Understanding the thermal activation behavior of montmorillonite and kaolinite at atomic level by ReaxFF molecular dynamics simulations

The origin of the difference in activity between calcined kaolinite and calcined montmorillonite is not well understood. Herein, reactive force field molecular dynamic (ReaxFF MD) simulations were adopted to study the thermal activation behavior of montmorillonite and kaolinite. It was found that during the calcination process, the removal of hydroxyl groups from kaolinite and montmorillonite not only leads to the break of Al (Mg)-O bonds, but also results in the formation of Si[5] and the break of Si-O bonds. The octahedral alumina sheet in kaolinite is more easily destroyed than that of montmorillonite. Most of the hydroxyl groups in kaolinite can be removed after calcination at 800 °C. While only 32.5% of the hydroxyl groups in montmorillonite are removed after calcination at 900 °C. The combination of free hydroxyl groups with Si[4] mainly causes the break of Si-O bonds in Si-O-Al(Mg) structure and the destruction of connection between the tetrahedral silica sheet and octahedral alumina sheet. While the Si-O-Si networks in both kaolinite and montmorillonite are difficult to disrupt. Disrupting the network structures of Si-O-Si sheet is the key to improving the activity of montmorillonite. This work provides an insight into the thermal activation of montmorillonite and kaolinite at atomic level, and is beneficial for exploring suitable pathways for simultaneous activation of kaolin and montmorillonite. •The removal of hydroxyl groups leads to the break of Al-O bonds, the formation of Si[5] and the break of -O bonds.•The octahedral alumina (AlO6) sheet in kaolinite is more easily destroyed than that of montmorillonite.•The Si-O-Si network in both kaolinite and montmorillonite are difficult to disrupt.•The violently moving ions Na+ ions do not destroy the Si-O-Si network of tetrahedral silica (SiO4) sheet.