Antagonistic Effects of Hydrated Lime and Calcium Formate on Early-Age Strength in High Volume Fly Ash Composites: Mechanisms and Engineering Implications

The utilization of high-volume fly ash (HVFA, ≥50% cement replacement) in concrete is pivotal for sustainable construction but hindered by low early-age strength. This study investigates the individual and combined effects of hydrated lime (HL) and calcium formate (CF) on the strength development, hydration kinetics, and microstructure of HVFA pastes (60% and 70% FA). Individual additions of 11% HL (HVFA60) or 14% HL (HVFA70) raised 28-day compressive strength by 18% and 22%, respectively, and shortened final setting from 10.0 h to 3.8 h. Similarly, 3% CF increased 28-day strength by 15% (HVFA60) and 12% (HVFA70) while cutting final setting to 2.1 h and 3.3 h. In contrast, combining HL and CF suppressed strength by 15–22% despite accelerating final setting to less than 1 h. Isothermal calorimetry showed a 40% reduction in cumulative heat release at 44 h for the combined system. XRD, TGA and SEM confirmed 20–30% lower C-S-H content, 25% less CH, and a rise in porosity when HL and CF were used together. These findings demonstrate that HL and CF act as competing accelerators, where rapid heat release compromises microstructural integrity. For practical applications using HVFA materials, individual use of HL or CF is recommended to enhance early-age performance, while combined application should be avoided to prevent strength reduction.