PSI - Issue 33

Francesco Freddi et al. / Procedia Structural Integrity 33 (2021) 371–384 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Fig. 1 Carbonation process example.

Ca(OH) 2 (s) → Ca (2) Eventually, the neutralization reaction takes place resulting in the formation of calcium carbonate (CaCO 3 ) Ca 2+ (aq) + 2OH - (aq) + CO 3 2 - (aq) + H + (aq) → CaCO 3 (s) + H 2 O (3) The carbonation process is modelled via the standard diffusion-reaction equation system. For the carbon dioxide, the governing equation is given by � � � � � ��1 � ��CO � �� � ∇ ∙ � �� � � � ∇�CO � �� � � � r � (4) The diffusion part is governed by the Fick law and multiplied by the term � ��1 � � as the gaseous concentration of CO 2 refers to the unit volume of the gas-phase of the pores. The diffusion coefficient depends on the carbonation state due to the changes of the porosity and on the damage state of the material as carbon dioxide can freely diffuse though cracks, and is equal to �� � � � � � 1.42 ∙ 10 � � � �.� �1 � ��� �.� ���r� � 0.�� 10 �� ���r� � 0.�� (5) The neutralization reaction rate term r n refers to the unit volume of the liquid phase of those pores partially filled with water and partially with air. Therefore, it is multiplied by � � which represent the volume of the liquid phase per unit volume of concrete and is given by r � � ���� � �OH � � �� �CO � ���� (6) For the calcium hydroxide and the calcium carbonate, only the reaction term is considered, and the governing equations are equal to �������� � � �� � � � � r � (7) ������ � � �� � � � r � (8) The carbonation reaction occurs in concrete if the relative humidity ranges between 50% and 70%. Lower values do not allow a complete dissolution of CO 2 in water while higher values considerably reduce the diffusion process through the concrete pores. 2+ (aq) + 2OH - (aq)