Multiscale Evaluation of the High-Temperature Performance of Cement Mortars Containing Class C and F Fly Ash

The incorporation of industrial byproducts into concrete is crucial for utilizing waste materials, reducing clinker use, and increasing the durability of cement-based materials. This study investigates the high-temperature exposure performance of cement mortars containing Class C and F fly ash from a multiscale perspective. Fly ash was used as a partial cement substitute at percentages of 5%, 10%, 15%, and 20%, and the prepared mortars were subjected to temperatures of 400, 600, and 800 °C. Flexural and compressive strengths, microstructural properties, X-ray microcomputed tomography (micro-CT) data, concrete petrography, and thermogravimetric analysis (TGA) characteristics were systematically examined to evaluate changes in mechanical performance, pore structure, microstructure, and thermal stability. The results indicated that the highest compressive strength values in mixtures subjected to 800 °C were obtained at a 10% replacement rate. Compared to the 5.64 MPa compressive strength of the reference mortar, this value was found to be 12.96 MPa in mortars containing 10% Class F fly ash and 11.86 MPa in mortars containing 10% Class C fly ash. Micro-CT and microstructural observations supported the finding that fly ash substitution reduced porosity and limited cracking, while thermogravimetric results supported the relationship between thermal stability and sustained mechanical performance. This study aims to contribute to the development of high-temperature-resistant cementitious materials by recycling fly ash, an industrial waste product.