| Nitrate sulfur gypsum is a kind of high purity decalcified gypsum from the preparation of compound fertilizer of nitric acid decomposing and phosphate rock, it has not received wide attention on the market because of limited scientific studies, desulfurization gypsum is mostly used in engineering field instead. Nitrate sulfur gypsum as a industrial by-product gypsum has been researched in this study, hydrothermal method has been used to prepare nitrate sulfur gypsum, the influences of different kind of organic crystal modifier and solution concentration, the reaction temperature and time, shaft speed of agitator and the mass fraction of nitrate sulfur gypsum were studied, and then, the particle size, the microstructural aspects, the mechanical strength and the mechanism were tested by laser particle analyzer, X-ray diffraction (XRD), scanning electron microscopy (SEM). After the experiment above, a good a-hemihydrate nitrate sulfur gypsum has been chosen to add retarder of barium salt and water reducing agent of compound and pure substance to test the effect of the additive to gypsum by XRD analysis, hydration reaction analysis and pore structure and porosity analysis.The study includes the following aspects:1.The effect of crystal modifier is first increased and then decreased with the increase of the number of carbon atoms among organic dicarboxylic acid, the crystal of a-hemihydrate gypsum with succinic acid appears the short column, small long-radial ratio, the size distribution homogeneously, the strength for hardening mechanical performance meets the high-strength gypsum standard, so succinic acid has the best effect of crystal; The effect of organic dihydric alcoho also showed a trend like organic dicarboxylic acid, but the overall effect is less good. Citric acid and EDTA are two kinds of organic polybasic acid, the study conducted an experiment on the effect of crystal during them and dicarboxylic acid under the same number of carbon atoms, it shows that citric acid is inferior to adipic acid, EDTA is superior to oxalic acid, even slightly better than succinic acid. The different effect of crystal is related to the adsorption between crystal modifier and gypsum through XPS analysis.2.Besides the crystal modifier, other factors of effect on the preparation of gypsum are reaction temperature, followed by reaction time, the mass fraction of nitrate sulfur gypsum, the concentration of crystal modifier and the shaft speed of agitator. In this condition, the crystal of a-hemihydrate gypsum with small long-radial ratio, the strength is inferior to high-strength gypsum.a-hemihydrate gypsum was mechanically ground by ball milling, this method made the long-radial ratio of crystal smaller, the strength reaches 39MPa which is twice before grinding and meets the high-strength gypsum standard, but the initial setting time of a-hemihydrate gypsum is shorter.3.The effect of two kinds of barium salt, BaCO3 and Ba2SiO4, on the performance of gypsum such as setting time, hydration rate, compressive strength, flexural strength and micro-morphology is determined. The results show that barium salt is highly effective in retarding and the retarding effect is enhanced with the increase of dosage. In the same amount, setting time and strength loss for gypsum containing Ba2SiO4 are better than that of BaCO3.Lots of retarders are produced to delay the hydration induction period. The study put forward that Ba2+and SO42- in gypsum can generate water-insoluble barium sulfate which could wrap on the surface of gypsum particles to inhibit the reaction process of hydration through XRD analysis.4.The study conducted an experiment on four pure water reducer:HC-esters, HC-ethers, FDN and calcium lignosulphonate. The results of water reducing effect are listed in the order:FDN> HC-esters> HC-ethers> calcium lignosulphonate, the compressive strength of nitrate sulfur gypsum mixed with FDN and HC-esters did 3-5MPa higher than blank sample, HC-ethers and calcium lignosulphonate make the improvement of the strength of gypsum not significant. Combining HC-esters with defoamer and doing experiment, the result shows that HC-esters still play a major role in water reduction in this kind of composite water reducer and the defoamer is not work. |
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