Research On Application Of Metakaolin Mineral Admixture Based On Low-heat And Early-strength Concrete

The construction period of modern project is getting shorter and shorter,which requires ordinary Portland cement to achieve early strength and high early strength under the condition of satisfying construction performance,which leads to the development of cement in the direction of increasing the content and fineness of early strength minerals(C3S)in clinker.However,the increase of cement fineness and early strength mineral content in clinker also leads to the acceleration of early hydration rate,obvious increase of hydration heat release and excessive internal temperature stress of concrete.in engineering,the early shrinkage cracking of concrete(especially mass concrete)and prefabricated parts becomes more and more prominent,the later strength does not increase or even shrink,the long-term durability is degraded and the life is greatly shortened.Large amount of mineral admixture or selection of low-heat cement can effectively reduce the early hydration heat,reduce shrinkage(crack resistance),and improve the durability and service life of concrete,but the early mechanical properties of concrete develop slowly and can not meet the requirements of the construction cycle.It is urgent to develop cementitious material system based on low heat early strength concrete.In this paper,metakaolin with high pozzolanic activity is selected as mineral admixture.the effects of mineral admixtures on the hydration process,composition,structure and durability of ordinary Portland cement,medium heat Portland cement,low heat Portland cement and its concrete were studied.The results show that the addition of mineral admixture has little effect on the mechanical properties of cementitious material system,but it can effectively reduce the heat of hydration and improve the durability of concrete.The mechanical properties of ordinary Portland cement were improved by adding metakaolin alone or mixed with metakaolin high titanium slag powder,and the mechanical properties of the cementitious material system were better when the content of metakaolin was 9%.The early(3 d)compressive strength increased by 30.85% and 10.64%,respectively.The addition of metakaolin greatly increased the early(3 d)compressive strength of medium heat cement,with a maximum increase of 21.51%.Metakaolin has no obvious effect on the compressive strength of low heat cement in the early stage(3 d).The single addition of metakaolin will advance the hydration induction period of ordinary Portland cement and increase its heat release rate.when the content of metakaolin is 9%,the cumulative heat release of hydration is higher than that of pure Portland cement.With the addition of high titanium slag powder,the working performance of ordinary Portland cement is improved,the hydration cumulative heat release of cementitious material system is reduced,and the adiabatic temperature rise of concrete is reduced.After metakaolin was mixed with medium low heat cement,the cumulative hydration heat of cementitious material system decreased in 3 d and 7 d,but the mechanical properties increased.When metakaolin is mixed with different types of cement,the hydration heat of the cementitious material is low,the shrinkage is reduced,the pore structure is refined,the porosity is reduced,the interface transition zone is denser,and the durability(sulfate resistance,freeze-thaw resistance)is improved.