The Influences Of Metakaolin On Volumetric Stability And Microscopic Structure Of Concrete

Metakaolin is a kind of artificial pozzolanic material with high-activity, which can take pozzolanic reaction in alkaline environment. When added in concrete, the metakaolin can bring some influences on workability, mechanical properties and durability of concrete. In this study, the volumetric stability and pore structure of concrete with metakaolin are investigated. The concrete with metakaolin, CaSO4 and fly ash was treated as a whole system, and the shrinkage rate testing and the mercury intrusion porosimetry(MIP) were employed to investigate the shrinkage and pore structure. The results show that:(1) When added into concrete, the metakaolin can reduce its shrinkage in late age significantly, and also decrease the total porosity, fine the internal aperture, decrease the average pore size, and minimize the content of large pore of concrete. Except that, the shrinkage resistance ability and density of concrete in the later ages can be enhanced obviously.(2) The decrease of the water to binder ratio can enhance the late stage shrinkage resistance ability of concrete and optimize the internal pore structure of concrete. When reducing the water to binder ratio of concrete, its shrinkage in early age will increase and its shrinkage in late age will decrease accordingly. Meanwhile, the decrease of the water to binder ratio can improve the density of concrete, reduce the total porosity and the average pore diameter values of concrete and optimize the distribution of concrete pore structure.(3) The early curing can bring some advantages to the volume stability of concrete with metakaolin. Extending the curing age can optimize the pore structure of concrete with metakaolin. Under sufficient curing conditions, the total porosity and average pore size of concrete will decrease fast in the early stage, especially the content of large pore.(4) Fly ash and metakaolin can both improve the volumetric stability and optimize the pore structure of concrete, but the improving effect of metakaolin is better than that of fly ash. When metakaolin, fly ash and Calcium sulfate were added into concrete, its volume stability and the distribution of pore structure can be improved efficiently.(5) With the increase of the content of metakaolin, some rules between the pore structure parameters and the shrinkage of concrete can be observed. When metakaolin content increase from 0% to 15%, the total porosity, average pore diameter values and proportion of harmless pore will reduce with the decrease of the shrinkage of concrete. When metakaolin content increase from 15% to 20%, the total porosity, average pore diameter values and proportion of harmless pore will increase with the decrease of the shrinkage of concrete. After fitting the data, it can be observed that when the metakaolin content is 17.8%, the total porosity of concrete reach the lowest value (0.0481 mL/g), and when the metakaolin content is 17.2%, the average pore diameter values of concrete reach the lowest value (13.1nm), and when the metakaolin content is 13.7%, the proportion of harmless pore of concrete reach the maximum value (46.8%).