Study On Properties And Utilization As Cement Admixture Of CFBC (Circulated Fiuidized Bed Combustion) Ashes

The Circulated Fluidized Bed Combustion （CFBC） ashes are by-products fromthe power plants with CFBC boilers which use sulfur coal and sulfur capture agentsuch as limestone burning and desulfurizing in the boiler at about900℃. As the rapidspread of CFBC technology, the yearly ash output is about80million tons andincreases rapidly; as a result, the utilization is urgent. As the development of cementindustry, the cement admixture demand is more and more. The CFBC ash as cementadmixture is one of its important use methods, which has little information and needsmore research.In this paper, the CFBC ash samples from different power plants were studied.The CFBC ash properties were investigated compared with the pulverized coal （PC）fly ash, a modified method was demonstrated to evaluate the reactivity of CFBC ashesand the reactivity origin was analyzed by XRD, SEM, and the feasibility as cementadmixture was initially studied on cement strength, setting time and soundness,compatibility with water-reducing agent and expansion performance, which laysemphasies on the influence of SO₃morphology and content on the cementperformance.The results showed that, compared to the PC fly ash, the CFBC ashes had morecontent of SO₃, f-CaO and loss on ignition. The particles of CFBC ashes wereirregular and loose textured. The SO₃existed as anhydrite â…¡-CaSO₄, which had thesame structure with natural anhydrite. A modified method of “clinker28dcompressive strength ratio” was demonstrated to evaluate the reactivity of CFBCashes according to the SO₃content, which showed that CFBC ash reactivity was highwhen the CFBC ash SO₃content was less than11.6%, satisfing the standardrequiments of PC fly ash used as cement active addition.The anhydrite of CFBC ash could completely replace gypsum to retard theclinker setting time, and the SO₃content should less than3.5%, otherwise, the SO₃soundness was unqualified.With high SO₃content and water requirement content, when the CFBC ashes were used as cement admixture, the permitted addition werelimited, at the same time, when the SO₃content and CFBC ash addition increased, thesaturation of water-reducing agent increased, the initial fluidity decreased, and theloss rate of fluidity as time increased, so the compatibility of CFBC ash withwater-reducing agent was bad. Consequently, when the CFBC ashes were used incement, it should better mix with PC fly ash or other admixtures, the admixtureaddition should be less than30%, and he cement SO₃content was less than3.5%.