Sulfate Resistance Of Cement-based Materials In Low Temperature Environment Under Long-term Immersion

The rapid construction of the "Silk Road Economic Belt" in the "Belt and Road" strategy has brought a large amount of infrastructure investment to the vast western region.The climatic and geological conditions in the vast western region are complicated and changeable,and the environment in which a large number of structures such as bridges,highways and houses are exposed to more complicated structural durability problems,among which the problem of sulfate attack on cement-based materials is an inevitable one.Sulfate Salt erosion issues will be of great help to improve the durability of the structure.In this paper,according to the scene of the Bai-Ming Expressway under construction in Gansu section of Jing-Xin Expressway under construction,the actual conditions of the bailing expressway are used to simulate the sulfate attack conditions in the natural environment of the Bailing expressway.From the following aspects,Erosion changes in performance and microscopic products were studied:1.Through laboratory simulation of low temperature erosion environment,the use of temperature simulation box temperature control at 5±1?,10±1?,and compare it with room temperature environment 20±1°C.Immersion way to choose fully immersion form,to simulate the project is completely in the underground erosion environment,selected four kinds of mixture ratio of a total of 12 specimens for test analysis;2.Through the test analysis of 40mm×40mm×160mm cement mortar samples,comparing the performance changes and microscopic products after the sulfate attack of cement-based materials with different cement types,different water-cement ratio,different mineral admixtures ratio;3.The change indexes of the cement-based materials after the sulfate attack are as follows: the change of the appearance quality,the change of the compressive strength and the compressive and the anticorrosion coefficient,the change of the flexural strength and the flexural and anticorrosion coefficient,the coefficient of dynamic elastic modulus loss;4.Analysis of microscopic products after low temperature sulfate attack on cement-based materials was carried out using scanning electron microscopy(SEM)and electron spectroscopy(EDS);Based on the analysis of the indexes after the sulfate attack of cement-based materials,the changes of the flexural strength which are more sensitive to the change of several indexes are selected,and the change rules of the flexural strength with immersion time are analyzed.Fracture strength prediction model.Through the research on the sulphate corrosion of fully soaked cement mortar specimens under the low temperature environment(5±1?,10±1?),the comparative analysis of the test indexes was carried out.The results showed that:1.Under long-term low-temperature soaking environment,the erosion at 5±1°C is more severe than that at 10±1°C.The cement variety has obvious influence on the sulfate resistance of cement-based materials,and the middle sulfate cement has excellent resistance to sulfate attack.Ordinary Portland cement;2.The water-binder ratio has a significant effect on improving the sulfate resistance of cement-based materials,and the 0.36 water-binder ratio is significantly better than the 0.5 water-binder ratio in resistance to sulfate attack.3.The addition of complex mineral admixture of mineral powder and silica fume can significantly improve the sulfate resistance of cement-based materials.The combination of 15% mineral powder and 3% silica fume has the best resistance to sulfate attack.4.The flexural and corrosion resisting coefficient is the most sensitive to the sulphate attack,and the dynamic elastic modulus loss coefficient is the slowest.The analysis of the microscopic product shows that the common portland cement product may be a mixed needle of ettringite and carbon xonotite.In crystals,the corrosion products of middle-sulfur-resistant cements were significantly reduced,and many of them were flaky.The amount of products after the addition of mineral admixtures was greatly reduced,and no obvious needle-like crystals were produced;5.There is a good correlation between simulations of changes in flexural strength after sulphate erosion by the second-order polynomial equation.