MODEL STUDY OF THE 1ST STEPS IN THE DETERIORATION OF CALCAREOUS STONE .1. INITIAL SURFACE SULFITE FORMATION ON CALCITE

An experimental set-up to investigate the reactions at ppm concentrations of pollutants with stone surfaces, involving a minimum of sample tampering, is proposed. In-situ Diffuse Reflectance Infrared Fourier Transform spectrometry (DRIFT) is employed to monitor the time evolution of products formed at a precipitated calcite sample. The powdered sample was exposed to a mixture of air and SO,(g) at different humidities. The nature of the initial sulphite formed is discussed in terms of anhydrous and hemihydrate calcium sulphite and sulphite in solution. In all three cases IR spectra are produced and compared to previous studies. The DRIFT spectra reveal the presence of at least two sulphite species on calcite at humid conditions, one chemisorbed and one dissolved. The relevance of this study to the understanding of calcareous stone deterioration is discussed.