Research On Anti-corrosion Characteristics And Mechanism Of Metakaolin Concrete In Coal Measures In Salt Lake Area

In recent years,with the implementation of the "One Belt,One Road" strategy,more and more concrete buildings have been built in the central and western regions of my country.The strong saline-alkaline geological environment in these areas has put the durability of concrete structures facing a huge test.Finding suitable mineral admixtures to form a composite cementitious material system to improve the corrosion resistance of concrete is an effective way to improve the durability of concrete,to achieve environmental protection and sustainable development of concrete.Coal-measure metakaolin is a highly active mineral powder produced by calcination of coal-measure kaolinite in coal gangue.According to the environmental conditions in the salt lake area of Yuncheng,Shanxi,this paper simulated the actual local working conditions through wet and dry cycles-corrosive liquid corrosion.Coal-series metakaolin was put into concrete cementing materials to explore the influence of coal-series metakaolin on the physical and mechanical properties of concrete before and after corrosion,such as the appearance,quality change,compressive strength and resistivity of the concrete.The chemical composition of the concrete before and after corrosion and the characteristics of the interface transition zone were studied,and the mechanism of the coal-series metakaolin on the corrosion resistance of the concrete to the saline soil was analyzed,so as to provide a test basis for solving the corrosion resistance of the concrete in the salt lake area.The main research contents of this paper are as follows:(1)In this paper,the flexural and compressive strengths of cement mortars of various ages of 3d,7d,28 d,60d,and 90 d were tested by replacing 0%,5%,10%,and 15% of cement with coal-based metakaolin.The strength development law of the metakaolin mortar specimens with different content of coal measures with different curing ages was explored,and the strength of the mortar mixed with 10% fly ash was compared.The results show that the flexural and compressive strength of the mortar gradually increase with the increase of the curing age.The increase in the flexural and compressive strength of the coal-based metakaolin to the mortar increases first and then decreases with the increase of its blending amount.When the amount is 10%,the lifting effect is the best;the coal-based metakaolin can increase the 3d flexural strength and 7d compressive strength of the mortar the most.Mortars mixed with 5%,10% and 15% coal-series metakaolin have a higher range of flexural-compressive strength and compressive strength at various ages than those mixed with10% fly ash.(2)The coal-based metakaolin composite cementitious material system was used to make concrete,and the plain concrete,10% fly ash concrete,10% fly ash and 5% or 10%coal-based metakaolin composite concrete were measured for 3d,7d,28 d,60d,90 d compressive strength,to study the influence of composite cementitious material system with different coal series metakaolin content on the compressive strength of concrete at various curing ages.The results show that the improvement effect of the mixed coal measure metakaolin on the compressive strength of each age is better than that of the single fly ash,and the improvement range for the 3d compressive strength is the largest,and the improvement range after 7d is reduced to some extent.Under the condition of mixed doping,10% metakaolin content of coal series is higher than 5% metakaolin content for the improvement of compressive strength of each age.(3)The dry and wet cyclic corrosion tests of plain concrete,concrete with single 10% fly ash,and composite concrete with 10% fly ash and 5% or 10% coal measure metakaolin were carried out using unmodified saline aqueous solution and 5wt% sodium sulfate solution as corrosive media.The apparent integrity,mass loss rate and compressive strength of concrete at 0d,30 d,60d,90 d,120d and 150 d under different corrosive environments were measured to study the corrosion deterioration law of concrete under different corrosive conditions and the influence of coal metakaolin composite cementitive material system on the corrosion resistance of concrete.The results show that the apparent deterioration,mass loss rate and compressive strength decrease of concrete corroded by undisturbed saline solution are smaller than those corroded by sodium sulfate solution.With the increase of corrosion age,the apparent integrity of concrete decreases gradually,and the compressive strength and compressive corrosion resistance coefficient increase first and then decrease.Under the same corrosion condition,the concretes of composite cementitious material system have higher apparent integrity,smaller mass loss rate and larger compressive strength and corrosion resistance coefficient.(4)LCR digital bridge was used to measure the resistivity of each concrete in content(3),and the influence of composite cementing material system with different metakaolin content of coal measures on the resistivity of concrete was studied,and the change rule of resistivity with the degree of corrosion and deterioration of concrete was explored.The results show that the initial resistivity of concrete can be improved by fly ash alone,and the improvement effect is greater by metakaolin mixed with coal measures;the improvement effect of 10%coal series metakaolin is better than that of 5% coal series metakaolin.Under the same corrosion conditions,with the increase of age,the overall resistivity of concrete increases first and then decreases,which is consistent with the change trend of concrete compressive strength.The corrosion deterioration degree of concrete can be inferred through the change of resistivity.(5)The composition of pure cement slurry,single 10% fly ash,single 10% coal metakaolin cementing slurry,and compound cement slurry of 10% fly ash and 10% coal metakaolin before corrosion and 150 days after corrosion of undisturbed saline solution or sodium sulfate solution with mass fraction of 5% were detected by X-ray diffraction.The influence of coal metakaolin on the composition of hydration products and its corrosion degradation of cementified slurry was analyzed.The results show that before corrosion,the calcium hydroxide diffraction peak of hydration product in cementified slurry mixed with coal metakaolin and fly ash is lower than the corresponding peak in cement paste,and ettringite diffraction peak is basically the same.After 150 days of corrosion,the diffraction peaks of calcium hydroxide,gypsum and ettringite in the composite cementing slurry are all lower than the corresponding peaks in the net cement slurry.The diffraction peaks of calcium hydroxide,gypsum and ettringite in the composite cementing slurry mixed with 10% coal metakaolin are the lowest.(6)Scanning electron microscopy was used to detect the appearance of interface transition zone of plain concrete,concrete with single 10% fly ash,concrete with mixed 10%fly ash and 10% metakaolin of coal measures before corrosion and after corrosion of undisturbed saline aqueous solution or 5% sodium sulfate solution for 150 days.The structural characteristics of interfacial transition zone were studied and the influence of mixed coal metakaolin on the weak interface and its corrosion resistance of concrete was analyzed.The results show that the cementitious slurry near the aggregate has many original defects,loose structure and poor compactness.The compactness of cementified slurry mixed with coal metakaolin and fly ash is relatively high,which is beneficial to improve the interfacial transition zone and enhance the corrosion resistance of concrete.In conclusion,coal metakaolin can improve the corrosion resistance of concrete,and the corrosion resistance of concrete mixed with 10% fly ash and 10% coal metakaolin is the best.The resistivity is an indicator of the corrosion of concrete.