Mechanism of Hydration of Cement Blended with Fly Ashes

Selective dissolution of different Belgian fly ashes (FAs) with water, hydrochloric acid (HCl), and hydrofluoric acid (HF) solutions was investigated by chemical and microscopical techniques.

In regard to the use of fly ash for blended cement fabrication, the behavior of FAs was also studied in lime saturated solution.

The release of different oxides from FA in water, hydrochloric acid, hydrofluoric acid, and calcium hydroxide (Ca(OH)₂) solution, gives useful data about the influence of Belgian FA in blended cement hydration. The chemical analysis of liquid phase (from 8 h to 28 days) confirms the pozzolanic activity of FAs in lime saturated solution. Chemical data and microscopical examination demonstrate that all the sulfate and most of the potassium oxide (K₂O) content of the FAs particles are in the superficial layers, whereas sodium oxide (Na₂O) is dispersed in all the amorphous phase.

The influence of FA substitution on the microstructure of hydrated normal portland cement (OPC) pastes was also studied. FA act to increase the total porosity. After 4 months of hydration, the pore size distribution of systems with low FA content is rather similar to that of the pure paste.

The pozzolanic activity of FAs was investigated by microscopical study and free lime determination of the mixtures. A significant pozzolanic activity of FAs was observed after 14 days.