Experimental Research On Cement Based Compound Shrinkage-reducing Admixture

The shrinkage cracking of concrete has been an important factor which decreases concrete durability for many years.Due to the hydration reaction,temperature and humidity changes the volume of cement-based materials always unstable,as the inherent property of cement-based material,which leads to material volume deformation and provide channel for the moisture migration and salt ions move,thereby lower the strength and durability of structures.The advent of shrinkage-reducing admixture caused extensive concern of the researchers,and generally accepted that shrinkage-reducing admixture mainly reduce shrinkage by reducing solution surface tension.At the same time,some scholars believe that by adding SRA can make the solution viscosity increase and decrease the evaporation rate,which are also conducive to the suppression of shrinkage of cement based materials.Surface tension,viscosity and evaporation rate of shrinkage-reducing admixture have been tested,combining with shrinkage performance of mortar specimens in 28 days to determine the mechanism of SRA.Comparing the microscopic pore structure and macroscopic performance of cement based materials with single component shrinkage-reducing admixture and compound shrinkage reducing admixture.The surface tension,viscosity and evaporation rate variation of the different shrinkage reducing admixture,thickening agent,moisturizing agent under different concentrations are compared.Through the result of orthogonal test calculate each agent's distinctiveness in changing the nature of solution properties.Based on the principle of high effect and low dosage to optimize and combine agents;by changing the surface tension or viscosity of the compound shrinkage-reducing admixture to explore the effect of solution properties on the shrinkage properties;through the testing of the properties of single and compound reducing admixture in aqueous solution and cement paste to determine the influence of the alkaline environment.Testing the contact angle variation of the compound shrinkage reducing admixture in water and cement paste in 28 days.Comparing the change of solution properties to determine long-term effectiveness of compound admixtures.Compare the influence of single and compound shrinkage-reducing admixture on the mechanical strength of mortar specimens.The effect of single shrinkage reducing admixture and compound shrinkage-reducing admixture on the pore structure and shrinkage of cement based material can be investigated which can be used to analyze the mechanism of admixtures.Through the above experiments,conclusions are that single component shrinkage-reducing admixture and compound shrinkage-reducing admixture can be long-term effective in alkaline environment in the solution properties.in the supernatant solution of cement paste the surface tension can be increased slightly.And the contact angle increased in the third day but returned to the initial valu e when it lasts to 28 th day,the increase of viscosity is in favor of increasing the solution contact angle.In the term of pore structure,both single component shrinkage-reducing admixture and compound shrinkage-reducing admixture are helpful to refine the pore within 10nm~50nm,the effect of compound reducing agent is better than the single component shrinkage-reducing admixture.The increase of solution viscosity will increase the total porosity.Shrinkage-reducing admixture can decrease the early strength of cement materials,the adverse effects of compound reduction on the strength are larger,but the strength of 28 days returns to normal.The increase of solution viscosity almost has no influence on the early strength of cement-based materials,but the strength of 28 days decreased.In terms of shrinkage,the same dosage of compound SRA on restraining the contraction is better than single component SRA,and the effect of restraining shrinkage depends not only on the surface tension,viscosity and evapo ration rate,also needs to consider the pore structure and the change of the contact angle.