Damage Machanism On Cement-based Materials To Thaumasite Sulfate Attack Under The Reaction Of MgSO₄

As we all know,sulfate attack is one of the most important factors that causes damage to concrete structures.Carbon Sulfonite type sulfate attack（TSA）is different from traditional sulfate attack,and its conditions are low temperature,high humidity,SO₄^2-,CO₃^2-,and the damage feature is a progressive disintegration type and easier to ignore.Especially in the saline area of Xinjiang Autonomous Region,sulfate/sulfite saline soil is rich in soluble ions such as SO₄^2-,Na⁺,Mg²⁺and so on.Combining with the climatic conditions in Xinjiang Autonomous Region,it is more likely to occur TSA destruction than in other parts of the country.Therefore,mastering the principle of TSA reaction is of great reference significance for further understanding of the damage-deterioration law of cement-based materials,especially the concrete structure in the saline area of Xinjiang.Raman spectroscopy and XRD phase searching were used to make a qualitative analysis of thaumasite in cement-based materials,and XRD was used for semi-quantitative analysis.Quantitative analysis was carried out on the content of thaumasite in cement-based materials by an improved combination of chemical analysis and K value method.The paper studies the change trend of the content of thaumasite in cement-based materials with age,the proportion of sulfate-eroded products of cement-based materials,and the ratio of water to cement and the content of thaumasite in cement-based materials by diffetent solution.The research results show that the content of thaumasite in cement-based materials increases slowly with the age;the sulfate-attack products of cement-based materials produced by the test specomen have the largest proportion of thaumasite;In the presence of external sulfate ions,the larger the water-cement ratio,the greater the content of thaumasite,and in the absence of external sulfate ions,the smaller the water-cement ratio,the greater the content of thaumasite.The fracture strength tests were performed on cement mortars of different attack ages,and the compressive strength tests were performed on cement pastes and mortars.The effects of TSA reaction on the relevant mechanical properties of cement-based materials were studied,and the plastic damage constitutive relationship was established.The results show that as the age increases,the compressive strength of cement paste and cement mortar tends to increase first and then decrease;when the presence of external sulfate ions,the compressive strength decreases more significantly and the damage is more serious.Combining the effect of TSA reaction on the relevant mechanical properties of cement-based materials and the content of thaumasite from quantitative analysis,the damage factor of cement-based materials caused by TSA reaction was derived,and a TSA damage constitutive model for cement-based materials was established.Compared with the plastic damage constitutive model,the damage and degradation law of concrete structure by TSA reaction is discussed.The research results show that the TSA damage factor is a non-decreasing function of age,and the value ranges from 0（no damage）to 1（complete damage）.A TSA damage constitutive model for cement-based materials is established to respond to TSA on concrete mechanics.The durability quality of concrete material was predicted.