دانلود رایگان مقاله مطالعه سینتیک خوردگی سیمان پرتلند سخت شده تحت حمله اسید

Abstract

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.

7. Conclusion

We have introduced in this article a methodology, including holographic interferometry, that gives access to the space and time resolution of heterogeneous reactions. In the case of the study of the degradation of hydrated cement, it enables to clearly discriminate between the dissolution and precipitation stages of the deterioration. This technique has been used to measure the dissolution rate constant of hardened Portland cement in pure aqueous solutions of hydrochloric, nitric and sulfuric acids at pH 2. Unexpectedly, the three values have been found to compare well, about 1 mg/m2/s. This is a direct demonstration that the degradation kinetics is mostly driven by precipitation. We have also measured the evolution of the reaction rate constant in nitric acid with the pH. Profiting from the resolving power of the technique, we have finally measured accurately the pH of the solution close to the surface, where it has been found to be always strongly alkaline (between 11.5 and 12.5), whatever the bulk pH. Now that ordinary Portland cement has been studied, this technique may be used to test thoroughly the resistance to acid attack of other cementitious materials, known to offer better acid resistance, like calcium sulfoaluminate cements, or Portland cement containing silica fume.