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The study of building materials constituting cultural heritage is fundamental to understand their characteristics and predict their behavior. When considering materials from archaeological sites, their characterization can provide not only relevant information for a broader understanding of the site and its importance and significance but can also increase knowledge about ancient materials and their performance. The Palace of Knossos is a very important archaeological site in the European history context, and its preservation benefits from the characterization of the constituent materials. Samples of mortars from this monument were collected under the scope of the H2020 HERACLES project, where a multi-analytical approach was chosen using established protocols for the different sample typologies. Instrumental techniques such as optical microscopy (OM), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and simultaneous thermogravimetry and differential thermal analysis (TG–DTA) were used for the chemical, mineralogical, and morphological characterization of these mortar samples. The results indicate that the majority are lime mortars, both aerial and hydraulic, but gypsum-based mortars were also identified. Differences in the chemical composition of the samples in distinct areas of the monument allowed us to reflect on the variety of materials used in the construction of the Palace of Knossos. [ABSTRACT FROM AUTHOR]

• Cement as a material used in the historical reconstructions of the Palace of Knossos. • Identification of the types of salts and crusts by WDXRF, XRD, FTIR and μ-Raman. • Formation of gypsum crusts and thenardite efflorescence resulting from the deterioration process of the cements. • Importance of material characterisation for the conservation of monuments. The Palace of Knossos, located on the island of Crete, Greece, is one of Europe's most important archaeological sites, serving as a testament to the Minoan civilization. Situated near the Mediterranean Sea, it is in close proximity to the seaport, airport, and industrial areas. Decay products commonly found in historical monuments within or near urban areas, such as black crusts and salt efflorescence, are also prevalent at the Palace of Knossos. To better understand the characteristics of the type of deterioration compounds found on cement in historical reconstruction zones, as well as their possible relationship with factors influencing the deterioration process, a multi-analytical approach was designed for the study of these materials. The results indicate that the black crusts primarily consist of gypsum and carbonaceous matter. However, the efflorescence salts are predominantly composed of thenardite instead of halite, despite the palace's proximity to the coastal area. These results may contribute to ongoing and future maintenance and preservation efforts for the monument. [ABSTRACT FROM AUTHOR]