Study On Modified Desulfurizated Ash Influence On Cementitious Properties And Structure Of Cement

With the rapid development of Circulated Fluidized Bed desulfurization technology, the ashes increased rapidly and its output is about 80 million tons in 2013. At present, this ash is still largely in stacked manner and the utilization is urgent. Desulfurization ashes belong to calcined clay substances and have pozzolanic activity, which can be used as cement mixture. As the demand of cement mixture is more and more, it is of important significance for resource recycling and environmental protection to use desulfurization ash in the production of cement mixture. Because the contents of f-CaO and SO3 in desulfurization ashes are high, when these ashes are used largely as cement mixture, the autoclave expansion value of cement is high and its stability is also failed. Considering these problems mentioned above, this paper focused on fundamental properties of desulfurization ash, some factors influencing the digestion effect of f-CaO in desulfurization ash and its microstructure(such as added water, modifier types, grinding time, digestion temperature) and cement properties & micro-structural changes. The main work and conclusion are as follows.The fundamental properties of desulfurization ash were detailedly investigated. It showed that the micro-morphology from Maoming Petrochemical CFBC ash is irregular, porous and rough, and grain size distribution of particles is under unreasonable level; that the percentage of particle size ≥65 μm reached 79.50% and D50 reached 113.67 μm. The total CaO content is 23.07% and f-CaO is about 8.49% in this ash, which contains special mineral components CaSO4 and CaO compared to common fly ash. With the increase of SO3 and f-CaO content, both the rigid strength and activity index of this ash increased first then decreased, while the linear expansion rate increases.The factors influencing the modified effect of desulfurization ash were systematically investigated. It was found that the major factors influencing f-CaO content of desulfurization ash are added water, then modifier type, digestion temperature, grinding time, etc. When added water was 30% and digestion time was 1 d, the f-CaO content decreased to 2.81%. When 1% of complexed modifier B composed mainly of Na2SO4 and 30% of H2 O were added, the content of f-CaO in desulfurization ash decreased to 2.04% after sealed curing for 8 h at 45 ℃. The autoclave expansion value of modified desulfurization ash decreased to about 2μm. The activity index of 3 d self-hardness strength increased from 78.28% to 82.35%, and 28 d from 80.69% to 84.89%. Ca(OH)2 produced by unmodified desulfurization ash covered on the surface of ashes and showed hexagonal plate-like. When 1% of complexed modifier B composed mainly of Na2SO4 and 1% of complexed modifier C composed mainly of triethanolamine were added, curing for 1d at 40 after 15 min, ℃Ca(OH)2 crystals produced on the surface of desulfurization ash grains had external growth like cluster, which inserted into interstitial space. Some flocculent materials formed on the grain surface and improved the surface structure of desulfurization ash.The property and microstructure of cement prepared by desulfurization ash were systematically studied. It was indicated that water-ash ratio of cement prepared by 30% of modified desulfurization ash decreased from 0.3045 to 0.285 compared to cement prepared by 30% of unmodified desulfurization ash and setting time accelerated significantly; that autoclave expansion value decreased from 0.5248 mm to 0.2789 mm; that the late development of cement mortar strength was significant and compression active ratio increased from 77.06% of 7d to 94.01% of 28 d. Based on SEM and XRD spectra of test block of cement paste after hydration 3 d and 28 d, it showed that hydration process accelerated for the cement prepared by ash modified by modifier B and the formation of ettringite was in advance. No chunks of CH was found after hydration 28 d. The combination of C-S-H gel and AFt became more close and the density of cement increased significantly after hydration.The modified desulfurization ash had higher activity and lower content of harmful components, which can be largely used as cement mixture. When the addition of modified desulfurization ash reached to 30%, the performance of cement meet the requirements of ASTM C.