Corrosion Mechanism Of Concrete Subjected To Sulfate Salt In Complex Environment

The existence of the North Sea coastal saline land area and a large area west of SaltLake sulfate ions, combined wet and dry cycle effects caused by freezing coldenvironment effects, the water level changes and the like.In these complexenvironmental effects, quick and easy concrete failure.To reveal the process of concretecorrosion damage in the above environment, especially the mechanism of corrosiondamage, the paper systematically studied the concrete in sulfate soaking wet and drycycle, as well as freeze-thaw environment corrosion damage.Comprehensive physical environment five categories: immersion,50℃immersion,freeze-thaw cycles, wet-dry cycle, ocean tidal zone, four types of erosion solution: water,water,5%sodium sulfate solution, complex solution (3.5%NaCl+5%sodium sulfate)orthogonal test.Using three types of analysis: macro mechanics and other performancetests, SEM-CT-MIP-DSC microscopic testing, ultrasonic nondestructive testing.Themain findings are as follows:(1)The solution temperature accelerates the corrosion of sulfate ion penetration rate,and promote the chemical reaction of cement hydration products and sulfatebetween.Compared to the normal temperature corrosion, sulfate ion corrosion of cementpaste at50°C reaction temperature higher than17%, but the corrosion products of thedecomposition of calcium alum Shifa Sheng, corrosion products are mainly gypsum.(2)Concrete wet cycle-under the action of sulfate corrosion, the main corrosionproducts for a large number of ettringite and a small amount of gypsum.Compared toimmersion corrosion, wet and dry cycle sulfate ions accelerated penetration rate, theapparent diffusion coefficient of concrete sulfate ions is about2times soakedenvironment.And the amount of sulfuric acid ions is higher than about10%immersionenvironment.(3)Freeze-thaw cycles during low temperatures reduces the diffusion rate ofconcrete sulfate ions and sulfate ions reaction volume, but the extent of freeze-thawdamage increase will result in an increase in concrete sulfate ion diffusion rate.Freeze-thaw cycles in the complex salt solution, the reaction capacity of the sulfate ion in solution is a sodium salt of freezing times of concrete to produce ettringite,thaumasite amount of gypsum and more corrosion products, increased capillary2.2times, the most probable pore size increases the8nm, concrete flaking worse.(4)Chloride ion concentration in the salt gradient freeze damage in concretedistribution, calculated in accordance with proposed expansion stress its surface is1.73times generated internally, causing chloride layer by layer thaw spalling of concretedamage. Sodium sulfate solution at low temperature ice expansion with itsconcentration increases, and higher than the chloride salt solution ice expansion; Butbecause concrete produces ettringite and gypsum in sulfate solution freeze-thaw process,solid corrosion products in volume expansion was less than the expansion rate at lowtemperatures, which leads to the degree of damage is less than concrete solution, andless complex salt solution and chloride salt solution.(5)Compressive strength of the salt solution in the absence of constraint status in0.17~0.3MPa, seawater freezes strength> sulfate> chloride, but less than the tensilestrength of concrete. Continuous freeze-thaw cycles of freezing and opening holesproduce fatigue loading, open hole wall erosion and eventually form a connectedmacroscopic cracks, water damage to the extent of the hole above the sodium sulfatesolution.