High strength geopolymers from fractionated and pulverized fly ash

Coal combustion residues are ideal precursors for the production of geopolymer cements. Activated in alkaline solutions, fly ash provides the necessary Si and Al for the polymer chains that form the matrix of the hardened product. Potassium silicate and blast furnace slag are added to enhance the strength developed. Compressive strength values of over 100 MPa are obtained in these matrices, much higher than observed with normal Ordinary Portland Cement binders. Besides the chemistry, also the physical characteristics of the fly ash are of importance for the development of high strength. The fine particle size and the high sphericity of the particles, both favor good and dense packing of the material into the matrix. In order to further benefit from the physical characteristics of fly ash, size fractions were separated and geopolymers produced from them. However, the relation between size and strength was not clear, mainly due to the fact that, when separating, the chemistry of the different fractions turned out to be different as well. This was clearly reflected by the difference in pH for the fractions (varying from 10.2 to 12.5). When size fractions were obtained by grinding, the chemistry was much less affected as shown by a much smaller pH variation of the ground fractions (11.1 to 11.6). Furthermore, a relation between geopolymer compressive strength and fly ash particle size becomes apparent. Workability seems to be influenced by free lime content of the ash. The addition of K₂HPO₄ helps to extend the working time for the paste.