Studies On A New Kind Of Mineral Admixture Of High Performance Concrete

A new high active admixture for preparing high-performance concrete-metakaolin (Abbreviated as MK) has been developed through systematic research on the chemical and mineral composition, active mechanism as well as other performance compared with silica fume based on the principle of high-performance concrete. In addition, the workability, mechanical properties, durability and etc. of high-performance concrete prepared by silica fume and metakaolin are also been studied. The results indicate that substituting metakaolin for silica fume to prepare high-performance concrete is completely feasible.1. The systematic research on preparing metakaolin exhibits that the reasonable chemical composition (mostly Al2O3 SiO2) and appropriate calcination temperature are the key to preparing metakaolin. The active composition of metakaolin is mostly Al2O3 and SiO2. The activity of metakaolin is different if the composition is different. When the content of Al2O3+SiO2 in metakaolin is high and the content of Al2O3 is also relatively high, plus the calcination temperature ranges from 700 to 800, the metakaolin shows good activity and can obviously improve the strength of cement.2. The hydration and harden mechanism as well as microstructure and feature of cement paste with metakaolin have been analyzed compared with that without metakaolin. The cement with metakaolin whose hydration products are mostly C-S-H, AFt and Ca(OH)2 has no significant difference from Portland cement. But in later period of hydration, the active Al2O3 and SiO2 react with the hydration products to form C-S-H gel and Aft filling the pores of paste to make the structure of cement paste compact thus contributing to the strength development of cement.3. The total pore ratio of cement paste with metakaolin under each age is lower than that of no metakaolin; that is to say, the strength of cement with metakaolin under each age is high than that of Portland cement. The pore distribution of cement paste with metakaolin is similar to that of silica fume. But the quantity of capillary above 100nm of cement paste with metakaolin obviously decreases, the quantity of gel pore greatly increases and the total pore ratio also significantly decreases with hydration. While for cement paste without metakaolin, only the quantity of capillary above lOOnm decreases, but it has no significant change for that less than 100nm with hydration,4. The research on the workability, strength and durability of concrete prepared by metakaolin and silica fume shows that the metakaolin can meet the technical requirements of high-performance concrete and is a good kind of admixture for preparing high-performance concrete.