Study On Directional Modification Of Cement-based Materials With Siliceous Additives And Its Action Mechanism

Cement-based materials,as the most widely used building materials,face damage caused by environmental factors.Calcium hydroxide in cement hydration phase is a kind of hexagonal sheet crystal slightly soluble in water.Its chemical properties are unstable,leading to calcium ion dissolution,calcium silicate hydrate gel(C-S-H)decalcification,sulfate erosion and chloride ion erosion and other diseases of cement-based materials in special environments such as flowing water,saline-alkali and ocean.The service performance of cement-based materials is degraded and the service life is shortened.Therefore,according to the physical and chemical properties of calcium hydroxide,the design of the corresponding modification method is of great significance to the prevention and treatment of calcium hydroxide diseases.Three silicon additives,lithium silicate(LS),nano silica(NS)and isoctyltriethoxysilane(IS),were selected to conduct directional modification of calcium hydroxide in cement-based materials.In order to solve the problem of low specific strength of foamed concrete,the foamed concrete test block was impregnated with lithium silicate solution with modulus of 20 and good permeability,and the calcium hydroxide in hydration phase was directional modified by silicate ion.The results show that the secondary hydration reaction between lithium silicate and calcium hydroxide produces more C-S-H,optimizes the pore structure,and makes foamed concrete have better mechanical properties and durability.After treatment with 4.5%,9.0% and 18.0% LS solution,the compressive strength of GROUP-800 increased by 16.0%,26.0% and 54.0%,respectively.After 30 freeze-thaw cycles,the mass loss of Group N3 decreased from 32.0% to 24.5%,22.0% and 18.0% with the increase of LS concentration.In order to further explore the lithium silicate modification mechanism of calcium hydroxide for calcium hydroxide content is higher in natural water rigid lime(NHL),the characteristics of lithium silicate adopted to carry out directional modification,calcium hydroxide was studied to C-S-H gel transformation affect the performance of the NHL mortar,reveals the lithium silicate directional modification mechanism of calcium hydroxide.The nano silica pre-dispersion solution was prepared by mechanical stirring and ultrasonic dispersion process,and its distribution state in cement mortar was optimized.The effect of dosage on the workability of the mixture was studied.The effects of NS on the chemical composition and microstructure of cement-based materials were studied,the modification mechanism of NS on cement hydration products was explored,and the effectiveness of mechanical mixing and ultrasonic dispersion was evaluated.The variation law between NS content and impermeability of NS mortar was studied by simulating the deep water environment.The relationship between pore structure and mechanical properties and durability was revealed.The results show that the secondary hydration between nano-silica and calcium hydroxide generates new C-S-H,improves the pore structure of cement-based materials,and greatly improves their physical and mechanical properties and durability.However,the agglomeration problem of nano-silica has not been solved completely,and further study on its pre-dispersion process is needed.In view of the physical properties of calcium hydroxide dissolved in water,vacuum impregnation process was adopted to create reaction conditions.The calcium hydroxide in cement-based materials was modified with isoctyltriethoxysilane,and the surface properties of cement-based materials were changed.The superhydrophobic coating was successfully prepared,and the formation mechanism of the coating was studied.The results show that isooctyl triethoxysilane forms a self-assembled molecular film on the surface of cement material through dehydration and condensation with Ca-oh,forming a superhydrophobic coating with a thickness of 1-3mm.The coating has significantly improved the waterproof,impermeability and freeze-thaw cycling resistance of cement-based materials,and its own weather resistance is good.