Influence of fly ash content on macroscopic properties and microstructure of high-performance concrete

To study the influence of fly ash content on the performance of high-performance concrete (HPC), Seven groups of tests were conducted to evaluate macroscopic properties (workability and compressive strength) and microscopic pore structure. Low field nuclear magnetic resonance (NMR) technology and SEM images were used to analyze the changes in the internal pore structure of the concrete. The results show that the workability of HPC initially increases and then decreases with the increase in fly ash content. When the content is 15%, the slump of HPC reaches a maximum of 264 mm, and the 28-day compressive strength increases by 23.2% compared to the 7-day compressive strength. The pore size distribution of the concrete varies with different fly ash contents, with over 95% of the pores in each group measuring less than 0.1 µm. At 15% fly ash content, secondary hydration of the fly ash is sufficient, refining the pores between 0 and 0.1 µm.