Effects Of Metakaolin On Mechanical And Electrical Properties Of Cement-based Materials

Metakaolin is a kind of auxiliary cementitious material with excellent properties.In recent years,with the popularity of green environmental protection and energy conservation and emission reduction needs,metakaolin has gradually attracted wide attention from all walks of life due to its high performance of volcanic ash.This thesis is based on the different dosage of metakaolin cement slurry for compression test,bending test,three point bending test,electron microscope scanning test and resistivity test.So that parameters such as strength and fracture toughness can be gotten to characterize its mechanical properties while resistivity in real time can be used to describe the development of the microstructure.Therefore,the influence of metakaolin content on the mechanical and electrical properties of cement-based materials can be analyzed.The main research results of this thesis are as follows:1.The optimum content of metakaolin was obtained and its mechanism was analyzed.Because of the physical effect,metakaolin can refine the pore size by filling effect,and produce more hydration products through pozzolanic activity effect which makes the structure more dense to improve the strength of cement-based materials and fracture toughness effect.When the content of metakaolin exceeds a certain value,the substitution of some cement clinker will produce dilution effect,which will affect the strength and fracture toughness of cement-based materials.When the content of metakaolin is 10%,the strength and fracture toughness of cement-based materials increase most obviously.2.Based on the Bazant size effect law and test results,the critical size and strength of metakaolin cement slurry are obtained.The calculated formula of size effect rate can predict the compressive strength of cement paste well and is suitable for small size and geometric similar specimens.For lower additive metakaolin,the critical size and the critical strength changes are not significant,which may depend mainly on the properties of the cement clinker.3.The mechanism of calcium hydroxide as basic activator to promote the secondary hydration of active oxides in metakaolin was investigated.The active oxide reacts with calcium hydroxide to form hydrated calcium silicate and hydrated calcium aluminate,and further forms calcium vanadite,which leads to obvious resistivity growth.With the increase of metakaolin content by 5%,the occurrence time of characteristic point P₃will be about 2 h in advance,which further indicates the accelerated effect of metakaolin on hydration reaction.4.The relation of 1 day resistivity for predicting the compressive strength of cement paste with metakaolin mixed with different ages is proposed.Due to the secondary reaction,the compressive strength of cement paste with different amount of metakaolin is actually affected by resistivity and age.As a non-destructive method,the non-contact resistivity method can characterize the early hydration process of cement-based materials in real time based on the change of microstructure such as porosity.Therefore,the compressive strength of cement paste with metakaolin can still be well predicted by resistivity.