Hydration products of fly ash cement-based material

Morphology, microstructure, chemical composition of hydration products, and pore structure for cement pastes with different fractions of fly ash were investigated by thermogravimetry-differential scanning calorimetry, scanning electron microscope, transmission electron microscope, energy dispersive X-ray analysis, high-resolution transmission electron microscope, and mercury intrusion porosimetry. Effects of different dosages of fly ash on characteristics of hydration products and pore structure were also discussed. Results show that the chemical composition of C-S-H gel can be improved by doping in larger fraction of fly ash. In late of hydrate stage, fly ash reacts with Ca(OH)₂ and C-S-H gel with high molar ratio of CaO and SiO₂ to produce C-S-H gel with lower molar ratio of CaO and SiO₂, which is called secondary pozzolanic reaction. The C-S-H gel can bind more alkali to alleviate alkali-aggregate reaction. The second pozzolanic reaction products can also fill in the pores, those are disadvantage to the strength and durability of cement paste. It can optimize the pore structure of cement paste to make the cement structure denser, which is helpful to the durability of concrete.