The Effect Of Metakaolin And Curing Methods On Performance Of Portland Cement Mortar

China is a country with abundant mineral resources.In recent years,the development of mineral resources has greatly increased,but the utilization rate of mineral resources is very low.Therefore,a lot of mineral solid wastes,especially tailings of mineral resources,such as common fly ash,silica fume,iron tailings and metakaolin have been produced.The accumulation of solid waste of mineral resources not only causes environmental pollution,but also brings a lot of waste of resources.Metakaolin and fly ash in China are rich in mineral resources and stable in physical and chemical properties,but their application rate is not high.Due to its small particle size and large surface area,metakaolin has a strong filling effect,which can refine the pore size and pore structure of the system,effectively reduce the number of harmful pores,and make the structure of the cement sample more compact,thereby improving the compressive strength of the sample.At the same time,a large surface area will increase the thickness of the water film on the surface of the particles and cause the decrease of the free water content in the system,which is equivalent to reducing the water-cement ratio in the system to a certain extent,and increasing the lubrication between the particles,thereby increasing the effective collision between the particles and accelerating the reaction speed and process.At the same time,metakaolin contains a high content of Al2O3 and SiO2 and has a strong pozzolanic activity,which can consume Ca(OH)2 in Portland cement system to generate more hydrated calcium silicate gel(C-S-H),promoting the reaction to the positive reaction direction,increasing the reaction process and speed of Portland cement,and further enhance the strength and durability of cement samples.In this paper,traditional Portland cement mortar was used as the control group to explore the influence of different substituted metakaolin(0,5%,10%,15%,20%)on the mechanical properties and sulfate resistance of cement mortar.The phase composition and microstructure of hydration products were analyzed and explained by X-ray diffraction and scanning electron microscope.The final results show that the standard curing condition,metakaolin can significantly enhance the compressive strength of Portland cement mortar,especially early strength,10%dosage of metakaolin to replace Portland cement specimen M10,the compressive strength in one day three times that of ordinary Portland cement mortar strength,close to the compressive strength of ordinary Portland cement sample 3 days.The hydration products of ettringite appeared in the cement sample with metakaolin compared with the ordinary Portland cement sample one day after hydration,indicating that metakaolin could accelerate the reaction progress and speed in the system.This will bring great benefits for the early demoulding of cement products,can realize the early molding demoulding of cement products,shorten the curing molding cycle,which will provide a strong theoretical reference for the realization of cement products in the practical production and application of construction engineering.Meanwhile,the compressive strength and durability of sample M10 are also higher than other groups,showing super comprehensive performance.Secondly,the research shows that the traditional silicate cement is highly dependent on water in the early stage,and the time period of wet curing in the early stage is long.The addition of metakaolin can greatly reduce the sensitivity of silicate cement to water and reduce the wet curing time of cement mortar samples in the early stage.In this paper,the effects of different curing methods and metakaolin content on the mechanical properties and sulfate resistance durability of traditional silicate cement mortar were investigated.At the same time,using the theory that the addition of metakaolin causes the increase of water film thickness on the surface of mortar concrete particles,a simple mechanism analysis is carried out on the results of the increase of cement strength by metakaolin.The final results show that the traditional silicate cement mortar is highly dependent on the wet curing time in the early stage,and the curing time cannot be less than 7 days.However,the incorporation of metakaolin can greatly reduce the sensitivity of silicate cement to water.Even the compressive strength of metakaolin cement mortar specimens with the wet curing time of the first three days can reach 49.1 MPa in 28 days.This can greatly shorten the early wet curing time,and the cement mortar specimens with metakaolin also show excellent durability against sulfate attack.This will provide a strong theoretical basis for the early demoulding of cement products in construction projects.At the same time,this study can also provide a theoretical reference for the construction of arid climate and saline soil areas,which has great reference value.Finally,this paper explores the influence of the mixture of metakaolin and fly ash in different proportions(1:1,1:2,1:3,2:1,3:1)on the mechanical properties and durability of cement mortar samples under standard curing conditions.The final results show that metakaolin has more advantages than fly ash in improving the properties of Portland cement in cement mortar samples mixed with metakaolin.At the same time,under standard curing conditions,the optimal ratio of metakaolin and fly ash is 3:1,and the optimal total replacement is 15%.Compared with other samples of the same curing age,the samples have the maximum compressive strength and durability.At the same time,the results show that the effect of metakaolin mixed with fly ash is better than that of metakaolin alone,especially in the durability of sulfate erosion.