Influence of Fly Ash Content on Macroscopic Properties and Microstructure of High-Performance Concrete

To investigate the influence of the fly ash (FA) content on the performance of high-performance concrete (HPC), seven groups of tests were conducted, aiming to evaluate both the macroscopic properties (workability and compressive strength) and microscopic pore structure. Low-field nuclear magnetic resonance (NMR) technology and SEM images were employed to analyze the changes in the internal pore structure of the concrete. The results showed that the workability of HPC initially increased and then decreased with the increase in the FA content. When the FA content was 15%, the slump of HPC reached a maximum of 264 mm, and the 28-day compressive strength exhibited a 23.2% increase compared to the 7-day compressive strength. The pore size distribution of the concrete varied with different fly ash content. At 15% FA content, secondary hydration of the FA was sufficient, refining the pores to between 0 and 0.1 µm. Excessive FA substitution deteriorated the internal structure of the HPC matrix and reduced the workability and mechanical properties of the HPC. When the content of FA was 35%, the slump of HPC decreased to 176 mm, while the macropores within the matrix significantly increased, resulting in porosity of 6.81%.

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