Analysis of the isothermal hydration heat of cement paste containing mechanically activated fly ash

Fly ash is a widely used supplementary cementitious material in the world. Mechanical activation, as a potential approach, can realize maximum bulk utilization of fly ash. In this study, the isothermal hydration heat flows of cement paste containing fly ash with or without mechanical activation are tested and modelled by Avrami equation, reaction order model and three-parameter exponential function at different stages. The results indicate that addition of 20% as-received fly ash retards the cement hydration at the nucleation and growth stage, because the aluminate ions in the pore solution might decrease heterogeneous nucleation sites of C-S-H and suppress its growth. Moreover, as-received fly ash impairs the reaction rate of the whole system at final tail stage owing to its low heat release. However, compared with as-received fly ash, mechanically activated fly ash has promoted cement hydration at the very early age due to the enhanced filler effect. More surface area supplied by mechanically activated fly ash promotes C-S-H growth and partially offsets the retarding effect of aluminate ions. The reaction rate of the whole cementitious system at the deceleration stage is also promoted by the improvement of pozzolanic reactivity after mechanical activation. Furthermore, three different mathematical calculation approaches have been adopted to calculate the ultimate hydration heat Qmax of the cementitious systems. In spite of different values for Qmax, the consensus can be achieved that mechanical activation towards fly ash can promote the reaction degree of the system.

Automated summary

The study found that mechanical activation can enhance the hydration reaction of fly ash with cement. Untreated fly ash can retard the cement hydration process, while mechanically activated fly ash can promote the hydration reaction of cement. This is because mechanical activation increases the filler effect of fly ash and increases its surface area, which is beneficial for the growth of C-S-H. Furthermore, mechanical activation can also improve the pozzolanic reactivity of fly ash and increase the reaction rate of the entire cement system.