CHARACTERIZATION OF CHANGES IN MATRIX OF SANDSTONES AFFECTED BY HISTORICAL FIRES Miguel Gómez-Heras* Instituto de Geología Económica (CSIC-UCM), Facultad de Ciencias Geológicas, Universidad Complutense de Madrid, 28040 Madrid, Spain. María José Varas Instituto de Geología Económica (CSIC-UCM), Facultad de Ciencias Geológicas, Universidad Complutense de Madrid, 28040 Madrid, Spain. Mónica Álvarez de Buergo Instituto de Geología Económica (CSIC-UCM), Facultad de Ciencias Geológicas, Universidad Complutense de Madrid, 28040 Madrid, Spain. Rafael Fort Instituto de Geología Económica (CSIC-UCM), Facultad de Ciencias Geológicas, Universidad Complutense de Madrid, 28040 Madrid, Spain, and Laboratorio de Petrología Aplicada, Unidad Asociada (CSIC-UA), Universidad de Alicante, PO Box 99, Alicante, Spain. Abstract Studies on damages of stone due to fire are usually focused on the variations of bulk mechanical or visual properties produced by the intense increasing of temperature during the fire event. The aim of this work has been to assess mineralogical and textural changes in the matrix of building sandstones affected by fires in the past. Two types of sandstone were selected in order to study the variations in both siliceous (argillaceous) and calcareous matrix. The comparison between the observations both in unaffected and affected areas focused the study. For this purpose, several analytical techniques were carried out including Polarizing Microscopy, Scanning Electron Microscopy with Electron Dispersive X-ray Analysis, X-ray Diffraction, Fourier Transform Infrared Spectroscopy, Differential Thermal Analysis, Spectrocolourimetry and Mercury Intrusion Porosimetry. Results show that the most affected sandstone constituent after a fire event is the matrix, against the skeletal grains, which are unaffected. Calcareous matrix was not severely affected and argillaceous matrix experienced the most intense changes. Fire produced processes of dehydration of clay minerals such as smectite and palygorskite. Temperatures were high enough to produce variations in morphology and distribution of the porous system of the stone, although were not high enough to promote irreversible dehydroxilation of minerals. Keywords: Building stone, fire weathering, palygorskite, smectite, porosity, Uncastillo sandstone, Villamayor sandstone. 1. Introduction Fire has been classically considered as an important weathering agent in natural environments. Early observations of Blackwelder (1926) mentioned the forest fires as a major factor causing scaling of rocks. * Author to whom correspondence should be addressed.