The present work investigates the chemical, physical and mechanical effects of high dosages of granulated blast furnace slag (85 percent) as an addition to Portland cements. Four samples were made, one of the reference standard cement known as CP IND and with the slag addition dosages, with 60 percent slag addition (usual on the market), 85 percent slag without additive (white) and 85 percent slag with 7.5 percent activating additive (Anhydrous Sodium Metasilicate). The materials were characterized regarding their chemical composition, surface area and particle size by Fluorescence, Blaine and laser particle size tests. To investigate the formation of the phases, x-ray diffractometry (XRD) and calorimetry analyzes were carried out. Finally, the hydrated cements were tested in uniaxial compression at ages of 1, 3, 7, 28 and 91 days to measure the compressive strength in the cement paste, and at 1, 7 and 28 days in concrete. The use of this activator guaranteed the alkaline reserve, increasing the precipitation of hydration products. The mixtures activated with 85 percent slag presented satisfactory compressive strength results for Portland cements Strength Class 25 MPa. The cement with 85 percent addition of slag activated with anhydrous sodium metasilicate presented a performance, in the cement paste, at the age of 1 day, more than 4 times higher when compared to cement with 85 percents lag addition without activator. This performance decreased as hydration progressed, when the opposite was expected, demonstrating that the percentage used and the granulometry of the activator may not have been the most appropriate, with the need for a more in-depth study both in this relationship and in the verification of the porosity of the mixtures.