The objective is the quantification of the durability in an acid medium of the cement obtained by the activation of blast furnace slag by means of sodium hydroxide or potassium hydroxide. The work focused on the analysis of pastes and the behaviour was compared to the Portland cement paste. For this, the techniques of measuring the acid neutralization capacity on pulverized paste and the measurement of the residual mass of paste specimens immersed in acid nitric media of a given concentration have been used. The most relevant conclusion is that alkali activated pastes have a greater durability than Portland cement pastes: C-A-S-H gels are more acid resistant than C-S-H gels.