Study On Deterioration And Its Mechanism Of Cement-based Materials In The Coexistence Of Electrical Field And Sulfate

Sulfate attack is a major threat for the durability of cement-based materials.When cement-based materials are exposed to the environment containing sulfate,the sulfate ions ingress into the materials,and subsequently react with hydrated cement products to form gypsum and ettringite which can cause expansion,cracking and loss of mechanical properties,or cement-based materials suffer from physical sulfate salt attack.Currently,there are numerous researches on the deterioration process and mechanism of cement-based materials exposed to sulfate attack and couple with other environmental factors.However,with the continuous development of economy and technology,electrical field is increasingly applied to or exists in the actual project environment,such as the stracy current of rail transportation and Electro-Osmotic Pulse technology.Under the influence of electrical field,the process sulfate attack on cement-based materials will become more complicated.Therefore,the deterioration behavior and mechanism of cement-based materials in the coexistence of electrical field and sulfate was investigated in this paper.The mian research contents and results are listed as below.The migration rate of sulfate ions in cement-based materials under electrical field was studied,and the influences of liquid environment and specimen composition were discussed.The content and distribution of products of sulfate attack were analyzed by using XRD and SEM-EDS.The results show that electrical field greatly accelerates the migration of sulfate ions.After 90 days,the migration depth and total content of sulfate ions in the specimen subjected to electrical field are 4.38 to 6.25 and 7.50 to 14.00 times higher than those without electrical field,respectively.The migrated sulfate ions react with cement hydrates to form a great number of gypsum and ettringite.The application of NaOH as the anode solution can avoid the acid corrosion of specimen.The adjustment of pH value of cathode solution around 7.0 is beneficial to the penetration of sulfate ions.The migration of sulfate ions is primarily controlled by the potential difference of electrical field,while the sulfate concentration difference has a little impact.Addition of fly ash has a certain restriction on the penetration of sulfate ions,while the use of limestone powder and increase in water/binder ratio have the opposite effect.The influence of electrical field on the cement hydration and products of sulfate attack was analyzed by measuring the temperature in the specimen and solution under electrical field,as well as using DSC-TG,FT-IR and XRD.The results show that the heat produced raises the temperature in the specimen,but the temperature rise is limited,aboute 1.5 to 4.9 oC,which should not markedly accelerate the later-stage hydration of cement.In addition,electrical field also has not changed the Si-O bond in C-S-H gel within a short period.The migration of Ca²⁺,SO42-and Al?OH?4-ions in the pore solution of specimen under electrical field results in the reduction of their number,causing the decomposition of ettringite formed from the early-stage hydration of cement and sulfate attack.The change rules of Ca²⁺ ions concentration in the external solution and calcium content in the cement-based materials under electrical field were analyzed.The change of composition and structure of cement hydrates caused by leaching of Ca²⁺ ions was investigated by using XRD,SEM-EDS and MIP.The results show that electrical field accelerates the leaching of Ca²⁺ ions from the specimen.The calcium hydroxide in the part of specimen close to cathode dissolves continuously and then C-S-H gel starts to dissolve.Therefore,the porosity and total pore volume increase and then the strength reduces.When water is chosen as the cathode solution,Ca/Si ratio of C-S-H gel in the section of specimen close to cathode decreases 5.1 to 13.5 percent,the porosity increase 20.9 to 27.4 percent,and the strength of specimen decreases 6.7 to 15.6 percent under electrical field after 90 days.The deterioration behavior and mechanism of concrete under electrical field combined with sulfate attack was systematically studied though compressive strength and chemical test as well as microscopic analysis of XRD and SEM-EDS.The results show that a great number of sulfate ions migrate into concrete and then suflate attack is accelerated under electrical field.At the same time,Ca²⁺ ions are continually leached from concrete,causing the porous microstructure,which further accelerates the penetration of sulfate ions.Therefore,the deterioration process of concrete is accelerated under this condition.The deterioration involves two major processes of sulfate attack and leaching,and the sulfate attack is dominated one.The rate of sulfate attack under electrical field is about 3.4 to 4.4 times higher than that of immersion in Na₂SO₄ solution.It is found that the electrical field accelerate the sulfate attack and leaching-induced damage of cement-based materials.This suggests that when the urban rail transportation is built and Electro-Osmotic Pulse technology is applied,close attention should be paid to the deterioration and damage of concrete structure caused by electrical field especially for the environment containing sulfate.The results also show that during the acceleration of sulfate attack,electrical field does not markedly affect the hydration degree of cement and change the Si-O bond in C-S-H gel.It is worth to further discuss whether electrical field is applied to rapidly evaluate the sulfate resistance of cement-based materials.