For many years, bentonite has been introduced in ceramic bodies to enhance plasticity in an economic way. However, its use in ceramics is not only bringing advantages but also some inconveniences related to its ability to increase plasticity. Its layer structure is responsible for these properties.
As it can be seen in the picture, we can think of bentonite’s structure as a stacked group of layers:
Image from BENTONITAS, PROPIEDADES y USOS INDUSTRIALES. CERÁMICA - Lic. Roberto Hevia, Director del Centro de Investigación y Desarrollo de Materiales del Intemin.
Both examples are typical silicate layers found in bentonites. Bonds between these layers are not so strong as the ones which form internally such layers. It means water can easily be retained between these layers, generating more free reactive surface, leading to an increase of interparticle space, hence to the mineral swelling.
This is also one of the main reasons for the easy “exfoliation” of these minerals.
hOW IS THIS AFFECTING MY DRYING PROCESS?
Once water is captched in between that inter-layer space, keeping in mind the tiny particle size that is also typical of these minerals, it cannot get out easily while drying. This is one of the main limitations when it comes to the its use in an industrial scale.
On the other hand, the presence of these minerals is interesting as they are both providing plasticity improvement and a better final appearance of the ceramic bodies prepared with them. This is mainly due to the fine particle size of these minerals, typically associated as well to its active surface, expressed as m2/g. Here we can see some examples of clay minerals compared to Bentonite:
|Mineral||Active Surface (m2/g)|
Based on this, it is quite common to use these minerals into tile formulations to improve the performance although there are other ways to optimize its efficiency.
WHAT CAN I DO TO KEEP A GOOD SURFACE AND TO DRY FASTER?
The use of bentonite can be complemented with other additives to improve the final formula performance, especially along the drying process. We can use other plasticizers to reduce the amount of bentonite we add, hence optimizing the drying of tiles.
Another option could be to optimize the pressing stage. Some additives can help to reduce the amount of pressure needed and, therefore, ease the drying process. Obviously, a high performance of the additives is required, and this will be guaranteed only if we have a reliable source.
This has also secondary effect in the black core removal, especially in the high iron content clays, as reduced pressure always helps improve oxygen accessibility during the firing process.