Lime Mortar with Natural Hydraulic Components: Characterisation of Reaction Rims with FTIR Imaging in ATR-Mode
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 aragonite 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.
KeywordsCalcium Silicate Hydrate Focal Plane Array Lower Wave Number Polished Thin Section Lime Mortar
This study was carried out within the framework an EU-Interreg project entitled “Interdisciplinary investigations of selected monuments as key examples for optimized planning of conservation measures” and a national project entitled “Historic building materials in Tyrol”. The funding for these projects, provided by the EU, the Austrian Federal Ministry for Education, Arts and Culture, the state of Tyrol and the Autonomous Province of Bolzano – South Tyrol is gratefully acknowledged.
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