| Slip casting is the common technology in sanitary ceramic industry for ceramic production due to the particular characteristics of ceramic body with large size, complicated structure and remarkably smooth surface. Ceramic slurry is transformed into ceramic body with special shape by porous model. Water absorbing is one of the most improtant properties of silp casting process including slurry pouring, water absorbing and embryo forming. China is rich in gypsum mineral resources. Besides, gypsum has many advantages, such as the moderater price, super water absorbing capacity, fast setting and hardening, micro expansion performance, clear edges and corners, stability dimension, simple application, Green Resources and so on. Therefor, gypsum models have always been used as the traditional models in ceramic slip casting technique for years. However, lower grade of gypsum mineral higher solubility, poor water resistance and higher porosity of gypsum make the strength decrease of gypsum models. In addition, gypsum models must be used in the moist environments by the requirements of slip casting technique. Models are dissolved and corroded by water and the electrolytes that are added into ceramic slurry to keep well fluidity and stability for more than two months. The property of water absorbing, mechanical property and the performances of water and electrolyte resistance are significantly reduced by the working time incease. In addition, the relative quality standards are absence in industry. Consequently, at present, gypsum models in our country could be used for only 60 times. The short working-life caused high economic cost for ceramic industry directly. The production efficiency of ceramic bodies and productions are decreased by the frequent replacement of gypsum models. Morever, the short working time makes the burden of operation work havier and the resources consuming waste seriously.As a consequence, in this paper, the relationship between the performances and the working times of gypsum models and the deterioration mechanisms were deeply analyzed on the basis of the previous surveys of the industry application conditions and labarotary simulation. Then, influence factors were discussed according to the changes of the macro-performances and the microstructure. Methods and its mechanisms were studied to improve the mechanical properties. Techniques and the mechanism which could enhance the corrosion resistance property of gypsum models were researched, as well. Finally, the optimization technique was determined by lab experiments and laboratory simulation. The technique was tested in the practical industrial application to analyse the working ability and the economic effects.Water absorbing, mechanical property, water-resistance and corrosion-resistance were all decreased severely in the whole working life of gypsum models. In the ten times, gypsum models had high water absorption. The water absorption kept steady between 10 th to the 40 th. Afterwards, the performances of gypsum models declined gradually. The properties of water absorption, water-saturated flexural strength, softeness coefficiency and corrosion ration approximately fell 13.06%, 31.60%, 33.33% and 3.65% at 60 times, separately. It was difficult to be used continuely.The effects of water and electrolytes on durable performances and surface condition were the main reason to make the deterioration of gypsum mould. Cystal structure of gypsum hardenites was loosened by the dissolution of water was the main deterioration factor of model gypsum. The corrosion by electrolytes made gypsum performances decrease further. In addition, porosity was reduced and pore diameter was thinned by the block of mud particles, which was the important factor to mke the water absorbing ability decrease.Mechanical performance requirements founded according to the periodic changes were that water absorption ≥37.89%, water-saturated flexural strength ≥ 1.41 MPa, softeness coefficiency≥0.23, corrosion ratio≤3.63%.Moulding process was the primary influence factor on the performances of gypsum models. The ratio of W/G played critically in hardenites poprosity related to the mechanical strength. The crystal morphology was greatly by hydration process affected by water temperature. Dihydrate gypsum(DH) became thin sheet at lower water temperature while the morphology changed to be sheet and rhombic when the temperature was above 40℃ owing to the rapid hydration. The compact net-like structure was destroyed to be loose with the crystal broken. The optimal moulding parameters were that: W/G 1:1.3~1:1.4, water temperature 15~20℃, 2min stirring, 1000r/min. Dihyrate gypsum became more gracile and the microstructure was more compact.The absorption of PC on the surface of gypsum made the hydration process prolonged. Consequently, the DH crystal became thin with larger ratio of length to diameter. Besides, the pore size was minified and the crystal structure was lossened. To satisfy the water absorbing capacity, 0.20% was the proper content. Polypropylene fibre with the 400:1 depth – diameter and the dosage of 0.04% made an important effect on the water-resistance property and toughness of gypsum hardenites. The performances of flexural strength and softeness coefficiency water increase by 18.02% and 44.44% separately. Tributyl phosphate(MSDC) was an effective defoaming agent to strengthen the flexural strength of gypsum hardenite with 0.07% content. All of the ordinary potland cement(OPC), aluminate cement(AC) and sulphoaluminate cement(SAC) had benefits on the mechanical performances of gypsum hardenites. Strength was significantly developed depending on the compact microstructure of DH, ettringite(AFt) and C-S-H/AH3 gel. In addition, AC with the content of 6% was optimum. The ratio of water-saturated flexural strength wans increased by 28.03%, and the water-resistance performance was increased by 13.79%. Silica sand with the fineness of 100 to 40 M at the ratio of 10% was efficiency to improve the abrasion-resistance property of gypsum hardenites.The new modification technology by salt solution was created in the paper. Water-resistance, corrosion-resistance and water-saturated strength were all dramaticly enhanced by immersed in sodium oxalate solution(0.15mol/L) for 2h. The softeness coefficiency was increased by 16.67% and the corrosion ratio was decreased from 2.09% to 0.33% for the modified hardenites. A water-resistance layer of calcium oxalate monohydrate(COM) with the thickness of 50~60μm was produced by the reaction of DH and sodium oxalate. COM covering on the surfaces of hardenite prevented the further dissolution of DH. As a result, the corrosion behavior from electrolyte was blocked. Moreover, inorganic COM layer had little influence on water absorbing capacity to satisfy the requirement of slip casting.A new-style gypsum models were prepared by the optimized technique above. All performances were tested more advanced than the original through laboratory simulation. Performances deterioration was slower to make the durable models more durable. Industrial test proved that the slurry suction time was shortened about 20 min in the first ten working times. Mechanical properties and working capacity could be still used continually when the original models were abandoned at 60 times. The water absorption, water-saturated flexural strength, softeness coefficiency and corrosion ratio were were 45.00%, 1.57 MPa, 0.31 and 2.67%. The durable performances were increased about 16.58%, 11.34%, 29.17% and 26.85% than the original models. And the decrease ratio of durable performances at 60 times were 38.13%, 8.85%, 16.29% and 26.85% lower than the original gypsum models. Meanwhile, durable performances were higher than the standards requirements about 18.76%, 11.35%, 34.78% and 26.44% separately. Moreover, the surfaces were kept clearer and less damaged. Finally, the working-life of new-style and durable gypsum models can be increased by 20 times in the actual industry. |
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