The role played by the reactive alumina content in the alkaline activation of fly ashes

This study explores the relationship between the chemical composition of fly ashes and the microstructural characteristics and mechanical properties of the cementitious materials resulting from the alkali activation of fly ashes (AAFA). Three reactive systems were prepared by mixing three F ashes, with an 8 M NaOH solution and stored at 85 °C. The main reaction product formed in three systems is an amorphous alkaline aluminosilicate gel. This gel (zeolite precursor), has a three-dimensional framework, with Al occupying Al(4Si) and Si occurring in a variety of environments Q4(nAl). After short thermal activation periods (2–5 h), an Al-rich gel was formed with silicon in tetrahedral Q4(4Al) units (intermediate phase), yielding low mechanical strengths. When the curing time increase (7 days) the gel changes into a more stable Si-rich phase with a greater mechanical strength. Finally, it has been shown that the amount of the reactive aluminium plays an important role in the aluminosilicate gel formation, from a kinetic point of view.