THE INFLUENCE OF CALCINED KAOLIN CLAY ON THE PROPERTIES OF PORTLAND CEMENT

The influence of kaolin clay additives calcined at 600 °C on the properties of Portland cement was investigated. At 505 °C, the mineral of kaolin clay – kaolinite – breaks down and transforms into an active form – metakaolinite. In samples of calcined kaolin clay, the pozzolanic reaction was found to take place between portlandite and the amorphous SiO2. The amount of forming calcium silicate hydrates increases with increasing amount of calcined kaolin clay additive (from 5 to 20 %) and the duration of hydration. The calcined clay additive reduces the compressive strength of the cement samples after 7 days of hydration. After 28 days of hydration, the compressive strength of samples with 10 % of calcined clay additive was the same as that of samples without additives, meanwhile the compressive strength of samples with 5 % of additive was slightly lower. After 3 months of hydration, the compressive strength of samples with 5 and 10 % of calcined clay additive was higher than that of pure Portland cement samples.