Preparation of geopolymeric materials from fly ash filler by steam curing with special reference to binder products

Abstract

In order to prepare geopolymeric materials, sodium disilicate solution (1.48 mol/L) added with caustic soda solution (15 N) in 3 : 1 proportion and fly ash were used as geopolymer liquor (W) and filler (S), respectively, and were mechanically mixed with ratio W/S 0.45. The mixtures were cast into molds and steam-cured at 80°C up to 16 h under variable humidity, 40%RH, 60%RH, 80%RH and 100%RH, to make monolithic materials. As a result, relatively higher strength was obtained under 40%RH and 100%RH curings, while relatively lower strength under 60%RH and 80%RH curings. On the contrary, relatively larger shrinkage was noted, when cured under 40%RH, while relatively larger expansion, when cured under 100%RH. Monoliths cured under 60%RH and 80%RH showed little expansion and shrinkage in the entire range of curing time. On the other hand, monoliths cured under 40%RH showed relatively lower bulk density despite the shrinkage, while monoliths cured under 100%RH showed relatively higher bulk density despite the expansion. Monoliths cured under 60% RH and 80%RH showed intermediate bulk densities. These two extreme phenomena were discussed in terms of hydration-polycondensation process of silica complexes of the geopolymer liquor containing foreign metallic ions leached out of the fly ash filler. SEM-EDX analysis showed that binder portions were zeolitic gels in amorphous state consisting of Na2O, Al2O3 and SiO 2 as main components associated with K2O, MgO, CaO and Fe2O3 as well as free C. In hollow type of fly ash particulates so called cenospheres, needle-like or blade-like crystallites were encountered, which were composed of chemical components similar to the binder portions and zeolite Y (faujasite) formation was suggested. Furthermore, the CaO component was concentrated both in zeolitic gels and in zeolitic crystallites.