Activation of rejected fly ash using flue gas desulphurization (FGD) sludge

Abstract

Low-grade fly ash (rejected fly ash, rFA), a significant portion of the pulverized fuel ash (PFA) produced from coal-fired power plants and rejected from the ash classifying process, remains unused due to its high carbon content and large particle size (>45 μm). But it is thought that the rejected ash may have potential uses in chemical stabilization solidification (S/S) processes which need relatively lower strengths and a lower chemical reactivity. Flue Gas Desulphurisation (FGD) sludge is a by-product of air pollution control equipment in coal fired power plants whose chemical composition is mainly gypsum. As there is no effective usage of both of these two materials, it is of interest to research on the possible activation of rFA using FGD. This paper presents experimental results of a study on the properties of rFA activated by the FGD in rFA-cement pastes. Different percentages of FGD were added into the mix to study the effects of the FGD on the reaction of the rFA blended cement pastes. The results show that FGD takes effect as an activator only at late curing ages. Adding Ca(OH)₂ enhances the effect of FGD on activating the hydration of rFA. Also, 10% FGD by weight of rFA is the optimal addition in the rFA-cement pastes. The results of the compressive strength measurements correlate well with the porosity results.

Keywords

rejected fly ash / PFA / strength development / activator / flue gas desulphurization sludge / FGD

References

[1] Langley W S, Carette G G, Malhotra V M. Structural Concrete Incorporating High Volumes of ASTM ClassF Fly Ash. ACI Material Journal, 1989, 86: 507-514.

[2] Horiuchi S, Kawaguchi M, Yasuhara K. Effective Use of Fly Ash Slurry as Fill Material. Journal of Hazardous Materials, 2000, 76: 301-337.

[3] Shi Caijun, Day Robert L. Acceleration of the Reactivity of Fly Ash by Chemical Activation. Cem Concr Res, 1995, 25: 15-21.

[4] Jueshi Qian, Caijun Shi, Zhi Wang. Activation of Blended Cements Containing Fly Ash. Cem. Concr. Res., 2001, 31: 1121-1127.

[5] Shi Caijun, Day Robert L. Acceleration of the Reactivity of Fly Ash by Chemical Activation. Cem. Concr. Res., 1995, 25: 15-21.

[6] Katz A. Microscopic Study of Alkali-Activated Fly Ash. Cem. Concr. Res., 1998, 28: 197-208.

[7] Fraay A L A, Bijen J M, de Haan Y M. The Reaction of Fly-ash in Concrete A Critical-examination. Cem. Concr. Res., 1989, 19: 235-246.

[8] Aimin Xu, Sarkar Shondeep L. Microstructural Study of Gypsum Activated Fly Ash Hydration in Cement Paste. Cem. Concr. Res., 1991, 21: 1137-1147.

[9] Poon C S, Kou S C, Lam L, Lin Z S. Activation of Fly ash/Cement Using Calcium Sulfate Anhydrate (CaSO₄). Cem. Concr. Res., 2001, 31: 873-881.

[10] Ma Weiping, Liu Chunling, Brown Paul W, Komarneni Sridhar. Pore Structures of Fly Ashes Activated by Ca (OH)₂ and CaSO₄.2H₂O. Cem. Concr. Res., 1995, 25: 417-425.

[11] Hyde A F L. Some Preliminary Test Data on Cement-pfa-anhydrite Mixes. Magazine of Concrete Research, 1984, 36: 174-180.

[12] Ma Weiping, Brown Paul W. Hydration Reactions of Fly Ash with Ca(OH)₂ and CaSO₄·2H₂O. Cem. Concr. Res., 1997, 27: 1237-1248.

[13] C S Poon, X C Qiao, Z S Lin. Pozzolanic Properties of Reject Fly Ash in Blended Cement Pastes.Cem. Concr. Res., 3, 2003, accepted for publication

[14] Lea F M. Cement and Concrete Chemistry, 1970 Third edition London: Arnold.