Study On Corrosion Resistance Of Fly Ash Cement-Based Materials Under The Action Of Dry-Wet-Sulfate

In recent years,with the continuous development of national economy and society,people pay more and more attention to environmental problems.On this basis,the CPC Central Committee and the State Council put forward the development concept of"Peak carbon and carbon neutrality"at successive sessions of the People’s Congress,general Secretary XI also put forward the"Two mountains"innovation theory of"Green Water and green mountains are mountains of gold and silver".But in our country,as a large thermal power generation country,a large amount of fly ash is produced every year.If it can not be well utilized,it will cause serious pollution to the surrounding environment and even endanger human health,this is obviously contrary to the"Two mountains"theory,and the Portland cement also produces a large amount of CO₂ in the process of preparation,contrary to the"Low carbon"concept.But if fly ash is mixed into cement as mineral admixture,it can not only make good use of fly ash,but also reduce the demand for cement.As an important transportation hub for the strategic development of Belt and Road,the Xinjiang region has a large demand for the infrastructure construction industry and a huge demand for concrete.In the process,a large amount of cement must be consumed,so in this area will fly ash into cement can better carry out the implementation of our country’s new development concept.In view of the service quality of steam-cured cement-based materials with large amount of fly ash in the complex environment of Xinjiang,the basic mechanical properties,hydration characteristics and corrosion resistance of cement-based materials with high volume fly ash by steam curing under the condition of dry-wet cycle-sulfate attack were studied,and the flexural compressive strength was measured,the microstructure and phase composition of fly ash cement-based materials were analyzed by XRD,DSC-TG and SEM.Finally,the optimum steam curing system,fly ash content and fine grinding time of cement-based materials with high fly ash content were determined.The main conclusions drawn from the above-mentioned studies are as follows:（1）For curing temperature.The effect of steam curing on the early flexural strength of high volume fly ash cement mortar is obvious,and the higher the steam curing temperature is,the more obvious the effect is,and in the curing stage,the higher the steam curing temperature is,the higher the flexural strength is,the higher the flexural strength is,the higher the steam curing temperature is,the higher the flexural strength is,the higher the flexural strength is,the higher the steam curing temperature is,the higher the flexural strength is,the higher the flexural strength is,the higher the flexural strength is,the higher the flexural strength is,the basic mechanical properties of steam-cured 80℃fly ash cement mortar are the best,and the compressive strength of steam-cured 90℃fly ash cement mortar will increase at the erosion stage,the remaining steam curing temperature of fly ash cement mortar will appear two stages of compressive strength growth。（2）For the content of fly ash.During the erosion stage,the compressive strength of the pure cement mortar specimens will increase in one stage,and that of the mortar specimens mixed with fly ash will increase in two stages,which is due to the continued hydration of fly ash during the erosion stage,increased compressive strength.At the same time,the addition of fly ash can well resist the strength deterioration of cement-based materials due to steam curing.（3）for the fineness of fly ash.The fly ash ground for 15 min is beneficial to the increase of 28d flexural strength of cement mortar,especially when the fly ash content is 15%,the flexural strength of the specimens raised by 11.24%under the condition of steam curing at 80℃;The pulverization of fly ash has little effect on the compressive strength of steam-cured specimens.When the content of fly ash is not more than 30%,the corrosion resistance of the specimen ground by fly ash for 15min is better,especially when the content of fly ash is 15%,the sulfate attack is finished,the compressive strength of the specimens with ground fly ash for 15 min was 123.3%of that with unground fly ash.（4）the results of XRD,DSC-TG and SEM show that steam curing can increase the hydration rate of cement,but it can decrease the hydration degree of cement at the later stage,and make the internal crack of the specimen can not be filled at the later stage,the mechanical strength of pure cement specimens is high in the early stage and low in 28d,especially 90℃,and the flexural strength of the specimens decreases by 5.7%from the early stage to the final stage.In addition,steam curing can promote the secondary hydration of fly ash,consume a large amount of Ca（OH）₂,generate C-S-H,fill the internal pores of the specimen,improve the interface transition zone,the results show that the compressive strength of fly ash mortar under steam curing condition of 70℃and 80℃is higher than that under standard curing condition,and the flexural strength of fly ash mortar under steam curing condition increases with the increase of fly ash content.（5）the results of XRD and SEM show that the ettringite,gypsum and sodium sulfate are the corrosion products of pure cement after steam curing in the wet-dry-sulfate environment,gypsum and sodium sulfate are the main corrosion products,and gypsum and ettringite are the main corrosion products of fly ash cement mortar after steam curing.（6）The parameters of mortar with better corrosion resistance are:steam curing 70℃,fly ash grinding15min,fly ash content 15%.