Mineralogical and geochemical studies of secondary mineral assemblages related to deterioration of building materials

Abstract

The study was aimed at mineralogical and geochemical characteristics of secondary phases related to deterioration of historic building materials. The investigations, carried out in the Holy Trinity Basilica in Kraków, Poland, focused on the southern facade of the 17th-century Myszkowskis Chapel, built of the Miocene Pińczów limestone. Lower part of the facade is covered with a cement render, and the exposed foundations are made of Jurassic limestone and Cretaceous sandstone, both of local origin from the Kraków region and neighbouring Carpathians, in the form of irregular blocks bound with a cement mortar. The wall surface exhibits clear signs of damage; from dark grey soiling and scaling to efflorescences. Sampled materials, deteriorated, altered crusts and efflorescences were analysed with optical and scanning electron microscopy, X-ray diffraction, and Raman microspectroscopy methods. The secondary minerals distinguished include abundant gypsum CaSO₄·2H₂O, less common thenardite Na₂SO₄ (and/or mirabilite Na₂SO₄·10H₂O), aphthitalite (K, Na)₃Na(SO₄)₂, darapskite Na₃(SO₄)(NO₃)·H₂O, ettringite Ca₆Al₂(SO₄)₃(OH)₁₂·26H₂O, monosulphite Ca₄Al₂O₆SO₃·11H₂O, as well as scarce nitre KNO₃, nitratine NaNO3 and halite NaCl. Gypsum usually forms surface crusts and fills the pores inside some materials. The efflorescences, sampled from the exposed foundations, consisted of thenardite and/or mirabilite, aphthitalite and darapskite, whereas ettringite and monosulphite were connected with cement renders. Traces of nitre, nitratine and halite were detected at various elements of the chapel facade and foundations. The origin of the salts is related to composition and physicochemical properties of the building materials, as well as to anthropogenic factors.