Research On Improving The Performance Of Cement And Hydration Mechanism Of Metakaolin

Coal series kaolin is widely distributed in China,it`s stable in physical and chemical properties and suitable for cement admixture when it has been calcined into metakaolin.This paper studied mineral characteristics of coal series kaolin and metakaolin;influence of content and calcination temperature of metakaolin on cement properties,microstructure,hydration products and other aspects,and discussed the function and mechanism of metakaolin as cement admixture in cement hydration reaction by XRD,SEM,TD-DSC and mechanical strength.Mineral properties of coal series kaolin were analysed by XRD,TG-DSC and FTIR.The results showed that the main mineral is kaolinite,containing a small amount of quartz,feldspar,pyrophyllite and anatase.The main chemical components of coal series kaolin are SiO<sub>2</sub> 49.03%,Al<sub>2</sub>O<sub>3</sub> 34.18%,TiO<sub>2</sub> 1.71%,Fe<sub>2</sub>O<sub>3</sub> 0.73%,SO<sub>3</sub> 0.23%and the loss of ignition is 13.5%.The influence of calcination temperature was also researched which the best calcination temperature is 800?.The highest flexural strength and compressive strength of cement samples are obtained by adding metakaolin.Flexural strength of 7d and 28 d are 6.4 Mpa and 7.4 Mpa respectively and increased 12.3%and 19.3%compared with non-additive portland cement specimens.And meanwhile increased18.5%and 29.8%compared with specimens mixed with coal series kaolin.The compressive strength of 7 d and 28 d are 37.7 MPa and 48.7 MPa respectively that were increased 21.6%and 18.8%compared with non-additive portland cement specimens and increased 17%and 35%compared with specimens mixed with coal series kaolin.Influence of the content of metakaolin on physical properties of cement was expolred.The cement mortar block reached the maximum at adding 12%of metakaolin.The flexural strength of 7 d and 28 d are 6.8 MPa and 7.6 MPa respectively that were increased 19%and 24%compared with non-additive portland cement block.The compressive strength of 7 d and 28 d are 43 MPa and 49 MPa respectively and increased similarly 38.7%and 19.5%compared with non-additive portland cement block.Metakaolin can be accelerated the early hydration and improve the early strength of cement.Ettringite,hydration products of cement,is length up to 3?m for 1 d in non-additive portland cement block,while it is small and length for 1-2?m in cement block with metakaolin.However it`s more small core for hydration products to grow and more ettringite in cement block with metakaolin.Because the grain size of metakaolin is very fine,mostly between 2-10?m.Metakaolin can be provided a amont of small active nucleation sites to accelerated the early hydration for cement.Calcium hydroxide interlayer contact is weak and it is easy to become a crack origin of cement block.Metakaolin can be reduced the content of calcium hydroxide and improved the strength of cement.Content of calcium hydroxide crystals in cement block decreases with the increase of metakaolin content.Content of calcium hydroxide crystals of 14 d and 90 d are 19.8%and 23.6%respectively in non-additive portland cement block.Content of calcium hydroxide crystals of 14 d and 90 d are18%and 17%respectively when adding 12%of metakaolin that were decreased 9%and 24%compared to non-additive portland cement.Amont of calcium hydroxide of14d and 90d are 17.24%and 16.8%respectively when adding of MK 800?which lead to 12.9%and 28.8%decreased compared to non-additive portland cement.Metakaolin has strong pozzolanic activity,the active SiO₂ and Al₂O₃ could be reacted with calcium hydroxide,cement hydration products,consume the calcium hydroxide in micro pore that was generated the higher strength of cementing material.Fine particles of metakaolin may be filled the gap of cement block and decreased the porosities of cement block and improved also the micromechanism of the concrete,refine pore structure and improving the cement density.Metakaolin could be improving the strength and durability of concrete.