10Micro-Raman spectroscopy has been used to follow the reaction of free lime (CaO) exposed 11 for 24 hours to moist air at 80 °C under conditions of different relative humidities (10 -80 % 12 RH). X-ray diffraction and SEM imaging were applied as complementary techniques. The 13 conversion of lime to calcium hydroxide and its subsequent carbonation to various calcium 14 carbonate polymorphs was found to strongly depend on the relative humidity. At low RH (10 15 -20 %), only Raman spectroscopy revealed the formation of early amorphous CaCO 3 which 16 in the XRD patterns was detected only at ≥ 40 % RH. However, XRD analysis could identify 17 the crystalline polymorphs formed at higher relative humidities. Thus, between 20 and 18 60 % RH, all three CaCO 3 polymorphs (calcite, aragonite and vaterite) were observed via 19 XRD whereas at high relative humidity (80 %), calcite was the predominant reaction product. 20The results demonstrate the usefulness of Raman spectroscopy in the study of minor cement 21 constituents and their reaction products on air, especially of amorphous character. 22 23
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