Material and structural characterization of alkali activated low-calcium brown coal fly ash

The waste low-calcium Czech brown coal fly ash represents a considerable environmental burden due to the quantities produced and the potentially high content of leachable heavy metals. The heterogeneous microstucture of the geopolymer Mn [-(Si-O)z-Al-O]n·wH2O, that forms during the alkaline activation, was examined by means of microcalorimetry, XRD, TGA, DSC, MIP, FTIR, NMR MAS (29Si, 27Al, 23Na), ESEM, EDS, and EBSD. The leaching of heavy metals and the evolution of compressive strength were also monitored. The analysis of raw fly ash identified a number of different morphologies, unequal distribution of elements, Fe-rich rim, high internal porosity, and minor crystalline phases of mullite and quartz. Microcalorimetry revealed exothermic reactions with dependence on the activator alkalinity. The activation energy of the geopolymerization process was determined as 86.2 kJ/mol. The X-ray diffraction analysis revealed no additional crystalline phases associated with geopolymer formation. Over several weeks, the 29Si NMR spectrum testified a high degree of polymerization and Al penetration into the SiO4 tetrahedra. The 23Na NMR MAS spectrum hypothesized that sodium is bound in the form of Na(H2O)n rather than Na+, thus causing efflorescence in a moisture-gradient environment. As and Cr6+ are weakly bonded in the geopolymer matrix, while excellent immobilization of Zn2+, Cu2+, Cd2+, and Cr3+ are reported.