Composition and microstructure of alkali activated fly ash binder:: Effect of the activator

The alkali activation of fly ashes is a chemical process by which the glassy component of these powdered materials is transformed into very well-compacted cement. In the present work the relationship between the mineralogical and microstructural characteristics of alkaline activated fly ash mortars (activated with NaOH, Na2CO3 and waterglass solutions) and its mechanical properties has been established. The results of the investigation show that in all cases (whatever the activator used) the main reaction product formed is an alkaline aluminosilicate gel, with low-ordered crystalline structure. This product is responsible for the excellent mechanical-cementitious properties of the activated fly ash. However the microstructure as well as the Si/Al and Na/Al ratios of the aluminosilicate gel change as a function of the activator type used in the system. As a secondary reaction product some zeolites are formed. The nature and composition of these zeolites also depend on the type of activator used. (c) 2005 Elsevier Ltd. All rights reserved.