Sodium Bentonite is one of the most common Bentonites. A Bentonite is an absorbent aluminium phyllosilicate clay that is usually produced by the natural weathering and compression of volcanic ash. 

Sodium Bentonite is capable of absorbing many times its own dry mass in water. Because of this unique property, it is commonly used in cement and mortar recipes as it allows for the modification of sagging, sealing, and rheological properties of the construction material.