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Tham khảo tài liệu 'handbook of corrosion engineering episode 1 part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 30 Chapter One alloying additions or protective coatings for corrosion resistance are associated with this steel. In simplistic terms concrete is produced by mixing cement clinker water fine aggregate sand coarse aggregate stone and other chemical additives. When mixed with water the anhydrous cement clinker compounds hydrate to form cement paste. It is the cement paste that forms the matrix of the composite concrete material and gives it its strength and rigidity by means of an interconnected network in which the aggregate particles are embedded. The cement paste is porous in nature. An important feature of concrete is that the pores are filled with a highly alkaline solution with a pH between 12.6 and 13.8 at normal humidity levels. This highly alkaline pore solution arises from by-products of the cement clinker hydration reactions such as NaOH KOH and Ca OH 2. The maintenance of a high pH in the concrete pore solution is a fundamental feature of the corrosion resistance of carbon steel reinforcing bars. At the high pH levels of the concrete pore solution without the ingress of corrosive species reinforcing steel embedded in concrete tends to display completely passive behavior as a result of the formation of a thin protective passive film. The corrosion potential of passive reinforcing steel tends to be more positive than about -0.52 V SHE according to ASTM guidelines.9 The E-pH diagram in Fig. 1.14 confirms the passive nature of steel under these conditions. It also indicates that the oxygen reduction reaction is the cathodic half-cell reaction applicable under these highly alkaline conditions. One mechanism responsible for severe corrosion damage to reinforcing steel is known as carbonation. In this process carbon dioxide from the atmosphere reacts with calcium hydroxide and other hydroxides in the cement paste following reaction 1.6 . Ca OH 2 CO2 CaCO3 H2O 1.6 The pore solution is effectively neutralized by this reaction. Carbonation damage usually appears as