Influence of SO2 deposition on cement mortar hydration

The chemical reactions that occur during the hydration of Portland cement can be modified in the additional presence of certain compounds, such as SO 2 , which may be present in areas of high pollutant gas concentration. This paper considers mortars that have been cured for 1 and 7 days and exposed to SO 2 gas under dry and wet conditions. The role of ozone as gaseous catalyst in the reactions is studied. Fourier transform infrared spectrum analysis of the surface of the samples revealed carbonation had occurred; however, X-ray diffraction of the bulk material from the samples revealed Ca(OH) 2 only when the samples were exposed to dry conditions. In wet conditions, in the presence of water, the dissolution of CO 2 gas is enhanced, the carbonation of the Ca(OH) 2 is promoted at higher levels, and thus no Ca(OH) 2 is detected. Under wet conditions, the degree of salt formation was greater than in dry conditions. In the presence of water, and due to Ca(OH) 2 carbonation, the pH of the pore solution is lower than 12; thus SO 3 2− formation is favored. For the mortars exposed to dry conditions, kinetic salts such as K 2 SO 4 are formed. However, in the additional presence of water, thermodynamically stable products as K 2 Ca(SO 4 ) 2 · 2H 2 O and CaSO 4 · 2H 2 O are detected. The results of scanning electron microscopy indicated that the morphological composition of C-S-H gel resembles needle-like structures radiating from a grain. This morphology has been described by Taylor as type I.