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Compared with being applied as Portland cement retarder, the production of sulphoaluminate cement can consume more phosphogypsum. In the production of sulphoaluminate cement, it is not only necessary to add proper amount of gypsum to ensure the formation of calcium sulphoaluminate minerals when raw materials are prepared, but also necessary to add proper amount of gypsum to participate in the hydration reaction of calcium sulphoaluminate when grinding cement. Compared with being used as Portland cement retarder, the usage amount of phosphogypsum increases significantly.
When it is used as a raw ingredient, the impurities in phosphogypsum will change under calcination, which can avoid or weaken the adverse effects on cement. The phosphogypsum should be fully tested and verified when it is used as post adding gypsum. The usage amount of phosphogypsum will be significantly increased if the acid production and calcium supply from phosphogypsum are also considered in the production of sulphoaluminate cement.
The traditional sulfoaluminate cement is produced by calcination at 1300-1350℃ with proper composition of limestone, bauxite and gypsum as raw materials, and has the characteristics of high early strength, slight expansion, corrosion resistance and good freeze resistance. Gypsum is one of the most important raw materials which occupies quite a large proportion in the production of sulphoaluminate cement, and it plays a guiding role in the sintering process of clinkers and the stabilization of mineral compositions.
While firing sulphoaluminate cement clinker by mixing Calcium raw materials and aluminum raw materials with sulphoaluminate rather than natural gypsum, the soluble impurities contained will react with the CaO produced by the decomposition of limestone to form a stable compound, and the impurities will be converted and dissolved solidly in cement clinker minerals in the inert form, which not only greatly reduces the adverse effects of phosphogypsum impurities, but also promotes the burning of cement clinker minerals and improves their burnability. Yang Lin made Belite sulphoaluminate cement clinker with high iron phase by using phosphogypsum and pyrite slags; the compressive strength of cement calcined at 1250 ℃ with phosphogypsum as the raw material is equivalent to that of cement calcined at 1300 ℃ with natural gypsum as the raw material. Both of the cases show that the phosphate, fluoride, organic matters and other impurities contained in phosphogypsum reduce the sintering temperature of clinker during the calcining process of cement clinker and play the role of mineralizers.
Phosphogypsum decomposition will happen under the condition where the sulphoaluminate cement clinker calcining atmosphere is reasonably controlled or the phosphogypsum is excessively used. As long as it can prompt the decomposition, but not the complete decomposition, of phosphogypsum in the clinker formation stage as much as possible, the CaO produced by the decomposition of phosphogypsum can replace some of the calcareous components provided by limestone, while the undecomposed phosphogypsum provides the calcium sulfate components which are necessary for the production of sulphoaluminate cement, and the SO2 produced can be collected and used in the production of sulfuric acid. In this way, it is possible to avoid the requirement of extremely high decomposition rate when producing Portland cement with phosphogypsum, which can greatly increase the usage amount of phosphogypsum in the raw materials of sulphoaluminate cement. The key to preparing sulphoaluminate cement by partial decomposition of phosphogypsum is to improve the decomposition rate of phosphogypsum.
Obviously, if the decomposition rate of phosphogypsum is not high enough, it is difficult to achieve the original purpose of resource utilization. Chai Junqing et al. studied the influence of high-temperature decomposition of phosphogypsum on the performance of fired sulphoaluminate cement. The decomposition rate reached 36.2% and the compressive strength of cement reached 58.5 Mpa at 28d when the content of phosphogypsum was 27.12% and the calcination temperature was 1300℃. Nevertheless, the CaO produced by the decomposition of phosphogypsum is still insufficient to effectively replace the CaO needed in the raw material ingredients, and the SO2 produced by the decomposition is difficult to collect for the co-production of sulfuric acid. So it is necessary to comprehensively consider the decomposition of phosphogypsum and the firing process of sulphoaluminate cement. If the intensive decomposition of phosphogypsum is controlled in a certain temperature section, while there are no decompositions in other temperature segments, it is conducive to collect SO2 and make acid.
In addition, maintain the phosphogypsum excess in the preparation of sulphoaluminate cement, and the excessive phosphogypsum will be in the clinkers in the form of high temperature anhydrite after calcination. This high temperature anhydrite has similar effect to the post adding gypsum, so it can replace the post adding gypsum as the component of sulphoaluminate cement.
Experts have explored the method and the results show that the adverse effects of impurities in phosphogypsum can be eliminated to a certain extent after a high temperature process. Obviously, with the method of replacing the post adding gypsum with this kind of high temperature anhydrite, there is no need to add gypsum in clinker grinding, which not only simplifies the technological process, but also helps to ensure the quality of cement.
