[en] Smectite clays, bentonite or not, are generally polycationic and require primary and secondary activations to meet the specifications for various industrial uses. These activations are usually made through Na₂CO₃. Recently several authors have obtained good results through secondary activations with MgO. This study aims to evaluate the effects of individual and joint additive for Na₂CO₃ and MgO in the rheological properties (VA, VP) and pH polycationic bentonite clays of the state of Paraíba, for use as viscosifier of water based drilling fluids and also as agent thickening of water-based inks. Polycationic bentonite clays have been used in the state of Paraiba occurrences of Boa Vista and Cubati, called Chocolate Boa Vista (CH BV) and Chocolate Campos Novos (CH CN) respectively. The physical, chemical and mineralogical samples was performed by the following methods: particle size analysis (AG), X-ray fluorescence (EDX), X-ray diffraction (XRD), cation exchange capacity (CEC), specific area (AE ), differential thermal analysis (DTA) and thermal gravimetric analysis (TG). These clays were treated with Na₂CO₃ and MgO alone and in combination. The results were compared with the specifications of PETROBRAS, API and ISO and specifications for use in paints base water that is also based on the specifications of the API and ISO. It concludes that the bentonite CH BV and CH CN meet PETROBRAS specifications and also the API, ISO and can be used as thickeners for water-based paints. (author)