Pozzolanic activity of metakaolins by the French standard of the modified Chapelle test: A direct methology

This research was focused on the use of the modified Chapelle test as a direct laboratory methodology to access the pozzolanic activity of both experimental and commercial metakaolins.At the same time, this test was used in the evaluation of experimental metakaolins.This chemical test, performed during 16 hours at 90 °C, allows the quantification of portlandite fixed by the metakaolin sample.The calcium hydroxide that was not consumed is quantified by acid titration (HCl), and the test result is expressed in mg of fixed calcium hydroxide by g of metakaolin.According to this test, the pozzolanic activity of a metakaolin should not be less than 700 mg Ca(OH) 2 / g metakaolin.The modified Chapelle pozzolanic activity of six commercial metakaolins was evaluated between 920 and 1560 mg Ca(OH) 2 / g metakaolin.From the seven experimental metakaolins produced between 750 ºC and 940 ºC, a material with modified Chapelle pozzolanic activity value of 1240 mg Ca(OH) 2 / g metakaolin was obtained, which is similar to some tested commercial metakaolins produced at industrial scale.The metakaolin produced at 800 ºC was ground, resulting in a particle size reduction of ≈ 4x less and a consequent increase of 21 % in the pozzolanic activity.