Production and characterization of high-reactivity metakaolins calcined in flash reactor

Abstract In this study, we used a flash reactor heated with electrical resistance to produce metakaolins (MKs) from high-purity kaolin. The study aimed to determine the effect of calcination temperature and kaolin influx on the properties of MKs, used as supplementary cementitious material (SCM). We characterized the kaolin starting material through X-ray diffraction (XRD), X-ray fluorescence, particle size distribution, and thermal analysis. The temperature range for complete MK formation was determined through a kinetic study using thermal analysis at different heating rates. Subsequently, the six MK specimens synthesized as above were characterized using the same techniques as for kaolin, whereas the amorphous phase was quantified by the Rietveld method. The pozzolanic characteristics were investigated by (i) testing the pozzolanic activity index using lime and Portland cement at seven days, and (ii) conducting the modified Chapelle test based on Brazilian standards. The results showed that four of the six MK samples obtained were at least 90% amorphous, indicating that the calcination method was efficient. Moreover, their pozzolanic activity was almost twice that required by the Brazilian standard. Finally, a correlation between the mass loss and the degree of amorphism of flash MK samples was established in this study.