Title: Activation of Fly Ashes: A General View
Author(s): A. Fernandez-Jimenez and A. Palomo
Publication: Symposium Paper
Appears on pages(s): 351-366
Keywords: alkaline activation; BSEM; fly ash; MAS NMR
Alkali activation of fly ashes is a chemical process through which the glassy structure of the material is transformed into a very well compacted cement. In general terms a variety of natural materials and industrial by-products rich in S'02 and Al2O3 may be activated with certain alkaline compounds to produce cementitious systems when cured under mild temperature conditions. The activation process may be considered as a set of destruction-condensation re-actions that initially lead to a series of unstable structure units and later produces the formation of tixotropic coagulation structures, which can condense to form the hydrated products. This paper describes a micro-structural study of a number of alkali activated fly ash matrices. MAS-NMR results has shown that the main reaction product in the activation of fly ashes is an amorphous aluminosilicate gel having a 3-dimensional structure in which the Si evolves in a variety of environments with a predominance of Q4(3Al) units when curing time is short, and Q4(2Al) units when curing time is long. Aluminium is essentially tetrahedrally co-ordinated. Microanalysis has provided additional information on the Si/AI ratio of the mentioned aluminosilicate gels too.