Improving the pozzolanic activity of metakaolin by urea intercalation technique

The aim of this study was to improve the pozzolanic activity of metakaolin used as mineral additive in concrete by urea intercalation technique. A precursor, urea-kaolin (U-kaolin) with the intercalation degree of 92%, was prepared by evaporating the solvent from suspension containing original kaolin (O-kaolin) and urea. Two series of metakaolin were obtained by calcining the O-kaolin and U-kaolin at nine different temperatures from 550 °C-950 °C for 2 h. The pozzolanic activity of the two series of metakaolin were evaluated and compared by Fixation of Calcium Hydroxide (FCH) and mortar strength as well. In addition, X-ray diffraction, Fourier Transform Infrared Spectroscopy, Thermogravity-Differntial Scanning Calormetry, Brunauer-Emmett-Teller method and Scanning Electron Microscopy were also applied to study the microstructure of the precursor and mechanisms for the pozzolanic activity improvement. The results show that the pozzolanic activity of metakaolin calcined by U-kaolin is 15% higher in the FCH test and 20% higher in cement mortar strength test than the normal one when the calcination temperature is 800 °C. U-kaolin presents lower dehydroxylation temperature due to the weakened interlayer attractive force, and forms less inert component at high temperature due to the looser structure of calcined products. The joint effects of intercalated urea molecules in making precursor and calcination process result in looser structure and higher specific surface to improve of the pozzolanic activity.