Abstract
The use of fly ash as a supplementary cementitious material in concrete is increasingly important due to environmental sustainability and enhanced performance requirements. This study examines the effect of fly ash replacement levels of 10%, 20%, and 30% by cement weight on the properties of B20-grade concrete. Fresh concrete showed reduced workability: slump decreased from 200 mm to 190 mm, 158 mm, and 136 mm (by 5%, 21%, and 32%), while air content dropped from 3.1% (control) to 2.8%, 2.9%, and 2.5% respectively. Hydration heat monitoring revealed significant reduction in peak temperature from 26.2°C (plain concrete) to 25.8°C, 25.7°C, and 25.0°C. Compressive strength at 28 days reached 97%, 86%, and 72% of the control mixture. The results confirm that 10–20% fly ash replacement optimally reduces hydration heat in massive structures while maintaining sufficient strength development, offering practical recommendations for sustainable concrete design in construction practice.