Study on the evolution of concrete pore structure and moisture transfer mechanism after high temperature based on different sequence NMR technology.

This study investigates the influence of elevated temperatures on concrete's pore structure and moisture transfer mechanisms. Concrete samples exposed to temperatures of 200℃, 400℃, 600℃, and 800℃ were subjected to capillary water absorption (CWA) tests, and their properties were analyzed using Scanning Electron Microscopy (SEM) and Nuclear Magnetic Resonance (NMR) techniques employing different sequences (CPMG and SE-SPI). The results reveal after high temperature four distinct categories of pores within the concrete: micropores, mesopores, capillary pores, and macropores. Mesopores are in the highest abundance, followed by capillary pores, macropores, and micropores. As temperature increases, hydration reactions lead to an upsurge in micropores and a decline in the proportion of other pores. With further temperature escalation, the cement slurry's volume alteration causes pore expansion, resulting in a rise in capillary pores and macropores and a decrease in micropores. Furthermore, NMR technology, employing CPMG sequences, identifies three stages in the concrete's CWA process: rapid moisture transfer, moisture transfer rate descent, and moisture transition stage. SE-SPI sequence-based NMR results show that, following high-temperature exposure, moisture transfer within concrete progresses as follows: initially, water rapidly permeates mesopores and capillary pores from the exterior. As CWA time increases, these pores tend to saturate, and water gradually infiltrates micropores and macropores. High temperatures significantly impact the CWA process, enlarging pore sizes and altering the type of pore filling during capillary water absorption in concrete. • The transfer mechanism of water in concrete pores was analyzed in two and three dimensions based on different sequences of NMR technique tests such as Carr-Purcell-Mebum-Gill (CPMG) and Spin Echo Single Point Imaging (SE-SPI). • Based on NMR and SEM techniques to explore the types and percentage of concrete pores after high temperature. • After high temperatures, the distribution of pores in concrete was in the order of mesopores > capillary pores > macropores > micropores in terms of quantity. • There are three different stages in the process of water absorption in concrete: rapid moisture transfer, moisture transfer rate descent, and moisture transition stage. • After high temperature, water initially penetrates into the mesopores and capillary pores and then saturates and enters the micropores and macropores. [ABSTRACT FROM AUTHOR]