Influence Of Autoclaved Curing On The Hydration And Products Volume Stability Of FBC Ashes

Circulating fluidized bed coal combustion is an advanced and clean coaltechnology, which combined combustion with sulphur-fix together. However, the factthat the anhydrite and free lime contained in circulating fluidized bed coal by products,namely circulating fluidized bed sulphur-fixed coal ashes(referred to as FBC ashes) hasa significant volume expansion in the process of hydration, limits the disposition andapplication of FBC ashes, thus the popularization and application of this technology areseriously affected. Autoclaved curing is a mature Wet-heat curing technology used inproducing silicate products with CaO-SiO2-Al2O3-CaSO4-H2O system, Hydrationproducts such as tobermorite (C5Si6H4,、C-S-H(B))with high strength and volumestability can be gained. Minerals in FBC ashes belong to theCaO-SiO2-Al2O3-CaSO4-H2O system, but the content of CaSO4in the typical system isusually less than5%, and CaSO4are dihydrate gypsum and anhydrite in the majoritysituations, FBC ashes contains II-CaSO4with a content of10%~30%(Calculated bySO3). In such case, the research on whether autoclaved curing can promote hydration,inhibit expansion and improve products volume stability like the typicalCaO-SiO2-Al2O3-CaSO4-H2O systemis of important theoretical and practical value.By studying the influence of different autoclaved systems on hydration, strengthdevelopment and volume stability of FBC ashes, and comparison with specimens curedin normal atmospheric pressure curing, the results shows that autoclaved curing caninhibit the volume expansion caused by anhydrite in FBC ashes, which mainly dependson the types and quantity of hydration products, the structure of hydration products andthe strength of the products after autoclaved curing；After the autoclaved curing, thestrength of FBC（or FBC ashes-clinker）products is2~3times than that of the specimenscured under natural conditions, with28-daycompressive strength reaching25MPa, andno ettringite forming in hydration products. However, tobermorite does form, andhydrates crystalline better. The parameters such as the heating rate, constant temperature,and time, and cooling rate for autoclaved curing have significant impact on the strengthand hydration products. With lower constant temperature and shorter holding time,ettringite and gypsum will be easily gained. Compared to room temperature curing,autoclaved curing can significantly inhibit volume expansion caused by anhydrite inFBC ashes, especially the volume expansion of FBC ashes products under water curing;The results show that the linear expansion of the specimen under water curing after autoclaved curing is only about10%that of the specimen without autoclaved curing,and the volume expansion under water condition after autoclaved curing occurs mainlybefore14d, with small changes afterwards, which is in favor of the late volume stabilityof FBC ashes autoclaved products.The XRD images of FBC ashes and slags production after autoclaved curing canprove the existence of anhydrite, but columnar crystal of anhydrite can’t be found inSEM images, and the distribution of sulfur can’t be found by EDS either. Thecalculation through chemical method and the theory of molecular dynamics shows thateach tobermorite lattice can accommodate four SO42-, and get more stable crystalstructure with lower energy, which can infer that SO42-can enter the crystal structure oftobermorite, The calculation of energy of system shows that SO42-enter crystal structureof tobermorite，resulting in lowing the energy of system and making the crystal structuremore stable. Combininng the conclusion obtained in the early research of lime-fly ashautoclaved products that SO42-can promote C-S-H (B) convertion into tobermorite andenter tobermorite lattice, it can be inferred that the anhydrite in FBC ashes underautoclaved curing conditions can promote C-S-H (B) convertion into tobermorite andenter tobermorite lattice to form a more stable tobermorite. Therefore, the anhydrites inFBC ashes improve the formation of tobermorite, increase the strength of autoclavedproducts and volume stability.Due to the complex composition of FBC ashes, the present research selectssimulate anhydrite with the crystal structure and properties similar to those of theanhydrite in FBC ashes as the research objects, with comparison with natural anhydrite.The research focuses on the influence of autoclaved curing on the solubility andstability and hydration reactivity of the two kinds of anhydrite.The XRD results ofnatural anhydrite and simulate anhydrite after autoclaved curing show that mineralcomposition of both has no change, but the height of the strongest diffraction peak andthe senior secondary diffraction peak significantly increases after autoclaved curing.They also tend to be complete. The stability of anhydrite after autoclaved curing hasbeen improved; Diffraction peaks rising indicates larger grain size. SEM results showthat the substance after autoclaved curing shows obvious phenomenon of agglomerationand particle increasing, which is one of the important causes of reducing solubility. AndTG results indicate that the thermal stability of anhydrite gets better; autoclaved curingdiminishes hydration capacity, which makes it able to convert to gypsum and react withothers to generate ettringite. And autoclaved curing can also help increase the stability and decrease dissolution rate of anhydrite in water, and hydration capacity of generatinggypsum and ettringite diminishes. When FBC ashes are used to produce autoclavedproducts, pure FBC ashes are not recommended, and enough calcium in the mixproportion is extremely necessary. In the process of producing FBC ashes autoclavedproducts, reasonable parameters should be as follows: water-solid ratio reaches0.15to0.20when pressing, molding pressure is25MPa, and static time is more than24hourswhich should be appropriately extended especially when pressing; the durability ofpressed FBC ashes autoclaved products is higher than that of the casted ones, and frostand carbonation resistance of FBC ashes autoclaved products decline with FBC ashescontent increasing. The durability can be significantly improved when FBC ashes aremixed with more than5%cement clinker. The process design and method of productionof FBC ashes autoclaved brick production line with an annual capacity of30million hasbeen built according to following conditions and parameters: using compressionmolding, the ratio of FBC ashes (pulverized sulfur residue): cement at80:20to90:10,water-solid ratio being0.15to0.20,25MPa molding pressure, and more than24hoursstatic time. The basic properties of autoclaved bricks meet the requirement of M25,M10autoclaved bricks in GB11945-1999"autoclaved lime sand brick" and JC239-91"fly ash bricks", and the stability of production is good enough.In a word, autoclaved curing promotes the hydration reaction of FBC ashes,changes hydration products, and forms tobermorite with high strength and volumestability(C5Si6H4), C-S-H（B）and so on. In addition, autoclaved curing reduces thehydration activity of anhydrite in FBC ashes, the rest anhydrous gypsum is stable. Bothimprove the volume stability of FBC ashes autoclaved products.