Study On Formation Mechanism And Inhibition Of Delayed Ettringite In Concrete Under Simulated Geothermal Environment

With the promotion of the"western development"strategy,China’s infrastructure projects extend to the subtropical zone from southern Tibet to Western Sichuan to Western Yunnan.The geothermal high-temperature environment will make deep buried and extra long tunnels face the impact of high-temperature heat damage.In the geothermal high temperature environment,the monosulfide sulphoaluminate（AFm）formed at the initial stage of hydration and hardening of concrete cement is easy to form ettringite with sulfate ions in the interface area of pores or slurry aggregate,resulting in concrete expansion and cracking,which lays a serious hidden danger for the safe service of western infrastructure projects.In order to clarify the formation conditions of delayed ettringite（DEF）in concrete and to restrain or slow down the potential harm of delayed ettringite to the project,this paper uses the solution method to simulate the temperature humidity coupling liquid phase environment of ettringite formation,determine the temperature limit of delayed ettringite,and determine the appropriate amount of gypsum in cement;The mechanical properties and deformation evolution of concrete under geothermal environment are studied,and the effective measures to inhibit or slow down the formation of delayed ettringite are put forward.The specific research contents and conclusions are as follows:（1）It is found that the geothermal high temperature environment will affect the growth of ettringite crystals on the（100）and（110）crystal planes,resulting in the gradual decrease of ettringite production with the increase of temperature.70～75℃is the key temperature range for the transformation from AFt crystal phase to AFm crystal phase,but ettringite can still be formed in the liquid phase environment above75℃,and the amount of AFm transformed will gradually increase with the increase of temperature.（2）Through the analysis of hydration heat and hydration products of gypsum cement with different dosage,it is found that the amount of AFt in static slurry increases with the increase of gypsum dosage,and about 4%is the appropriate dosage of gypsum;2%gypsum content can not play a role in retarding the setting,but will promote the hydration reaction and shorten the setting time.If the content of gypsum is too high,it will not only generate more AFm under high temperature environment,but also increase the concentration of Ca²⁺and SO₄^2-in concrete pore solution and promote the delayed ettringite reaction of hardened cement paste.（3）Through standard curing and 90℃ high temperature curing of different types of concrete,it is found that high temperature curing in geothermal environment can accelerate the hydration rate of cement and improve its early strength;It has little negative impact on the later strength;In the early stage of hydration,the water in the slurry evaporates rapidly under the action of high temperature,forming various micro cracks;As the hydration reaction enters the acceleration period,the generated hydration products fill the gaps between cement particles,and the cracks at the edge of the pores gradually disappear and become flat.By mixing fly ash and metakaolin in concrete,we can complement each other,improve the high-temperature mechanical properties of concrete and reduce the number of connected pores;Through the ring test of mortar specimens under different curing temperatures,it can be seen that the early restrained shrinkage stress of concrete specimens increases with the increase of curing temperature（the compound mixing ratio is particularly obvious）,but the corresponding early tensile strength is also improved,and the shrinkage stress of compound ratio does not exceed its tensile strength.（4）By testing the expansion rate of Portland cement and high sulfate resistant saline mud based concrete,it can be seen that the expansion rate of Portland cement concrete cured by geothermal high temperature reaches 0.039%after 210 days.The increase of gypsum content in concrete leads to the further increase of expansion rate.The expansion rate of concrete increases exponentially with the increase of sulfate ion content.By adding fly ash and metakaolin,the secondary C-S-H gel generated by reaction can fill the connected pores in concrete,inhibit the release and transport of sulfate,and effectively inhibit the formation of DEF in geothermal high temperature environment,and the effect is more obvious when high sulfate content is applied.（5）By testing the internal surface strain of hollow reinforcement restrained by the end in different types of concrete cured at high temperature and calculating the expansion stress borne by the concrete,it can be seen that the expansion stress borne by Portland cement concrete at 210d has reached 1.42 MPa,and can reach 5.62 MPa after adding 5%gypsum;adding fly ash and metakaolin into Portland cement can reduce the expansion stress produced by DEF.Replacing Portland cement with high sulfate resistant cement can reduce the C₃A content of concrete system and reduce the expansion stress produced by DEF.It is more economical and effective to use Portland cement,fly ash and metakaolin to reduce the expansion stress of delayed ettringite.