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Abstract
Geopolymers can be categorized as synthetic zeolitic monoliths. The amorphous structure of this material is developed by dissolution of an aluminate and silicate-rich source, like fly ash, in a highly alkaline solution and then interlinking and crosslinking of Al and Si structures. These processes provide geopolymers with low permeability and resilience to physical and chemical attacks. The low permeability of geopolymers makes them a good candidate for solid waste immobilization. A host of research projects are dedicated to the immobilization of municipal waste incineration residue, carbon electric arc furnace residues and metal anions. However, the primary trials to immobilize flue gas desulfurization wastewater (FGD) have led into the disintegration of the geopolymer matrix. It is hypothesized that precipitates and crystals formed with anions (Cl-, CO3(2-) , C2O4(4-) and PO4(3-) ) and halides (Cl and Br) andgel components interrupt the formation of aluminosilicate gel. Also, hydrolytic attack, in which water molecules break the Si-O-Si structures, can be the cause of low durability of the geopolymer matrix.The thermodynamic models developed with different mix designs and varying doses of FGD wastewater showed that low strength chemical species formed as a result of reaction between the geopolymer gel and anion-saturated wastewater cause discontinuity in the tetrahedral chains in alkali-activated fly ash (geopolymer). These species include Alunite KAl3(SO4)2(OH)6, anhydrite CaSO4, thenardite Na2SO4 and halite (NaCl). Moreover, the water-soluble species like thenardite and halite cause disintegration of samples in aqueous environment. On the other hand, the formation of these species result in decrease in charge-balancing ions (Na+, K+ and Ca2+) that help in the formation of zeolites. Addition of fly ash type C improved the integrity of the material by formation of species similar to the ones found in hydrated portland cement like calcium silicate hydrate (C-S-H), calcium (alkali) aluminosilicate hydrate (C-(N-)A-S-H) and Fe monocarbonate (C3FH6). These results are in agreement with the models developed by previous studies with alkali-activated blast furnace slag and naturally-occurring zeolites in saline-alkaline lakes and cavities of mafic rocks.