| Gypsum as a kind of molding material with low cost,convenient molding and excellent water absorption performance is widely used in slip casting process of sanitary ceramics.However,the slurry suction property becomes worse with increasing the slip casting molding time in practical applications due to the slight solubility,low corrosion(Na2SiO3 corrosion in slurry)of gypsum itself,resulting in short service life.It is thus important to investigate the corrosion mechanism of gypsum mold,and enhance its corrosion resistance and service life via adding some modification additives.In this dissertation,the properties of gypsum powder,the molding performance and corrosion mechanism of gypsum mold were investigated.The effects of fly ash,sodium polycarboxylate cellulose(CMC),polyvinyl alcohol(PVA)and their composite additives on the corrosion rate of gypsum mold were analyzed.The experimental results show that the lowest corrosion rate is 3.6%,which is 1/4 times lower than that of the blank sample(i.e.,16.2%),when the content of fly ash is 7 wt%.The service life of the modified gypsum mold is more than 4 times greater than that of the conventional gypsum mold,and its water absorption rate is 53.82%,and the compression strength reaches 6.6 MPa(±0.3MPa),which fully meets the requirements for the medium-compression strength of 5 MPa,based on the National Standard GBT9776-2008.Also,the effects of type and content of fly ash,sodium polycarboxylate cellulose and polyvinyl alcohol additive on the gypsum sample and the molding properties of gypsum mold were investigated.The material of modified gypsum sample was designed.An orthogonal experiment with various additives and different contents of each additive was carried out.The superior molding performance and corrosion resistance rate can be obtained under the optimum conditions,i.e.,fly ash of 7 wt%,poly carboxylic acid sodium of 9 wt.% and polyvinyl alcohol of 5 wt%,and gypsum/water ratio of 1.25 g/ml.Furthermore,the corrosion mechanism of gypsum mold was analyzed qualitatively and quantitatively.The influence of pore size distribution on the corrosion rate of gypsum was investigated,and the kinetics of water-absorption process for gypsum was discussed as well.Based on the results by surface scanning analysis,Na2SiO3 deposits on the surface of fly ash,preferentially reacts with the fly ash,and reduces the reaction with gypsum(i.e.the corrosion rate of gypsum)and has an effect on the corrosion resistance.The microstructures of fly ash,poly carboxylic acid sodium and polyvinyl alcohol were characterized.The corrosion rate,water absorption,compression strength and microstructure of gypsum were determined.It is indicated that the greater specific surface area of fly ash can promote its reaction with Na2SiO3 in slurry to further hinder the corrosion of gypsum.The effects of CMC and PVA in gypsum mold were analyzed.The results show that CMC has a good thickening effect,and can improve the strength of gypsum mold as it reduces the growth rate of gypsum crystal on the C axis.CMC forms a layer of polymer film on the surface of the crystal to reduce the corrosion rate of gypsum mold for the improvement of the service life of gypsum mold.PVA changes the microstructure of gypsum to produce different capillary forces.PVA has a positive effect on the reduction of the corrosion rate,thus improving the service life of gypsum mold.Finally,it can be concluded that the gypsum mold reaches the lowest corrosion rate of 3.6%,when the fly ash content is 7 wt%,and the corrosion rate of gypsum mold of 4.5% and the superior molding performance can be obtained at fly ash of 7 wt%,CMC of 9 wt%,and PVA of 5 wt%. |
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