Firing and post-firing dynamics of Mg- and Ca-rich bricks used in the built heritage of the city of Padua (northeastern Italy)

Diverse types of bricks from monuments in the city of Padua (northeastern Italy) were studied using a multi-analytical approach based on spectrophotometry, X-ray fluorescence (XRF), X-ray powder diffraction (XRPD), polarized-light optical microscopy (POM) and/or high-resolution scanning electron microscopy with coupled energy-dispersive X-ray spectroscopy (HRSEM-EDS). The most representative bricks were yellow or beige and in well-preserved condition. The results showed that they were made of Mg- and Ca-rich illitic clays, were fired at high temperatures (from 900 to over 950 C), and achieved an incipient vitrification. Two main processes took place during firing: (i) the development of a Ca-aluminosilicate amorphous phase where very abundant pyroxene-type crystals were nucleated and (ii) the transformation of the pristine Mg-rich clayey grains into Mg-silicate mineral phases. The analyses suggest a firing dynamic within a highly reactive and supersaturated unstable system, particularly rich in calcium and magnesium. There are also signs of the rapid heating and/or soaking of the bricks and the irregular heat distribution and/or different residence times inside the kilns. The formation of zeolite and calcite secondary phases was also observed. The former was largely promoted by the high calcium content of the bodies and the very humid conditions, while the latter was mainly precipitated from Ca-rich solutions. The preservation of the bricks was enhanced by processes that took place both during and after firing. Firstly, the significant development of a Ca-rich amorphous phase and of high-temperature pyroxene-type crystals has provided strength to the bricks. Secondly, the porosity yielded by the firing of the carbonate-rich clays was almost filled by secondary calcite, which acted as a cementing agent. The information attained has increased the knowledge of (i) the mineralogical and microstructural changes that take place during the firing over 900 C of Ca- and Mg-rich illitic clays and (ii) the formation of secondary phases within highly calcareous bricks laid in very humid environments and affected by Ca-rich solutions. The key role of the Ca- and Mg-rich raw clays and of the high firing temperatures, in producing high-quality bricks, and of the secondary calcite, which increased their durability, is highlighted. All these factors have contributed to the better preservation of the built heritage of the city.