Research On Pore Regulation And Repair Of Mortar By Pressurized Osmotic Crystallization Reaction

Cement-based materials are the most used materials in construction projects.But the material inevitably produces a large number of pores during the hydration process of cement.These pores become the infiltration channels of harmful substances,which not only seriously affect the mechanical properties of cement-based materials,but also reduce the impermeability,durability and service life.The use of permeable active substances to repair pores is a method to improve the impermeability and durability of cementitious materials,which can react with calcium hydroxide to repair pores.At present,the main method of infiltrating active substances is to prepare a slurry or solution of active substances to paint or soak cementitious materials,but these two common infiltration methods have the problem of insufficient penetration depth and penetration amount.The depth of penetration of the active substance in the cementitious material and the amount of penetration are increased by means of pressure infiltration,which further enhances the effect of the active substance on the repair of pores to meet the requirements of some buildings in corrosive environments and increase their service life.In this thesis,the effect of water absorption and compressive strength on the repair of mortar pores was investigated by pressurizing and infiltrating active substances under different conditions and combining with microscopic characterization to investigate the repair mechanism.(1)Experiments were conducted to explore the process parameters of pressurized permeation by screening the screened methyl silicate.The results show that the test blocks after pressurized infiltration showed a 55.25% decrease in water absorption and a 17.77% increase in compressive strength compared to normal pressure infiltration.The penetration depth and infiltration volume under 1 MPa pressure for 1h were increased by 400.00% and 148.92%,respectively.The optimal pressurized permeation process parameters were obtained with an osmotic pressure of 1 MPa,a permeation time of 1 h,a concentration of100% methyl silicate,two permeation times,and a permeation time interval of 7 d.The compressive strength of methyl silicate compounded with magnesium fluorosilicate can be increased by 91.88%compared with the blank group.It can be observed by scanning electron microscopy that after pressurized penetration of methyl silicate,it can generate fibrous C-S-H inside the mortar.By thermogravimetric as well as piezometric analysis,it was demonstrated that calcium hydroxide was consumed and mortar porosity was reduced by 0.91%,and the percentage of harmful pores in the range of 50-10000 nm was reduced.(2)Methyl silicate is expensive and not easy to transport as a liquid.In the case of adding cetyltrimethylammonium bromide,urea,which is the closest active substance to methyl silicate in reducing the water absorption of mortar and is cheap,was screened.Urea was most effective when compounded with admixture of 0.5% concentration of hydroxyethyl cellulose,reducing water absorption by 38.11%compared to the blank group.Urea + hydroxyethyl cellulose different porosity of mortar has different repair effect,porosity is larger or smaller will make the repair effect of urea + hydroxyethyl cellulose reduced.It can be observed by SEM that calcium carbonate was generated inside the mortar after pressure infiltration of urea.By thermogravimetric as well as piezometric analysis,it was demonstrated that calcium hydroxide was consumed and the resulting calcium carbonate clogged the pores resulting in a 0.66%reduction in mortar porosity.(3)Infiltration of methyl silicate and urea + HEC can improve the mortar’s resistance to sulfate ion attack,chloride ion attack,freeze-thaw resistance and acid attack resistance,and pressurized infiltration can further improve mortar durability than normal pressure infiltration.(4)The reaction mechanism was elucidated by simulating the reaction of methyl silicate and urea with calcium hydroxide separately to determine the reaction products.The consumption of calcium hydroxide and the generation of C-S-H gel and calcium carbonate were determined by SEM and XRD analysis,respectively,and these reactions produced C-S-H and calcium carbonate to fill the plugged pores to complete the repair of the pores.Properly increasing the reaction temperature can promote the degree of reaction between methyl silicate and calcium hydroxide.When the temperature is increased,the hydrolysis rate of urea becomes faster and the reaction rate with calcium hydroxide is also faster.