Characterization of alteration phases on Potash–Lime–Silica glass

• The alteration resistance of Potash–Lime–Silica glasses is very low.
• Glasses with different K2O content were aged in near-neutral and acid conditions.
• Neo-formed phases were found depending on the media pH, exposure time, K-content.
• Phases were characterized by Raman spectroscopy, SEM–EDS and XRD.

Glass with Potash–Lime–Silica (PLS) composition was typically produced for the cathedral windows during the Middle Age in Central and Northern Europe and is known to be very sensitive to the attack of the atmospheric pollutants. PLS glass, containing different amounts of K2O (sample V1 = 15 wt%, V2 = 20 wt%, V3 = 25 wt%), was produced and aged in concentrated boiling H2SO4 and in high temperature-high pressure water, to simulate separately the two alteration processes of leaching (ion exchange) and dissolution, respectively. The aged samples were studied by micro-Raman spectroscopy, by XRPD and by SEM–EDS technique to identify the neo-formed crystalline phases, to get deeper insight into degradation and stability of these systems.The behavior of the glasses as revealed by Raman spectroscopy depends on the K2O content. Gypsum (CaSO4·2H2O), bassanite (CaSO4·1/2H2O) and anhydrite (CaSO4) are found on the alteration layer of the V2 and V3 leached samples. The water aged V1 and V2 glasses show the Raman features of the phyllosilicates gyrolite (Ca16Si24O60(OH)8·14H2O) and reyerite (KCa14Si24O60(OH)5·5H2O) together with hydroxyapatite (Ca5(PO4)3OH). In the alteration layer of the V3 glass, the Raman spectra suggest a mixture of the Ca–K silicate charoite (K5Ca8(Si6O15)2Si2O9(OH)·3(H2O)) and hydroxyapatite.

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