Lime mortar with natural hydraulic components – characterization of reaction rims with FTIR-imaging in ATR-mode

Abstract eng:
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 m. IR spectra show two regions with main absorption bands at 1280– 1580cm-1 and 900–1120cm-1. Bands at 1450cm-1 and 1396cm-1 correspond to the asymmetric stretching of CO32-, indicating two different forms of CaCO3; the 900– 1120cm-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. Our investigation shows that the reaction rims consist of CaCO3, C–S–H phases and SiO2 gel in variable proportions. In areas with low carbonate-to-C–S–H phase ratios, carbonate is mainly present as metastable aragonite. The associated C–S–H phases show a comparatively low Ca/Si ratio and a high degree of polymerization. In areas with high carbonate-to-C–S–H phase ratios, both, aragonite and calcite are present. The C–S–H phases show a comparatively higher Ca/Si ratio combined with a lower degree of polymerization. The presence of SiO2 gel and aragonite indicate carbonation of C–S–H phases. The matrix outside of the cloudy reaction rim consists mainly of calcite. FTIR imaging measured in ATR-mode is a promising method to gain information on the hydraulicity of natural hydraulic mortars in its spatial distribution on a micrometer scale and to characterize the hydration and carbonation products as well.

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