Local study of the corrosion kinetics of hardened Portland cement under acid attack

Whereas the chemical reactions occurring during the acid corrosion of cementitious materials are now well known, their reaction rates have still not been measured. We propose here a methodology using digital holographic interferometry to access to these pure surface reaction rates. This method makes possible the differentiation between the dissolution and precipitation steps. We use it to measure the reaction rate constant of the dissolution of hardened Portland cement in aqueous solutions of nitric, sulfuric and hydrochloric acids at pH 2. This quantity is seen to be unexpectedly similar for the three acids, with a value of the order of 1 mg/m2/s. We have measured the evolution of this reaction rate constant with the pH in nitric acid. We have also measured the real pH of the solution at the material surface, which is always alkaline (pH  > 11), even for attacks by solution with pH as low as 1.