How to improve the quality of fly ash?

The hydration reaction of fly ash lags behind that of cement, and the reaction is relatively slow, so its early activity is low and its contribution to the early strength of concrete is not significant. It is necessary to improve the mid to late strength and durability of concrete by adding high-efficiency water reducing agents and reducing the water cement ratio. However, it cannot fundamentally solve the problem of low early strength of fly ash concrete. Therefore, it is necessary to consider tapping into the potential of fly ash itself and suggest taking corresponding measures. Practice has proven that using methods such as mechanical grinding and chemical excitation to improve the early activity of fly ash is undoubtedly of great significance for the promotion and application of fly ash.
After mechanical grinding, the thin-walled hollow particles of undisturbed fly ash (FA) are crushed, and the small fish egg like particles inside are exposed and dispersed. Sponge like porous composite beads and various cohesive substances are easily crushed, exposing new surfaces. The coarse obtuse angle particles and debris are refined, and the particle shape is improved. Due to the increase in specific surface area of fly ash particles and the presence of external damage, its early hydration reaction is greatly accelerated.
The use of fly ash activator can improve the early strength of fly ash and is crucial for achieving high dosage fly ash technology. Among commonly used active activators, aluminates and strong acid salts have better excitation effects. For high dosage fly ash cement systems, organic and inorganic composite activators mainly composed of Na2SO3 · UEA · CaO, triethanolamine, etc. can achieve a significant increase in early strength and a sustained increase in later strength.
The fly ash HPC prepared by replacing 20-45% cement and high-efficiency composite water reducing agent with fly ash composite ultra-fine powder (PFAC) has been proven to have excellent workability, mechanical properties, volume stability, and durability. The so-called PFAC refers to the composite modification of ultrafine ground fly ash by adding a small amount of certain inorganic active activator CF.
In summary, mechanical grinding has a certain effect on improving the flowability and strength of fly ash mortar, while the active activator CF can significantly improve the compatibility between cementitious materials and high-efficiency water reducing agents, enhancing their early strength and flexural strength. Comparing the dry shrinkage rates of FA and FAC, it can be seen that the addition of CF greatly reduces the dry shrinkage of mortar specimens and improves their volume stability. However, comparing the dry shrinkage rates of FAC and PFAC indicates that from the perspective of dry shrinkage, the fineness of fly ash should not be too large. In addition, it is known from the experiment that CF also has a significant enhancing effect on the wear and corrosion resistance of concrete.