Soda ash is the common name for sodium carbonate (Na2CO3). It is a weak base that is soluble in water and dissociates into sodium (Na+) and carbonate (CO3)2- ions in solution, & helps to reduce calcium hardness in mix water and to treat calcium contamination while drilling.
Soda Ash is alkaline (pH 10.5), is best for treating calcium contamination due to anhydrites because it neutralizes the sulfate ions (SO4)2- and maintains mud alkalinity at acceptable levels.
Carbonate ions cause reactive clays to flocculate, thereby increasing yield point and gel strengths. But organic deflocculants do not function properly because of the chemical imbalances created when carbonates react with the hydroxyl ions in solution.