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Water-saturated Fluorgypsum Modification with Modifiers for Building Gypsum Powder under Calcination

Author ZhangKang
Tutor GuoChaoZuo
School Central South University
Course Environmental Science and Engineering
Keywords water-saturated fluorgypsum phase transformation forgypsum calcinations dehydration modifier activation hemihydrated gypsum
CLC
Type Master's thesis
Year 2013
Downloads 13
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Fluorgypsum is a by-product from hydrofluoric acid and sodium aluminate fluoride processing. It is a key problem for reuse of water-saturated fluorgypsum in China due to the waste of huge land with large amounts of historical heaps. In the study, water-saturated fluorgypsum was modified by modifier combined with calcination at low temperature for building gypsum powder. More, the mechanisms of chemical phase transformation and activation of fluorgypsum were studied. The main results are as follows:The chemical compositions of water-saturated fluorgypsum are calcium sulphate dihydrate (CaSO·2H2O) and the content of calcium sulfate is around74.7%. According to the DSC-TGA analysis, the dehydration from CaSO4·2H2O phase occurred from80℃to90℃, and transformed to CaSO4·1/2H2O phase from120℃to170℃, then transformed to soluable anhydrite phase from170℃to340℃and to P-anhydrite III phase from340℃to360℃. The results suggestted that calcination temperature for hemihydrated gypsum from water-saturated fluorgypsum is appropriately ranged from120℃to300℃.Based on the results from the optimization for modifiers and calcination temperature, sulphate, compound containing silicon, compound containing aluminum and compound containing calcium are effective for fluorgypsum modification, respectively. It is available to meet the demand for gypsum modification when conditions are particle size of fluorgypsum powder of0.18mm, calcination time of2h, calcination temperature from150℃to300℃and heating rate of8℃min-1.The water-saturated fluorgypsum modification by modifiers added with two-step method has been greatly optimized. The results showed that water-saturated fluorgypsum powder of particle size0.18mm was calcined2h under230℃with sulphate of0.30%and compounds containing silicon of1.00%, respectively, then, the addition of1.00% compounds containing calcium was mixed again. The initial and final setting time for modified fluorgypsum was9.1min and14.5min, and the flexural strength and compressive strength after2h reached1.81MPa and5.90MPa, respectively, which meets the National Standards for Building Gypsum (GBT9776-2008). Hemihydrated gypsum content of modified fluorgypsum reached71.2%, and the main hydration phases of modified fluorgypsum are CaSO4·2H2O and C-S-H gel. The hydration hardening of hemihydrated gypsum rapidly formed CaSO4·2H2O, and continuously formed the cement materials of xCaO.SiO2.aq with modifier of compounds containing silicon, that is to say, C-S-H gel.The water-saturated fluorgypsum modification by modifiers added with one-step method has also been further optimized. The results showed that water-saturated fluorgypsum powder of particle size0.18mm was calcined2h under170℃when additions of sulphate, compounds containing aluminum and calcium, and calcination temperature were0.80%,1.00%,1.00%, respectively. The initial and final setting time for modified fluorgypsum was7.6min and10min, and the flexural strength and compressive strength after2h reached1.88MPa and6.00MPa, respectively, which meets the National Standards for Building Gypsum (GBT9776-2008). Hemihydrated gypsum content of modified fluorgypsum reached73%, and the main hydration phases of modified fluorgypsum are C-S-H gel and unstable salt (salt-mCaSO4-nH2O), which contains the compounds containing aluminum and calcium, enhancing the condensation and mechanical strength of hydration body. The flexural strength and compressive strength reached2.48MPa and7.30MPa after curing28days. Figures:37, tables:13, references:78.

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