Reaction rims of natural hydraulic relicts in historic mortars were investigated using a novel technology; a FTIR-spectrometer equipped with a focal plane array detector enabling in ATR-mode IR imaging with a spatial resolution of 1.0 μm. IR spectra show two regions with main absorption bands at 1,280-1,580 cm~(-1) and 900-1,120 cm~(-1). Bands at 1,450 and 1,396 cm~(-1) correspond to the asymmetric stretching of CO_3~(2-), indicating two different forms of CaCO3; the 900-1,120 cm~(-1) group of bands is assigned to Si-O stretching vibrations indicating C-S-H phases. The ratio of the integral absorbance of these two main regions of absorption bands shows an inhomogeneous spatial distribution in the reaction rim. From this variation we conclude that the reaction rims consist of areas containing both calcite and ara-gonite in addition to C-S-H phases and areas containing aragonite and C-S-H phases, the latter with a lower Ca/Si ratio and a higher degree of polymerization. SiO2 gel is present in both areas.
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