CFB鍋爐燃石油焦脫硫飛灰活化水淬爐石粉漿體之水化及硫酸鹽侵蝕微觀研究;Microscopic study on hydration and sulfate attack of blast furnace slag cementitious paste activated by the desulfurization fly ash obtained from CFB boiler firing petroleum coke

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Title:	CFB鍋爐燃石油焦脫硫飛灰活化水淬爐石粉漿體之水化及硫酸鹽侵蝕微觀研究;Microscopic study on hydration and sulfate attack of blast furnace slag cementitious paste activated by the desulfurization fly ash obtained from CFB boiler firing petroleum coke
Authors:	陳冠宇;Chen, Guan-Yu
Contributors:	土木工程學系
Keywords:	CFB脫硫飛灰;外部硫酸鹽侵蝕;內部硫酸鹽侵蝕;延遲性鈣礬石形成;CFB desulfurization fly ash;External sulfate attack;Internal sulfate attack;Delayed ettringite formation
Date:	2023-01-09
Issue Date:	2024-09-19 14:08:49 (UTC+8)
Publisher:	國立中央大學
Abstract:	本研究以循環式流體化床鍋爐燃石油焦脫硫飛灰(CFB脫硫飛灰)作為激發水淬爐石粉卜作嵐反應之輔助膠結材料，依照規劃之摻配比例及混合少量水泥製作純漿試體，透過電子顯微鏡(SEM)、X光繞射分析(XRD)、熱重分析(TG/DTG)等觀察漿體在不同齡期下形成的水化產物種類與微觀結構發展;此外，由於CFB脫硫飛灰材料中有較高含量的SO3與free-CaO等容易形成硫酸鹽反應之化合物，因此分別依據ASTM C1012進行「外部硫酸鹽侵蝕」試驗及ASTM C1038進行「內部硫酸鹽侵蝕」試驗，針對受硫酸鹽侵襲之漿體進行微觀分析。　　水化反應調查結果顯示CFB脫硫飛灰活化爐石粉漿體之水化產物以鈣礬石(AFt)發展為主體，與脫硫飛灰摻配比例正相關;而Ca(OH)2形成則主要受水泥摻配影響;二水石膏僅在水化初期少量形成，未被消耗之硫酸鹽仍以硫酸鈣(CaSO4)型態留存。在水化發展上，較低的脫硫飛灰摻配量會使晚期強度與水化產物發展受到限制，較高的摻配量(≥30%)則會導致早期發展相對緩慢，但水化後期反應則較為活躍;而水泥的摻配能夠形成充足的C-S-H膠體來協助鈣礬石填補孔隙，使微觀結構的緻密程度更佳。　　外部硫酸鹽侵蝕部分，SEM觀察結果顯示劣化區域主要在漿體表面與近表面之孔隙及裂縫處，反應產物為Ca(OH)2、CaCO3與部分的二水石膏結晶，並且摻配部分水泥時會導致外部硫酸鹽侵蝕之反應程度較為顯著;而漿體內部雖有少量的二次鈣礬石反應結晶形成，但對其原有孔隙結構並無明顯劣化和侵蝕之跡象。　　而在內部硫酸鹽侵蝕作用影響下，漿體從表面至內部之孔隙均有明顯的侵蝕跡象且形成大量高S/Ca比的延遲性鈣礬石大型結晶，造成微觀結構之劣化與水化產物的降解與破壞;劣化程度隨脫硫飛灰摻配用量增加而加劇，而適量摻配水泥則有助於提升漿體緻密性，從而減少劣化深度和延遲性鈣礬石的形成。　　透過比較各項微觀分析結果，顯示對於脫硫飛灰活化爐石粉漿體而言，內部硫酸鹽侵蝕問題明顯比外部硫酸鹽侵蝕之影響更為嚴重、更具破壞性，因此需要合理控制脫硫飛灰之摻配用量，避免影響硬固結構的穩定性。;In this study, the desulfurization fly ash obtained from circulating fluidized bed boiler firing petroleum coke (CFB desulfurization fly ash) was used as a supplementary binder material to activate ground granulated blast furnace slag (BFS) for use in concrete. Specimens were made according to the planned blending ratio with a small amount of Portland cement. The hydration products and microstructural development of paste specimen at different ages were observed by scanning electron microscopy (SEM), X-ray Diffraction (XRD), and thermal gravimetric/derivative thermogravimetric (TG/DTG) analysis. Since the high content of SO3 and free-CaO in the CFB desulfurization fly ash can easily form sulfate reaction compounds, the external sulfate attack test was conducted according to ASTM C1012 and the internal sulfate attack test was conducted according to ASTM C1038 on mortar specimens. Then, microscopic analysis was carried out on the paste specimens subjected to sulfate attack. 　　The results of the hydration investigation show that the hydration products of CFB desulfurization fly ash activated BFS paste were dominated by ettringite (AFt) development and positively correlated with the proportion of CFB desulfurization fly ash in the blends. The formation of Ca(OH)2 is mainly related to cement blending. Gypsum is formed in a small amount at the early stage of hydration only. And the non-consumed sulfate remains in the form of anhydrite (CaSO4). 　　In the development of hydration, a lower proportion of CFB desulfurization fly ash blending will limit the development of late compressive strength and hydration products, while a higher proportion of blending (≥ 30%) will lead to a relatively slow development in the early ages, accompanied by a more active reaction in the later age of hydration. The blending of cement in the mixture helps produce sufficient C-S-H gel to assist ettringite in filling the pores, resulting in a better denseness of the microstructure. 　　In the external sulfate attack test, SEM observation results show that the degraded area is mainly in the pores and cracks on and near the surface of the paste specimen. The reaction products are Ca(OH)2, CaCO3 and some gypsum crystals. The blending of cement will cause more significant external sulfate attack reaction. Although there is a small amount of secondary ettringite formation inside the paste, but there is no obvious signs of degradation or erosion of the original pore structure. 　　Under the influence of internal sulfate attack, the paste specimen shows obvious signs of erosion reaction from surface to internal pores and a large amount of delayed ettringite formation (DEF) with high S/Ca ratio, resulting in microstructural deterioration and degradation of hydration products. the degree of deterioration increases with the amount of CFB desulfurization fly ash in the blend. A suitable amount of cement blending helps to enhance the denseness of the paste, thus reducing the depth of deterioration and the formation of delayed ettringite. 　　By comparing the microscopic analysis results, it is shown that for CFB desulfurization fly ash activated BFS paste, the internal sulfate attack is obviously more serious and destructive than the external sulfate attack. Therefore, it is necessary to reasonably control the proportion of CFB desulfurization fly ash in the mixture to avoid affecting the stability of the hardened structure.
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