This study aims to examine the thermomechanical properties of high-volume fly ash (HVFA) concrete for mass concrete applications. To achieve this, compressive strength, elastic modulus, adiabatic temperature rise, and thermal conductivity tests were conducted by varying the fly ash substitution ratio and Blaine fineness, along with the addition of limestone powder. Furthermore, 1200 × 1200 × 1200 mm cube-shaped mock-up members were fabricated to measure the temperature history and analyze the thermal crack index. Finite element analysis (FEA) was also performed to assess the temperature distribution and thermal stresses in HVFA concrete. The experimental results demonstrated that HVFA concrete achieves long-term compressive strength development due to the pozzolanic reaction of fly ash—a process that can be accelerated by utilizing more finely ground fly ash. Additionally, the compressive strength of HVFA concrete exhibited greater sensitivity to variations in curing temperature compared to that of normal concrete. The maximum adiabatic temperature rise of HVFA concrete decreased to 22.3 % of that of normal concrete, with this reduction becoming more significant as the fly ash substitution rate increased. The high fineness of fly ash and the addition of limestone powder increased the adiabatic temperature rise by 31.9 % and 14.9 %, respectively, compared to HVFA concrete with the same substitution rate. The mock-up test revealed that the temperature at the center of the HVFA concrete specimen decreased by 32.9 % relative to that of normal concrete and that resistance to thermal cracking improved with higher fly ash substitution rates. Further FEA validated the experimental results, demonstrating that fly ash substitution helps mitigate the risk of cracking by reducing thermal stresses in mass concrete structures.