| As an ancient building material,gypsum has excellent performance of environmental protection,energy saving,light weight,heat insulation,fire resistance,sound absorption,dimensional stability.However,gypsum material has poor water resistance,which greatly limits its development in various fields.In order to solve the problem of gypsum water resistance,this paper analyzed the causes of poor water resistance of gypsum,and summarized the research status of gypsum modification at home and abroad.The modification performance was evaluated from the aspects of the density,water absorption,volumetric stability,flexural strength,compressive strength and softening coefficient by mixing and coating phosphate.The experimental results show that the foaming and retarding phenomenon appeare after single mixing of phosphoric acid in gypsum which leads to the phenomenon of connecting holes and collapse.After the incorporation of phosphoric acid,the water absorption is increased due to foaming.When the dosage is more than 2%,the water absorption decreases with the increase of the content.XRD and SEM analysis show that a water-insoluble calcium hydrogen phosphate generats after the addition of phosphoric acid monocalate,which cover the surface of the gypsum crystal,resulting in the improvement of the water resistance of the gypsum.The experimental results of PVA and dihydrate gypsum show that the effect of stabilizing foam after adding PVA is better than other contrast group.The initial and final setting time are greatly shortened after adding dihydrate gypsum,.The effect of the gypsum adding with PVA and dihydrate gypsum is the best when the content of the phosphoric acid is 2%.Gypsum density is reduced by 14.3%,water absorption is reduced by 12.8%,volume stability is improved by 50%,however the dry flexural strength is reduced by 75.3%,and dry compressive strength is reduced by 85.6% compared to the comparative gypsum.The experimental results show that perlite can effectively improve the mechanical properties and softening coefficient of gypsum.The optimum amount of expanded perlite is1.5%.With the incorporation of expanded perlite,the dry flexural strength of gypsum is increased by 46.9%,the dry compressive strength is improved by 27.0%,the flexuralsoftening coefficient is improved by 12.9% and the compressive softening coefficient is increased by 3.0%.The experimental results show that the optimum fiber content is 2.5%,adding in which,the dry flexural strength is 116.8% higher than that of the non-wheat straw fiber,and the dry compressive strength is 183.9% higher than that of the non-wheat straw fiber.The SEM test shows that the wheat straw fiber and gypsum are well bonded and that the addition of PVA prevents the wheat straw fibers from being corroded by the acid.The experimental results of the aqueous solution of phosphoric acid shows that the optimum dilution ratio of phosphoric acid and water is 1: 4.At this point the improvement of water resistance of gypsum is better,and the water absorption is reduced by 8.5%.The flexural softening coefficient is 0.57,which is 54.1% higher than that of the blank gypsum,The compressive softening coefficient is 0.52,which is 44.4% higher than that of the blank gypsum,and the flexural and compressive strength decreased slightly.The optimum coating thickness is four layers.The water absorption of the blank gypsum decreases by 9.3%,and the dry flexural strength increases by 4.9%,and the dry compressive strength increases by1.8%,and the flexural softening coefficient increases by 18.9%,and the compressive softening coefficient increases by 25%.After coating with phosphoric acid,the SEM test shows that the surface of the gypsum is covered with a layer of material.Compared with the blank gypsum,the crystal and the inter-crystal pores are reduced,and the crystal contact points are increased,and the crystal becomes more compact. |
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