Hydraulic clinkerless binder on the fluid sulfocalcic fly ash basis

Abstract

Sulfocalcic fly ash (mostly called FBC fly ash) is formed when a dry process of limestone injection is applied to the desulfurization of flue gases. The hydration of sulfocalcic fly ash results in the formation of an expanding low-strength body. Its volume instability prevents its use as an admixture to the concrete. The mixing of sulfocalcic fly ash, PCC fly ash and Ca(OH)2 in a presence of a plasticizer gives the possibility to obtain a ternary binder possessing strength values comparable to those of Portland cement (PC). The ternary binder is characterized by a long-term volume stability thanks to the presence of the plasticizer based on polycarboxylates that modifies the habitus of ettringite crystals; the expansion stresses due to the ettringite modification are thus eliminated. The SEM/EDX analyses have revealed that the composition of the amorphous parts does not correspond to that of the C-S-H phase because even Al is present in this phase. It was found, the hydrated ternary binder contains an amorphous C-A-S-H phase, a crystalline ettringite phase as well as residues of crystalline phases (as mullite, quartz, hematite) from the fly ashes. The C-A-S-H phase in the hydrated ternary binder can also be characterized by 21Al MAS NMR as a geopolymer formed by alkaline activation of amorphous parts of ashes under the action of Ca(OH)2. The hydrated ternary binder is obviously similar to the Roman concrete because it contains an amorphous C-A-S-H phase. The ternary hydraulic binder would be a promising perspective for the use of the sulfocalcic fly ash that is nowadays predominantly deposit in the ash landfills.