Study On Properties And Mechanism Of Polyvinyl Alcohol Modified Cement Composites

Polyvinyl alcohol is a water-soluble polymer widely used in different industries due to excellent chemical resistance and physical properties.Polyvinyl alcohol in the form of aqueous solution can improve workability and adhesive properties of cement-based composites.The physical and chemical properties of polyvinyl alcohol mainly depend on degree of hydrolysis and polymerization.Although polyvinyl alcohol with varying degree of hydrolysis and polymerization has been used,there is limited knowledge of the effect of degree of hydrolysis and polymerization of polyvinyl alcohol on the properties of fresh and hardened cement based composites.In this paper,the effect of polyvinyl alcohol with different degrees of hydrolysis and polymerization on fresh and hardened cement-based composites is investigated.Besides,the mechanism of polyvinyl alcohol on cement-based composites is discussed.The main results and conclusions are as follows:1.The effect of degree of hydrolysis and polymerization of polyvinyl alcohol on early workability and hydration process of cement-based materials was studied.The results showed that the rheological model of polyvinyl alcohol modified cement composites conformed to the modified Bingham model.With increasing degree of polymerization of polyvinyl alcohol,the yield stress and plastic viscosity of modified cement mortar in fresh state increased sharply.However,the degree of hydrolysis of polyvinyl alcohol had no obvious effect on the yield stress and plastic viscosity of modified cement mortar.The addition of polyvinyl alcohol retarded cement hydration and decreased early hydration rate.As the degree of polymerization of polyvinyl alcohol increased,the retardation for cement hydration was intensified.Compared with fully hydrolyzed PVA,partially hydrolyzed had greater retardation on cement hydration.2.The effect of degree of hydrolysis and polymerization of polyvinyl alcohol on mechanical properties of cement-based materials was studied.The results showed that the effect of degree of polymerization of polyvinyl alcohol on flexural strength of cement mortar was affected by the amount of polyvinyl alcohol.As degree of polymerization of polyvinyl alcohol increaseed,compressive strength of cement mortar increaseed.Compared with partially hydrolyzed PVA,fully hydrolyzed PVA modified cement mortars had greater compressive strength and flexural strength.The addition of polyvinyl alcohol could improve adhesive properties of cement mortar significantly.With increasing degree of hydrolysis and polymerization of polyvinyl alcohol,bond strength of modified cement mortar increased gradually.3.The mechanism of polyvinyl alcohol on cement-based materials was studied.The results showed that the addition of different polyvinyl alcohol slightly increased overall degree of cement hydration and affected the structure and morphology of Ca?OH?₂ and C-S-H.The addition of polyvinyl alcohol increaseed porosity of cement-based materials but refined pore size distribution.As degree of polymerization of polyvinyl alcohol increased,the porosity of cement composites increased.Besides,it could be found that polyvinyl alcohol film covered the surface of cement hydration product,filled in pore structure which refined pore structure and appeared between Ca?OH?₂ layers.The formation of polyvinyl alcohol film,the decrease in content of Ca?OH?₂ and finer pore distribution of polyvinyl alcohol modified cement composites were beneficial to the enhancement of bond strength and flexural strength.