraining agent dosage and freeze-thaw on the microporous characteristics of concrete were systematically investigated. By testing the working performance of fresh concrete, the change of elastic modulus after freezing and thawing, combined with nuclear magnetic resonance technology, the mechanism of air-entraining agent on pore structure parameters was revealed. The test found that the appropriate amount of air-entraining agent could improve the workability of the mixture and enhance the frost durability of concrete. With the increase of the amount of air-entraining agent, the internal porosity of the material continued to increase, and the pore size distribution showed an evolution law of first optimization and then deterioration. When the dosage of air-entraining agent was 0.04%, after 250 freeze-thaw cycles, the proportion of beneficial pores in concrete reached a peak, which was higher than that of other dosage groups. However, when the dosage increased to 0.06%, not only the number of harmful pores increased significantly, but also the excessive porosity and complex pore morphology led to the deterioration of frost resistance. This study provides an important technical basis for the design of engineering concrete mix ratio in cold areas.