Hydration and Air Entrainment Challenges of High-Volume Fly Ash Concrete Pavement

The goal of high-volume fly ash concrete (HVFAC) is to produce concrete pavements at a lower cost and carbon footprint while maintaining its desired durability. Previous research has demonstrated that the required fresh and hardened concrete properties can be achieved at higher replacement rates of cement with fly ash such as 40%. However, most transportation agencies do not permit more than 30% cement replacement with fly ash primarily because of the potential inconsistencies in early-age properties such as variable air entrainment, delays in setting times, and lower strength gains. In this paper, the heat evolved during hydration of HVFAC are presented with respect to the source of the cement and fly ash, the variability of fly ash from the same source, and addition of nano limestone. Isothermal calorimetry showed longer setting times were dependent on the specific fly ash-cement combination as well as the degree of sulfate imbalance. For this study, HVFAC mixes with class C fly ash had a larger sulfate imbalance than class F fly ash with final setting times 4.5 hours and 1.9 hours longer than straight cement system, respectively. Replacing cement with 10% nano limestone in HVFAC system accelerated the initial set time by 3.2 hours which was much greater than the set time acceleration (1.3 hours) with the replacement of straight cement with 10% nano limestone. The various types of inorganic and organic carbons in fly ash remain a challenge for predicting and maintaining air content but the foam index still offers a rapid and straightforward quality control test with operator variability within ±1 µL AEA/gm fly ash.