Thermal stability of Friedel’s salt from metakaolin origin by DSC and HTXRD techniques
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Friedel’s salt, 3CaO·Al2O3·CaCl2·10H2O, is a compound formed in Portland cement (PC)-based conglomerates with pozzolans exposed to chlorides presence from the thaw salts or marine environments. The constituents that chemically bond chlorides are tricalcium aluminate, 3CaO·Al2O3, of PC and reactive alumina, Al2O 3 r− , of pozzolan. In this research, thermal stability of Friedel’s salt from the metakaolin origin (Guadalajara, Spain) is studied by DSC up to 1000 °C and HTXRD technique at 25, 35, 50 and 75 °C. The course of the structural transition between its two crystalline modifications: monoclinic (space group C2/c of low temperature α-form, < 35 °C) and rhombohedral (space group R-3c of high temperature β-form, > 35 °C), is verified. Both DSC signals and HTXRD patterns confirm the absence of the monoclinic form; instead, the rhombohedral structure of the compound is modified in the function of temperature. In return, the cell parameters of the precipitated rhombohedral Friedel’s salt crystals undergo insignificant modifications that result in a decrease in the unit cell volume (a total of 0.5%, from 25 to 75 °C). Importance of the results is essential for the active mineral additions as metakaolin, and both mechanical strength and chemical durability against the sea spray and de-icing salts (chlorides strikes the reinforced steel concrete causing them electrochemical corrosion by “bite”) attack that may suffer its resultant PC-pozzolan-based binder products.
KeywordsFriedel’s salt Thermal stability Metakaolin Portland cement Chlorides chemical attack Rietveld method
Research data obtained within the framework of the Ph.D. dissertation experimental scientific works of the author on: “Quantitative determination of reactive alumina content, natural and artificial, by thermogravimetry and Rietveld method,” defended on January 30, 2014, at Technical University of Madrid in Spain (oa.upm.es/22743/) were financially supported by Technical University of Madrid (Spain), Ph.D. Grant No. RR01/2008. Research results dissemination was funded from the European Union’s Horizon 2020 Marie Skłodowska Curie—Individual Fellowship research and innovation programme under Grant Agreement No. 746830 (H2020-MSCA-IF-2016).
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