Deterioration Mechanism Of Cement-based Materials In Complex Salt Environments

China has a vast territory,containing a large amount of sulfate ions in the seawater of the southeast coast and the saline-alkali zone in the northwest inland.Sulfate ions enter the concrete and react with certain components of the cement hydration product to form ettringite and gypsum.The expansive product causes the concrete to crack,flake and accompany a decrease in strength.The building will be withdrawn from service early when it has not reached the designed service life,causing great economic losses.The content of tricalcium aluminate in cement has an important influence on the resistance of concrete to sulfate attack.In the actual environment of sulfate-eroded concrete,it is often accompanied by the presence of other harmful ions,such as Mg²⁺,Cl^-,etc.,and many cracks in the process of sulfate attack on concrete will undoubtedly accelerate the penetration rate of these harmful ions into the concrete.These harmful ions themselves will corrode the concrete,and when coexisting with the sulfate,it will have different effects on the performance degradation of the sulfate-eroded concrete.Therefore,in this experiment,after preparing tricalcium aluminate?3CaO·Al₂O₃,C₃A?,the deterioration law and mechanism of cement stone prepared by tricalcium aluminate in different sulfate solutions?Sodium sulfate and magnesium sulfate?were studied in the third chapter.Furthermore,in the fourth chapter,the effects of tricalcium aluminate content in cement on the deterioration of mortar performance under the different salt environments of sodium sulfate and magnesium sulfate were explored.Based on the test results of the first two parts,in Chapter five,the performance degradation of mortar under the combination of chloride salt and different sulfates is explored.The above research results show that:?1?The cement stone made of tricalcium aluminate may undergo swelling damage in both sodium sulfate and magnesium sulfate solutions.The formation of ettringite and gypsum is the main reason for the expansion cracking of cement stone in two solutions respectively.?2?The main erosion product of the mortar in the sodium sulfate solution is ettringite and gypsum.When immersed for 300d,when the content of tricalcium aluminate in cement increased from 6.7%to 15.1%,the amount of ettringite increased from 21.66%to 42.26%.?3?In magnesium sulfate solution,the growth of ettringite in mortar is the main reason for its expansion and mass growth in the early stage of erosion.In the later stage of erosion,gypsum formation is the main cause of damage.?4?When the mortar is immersed in a composite solution of sodium sulfate and sodium chloride,chloride ions will penetrate into the mortar before the sulfate ions,and react with the hydrated calcium aluminate to form hydrated calcium chloroaluminate,which inhibits calcium.The formation of vermiculite reduces the damage and deterioration of the performance of the mortar in the sodium sulfate solution.?5?When the mortar is immersed in the composite solution of magnesium sulfate and sodium chloride,the presence of chloride ions accelerates the deterioration of the mortar in the magnesium sulfate solution,which is mainly related to the large amount of gypsum generated in the later stage of erosion.