Simulated Standards for the Characterization of Dolomitic Mortars
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Simulated Standards for the Characterization of Dolomitic Mortars C. Montoya, J. Lanas, M. Arandigoyen, I. Navarro, P.J. García Casado and J.I. Alvarez Dept. of Chemistry and Soil Science, University of Navarra, 31080 Pamplona, Spain. ABSTRACT In order to clarify the structure underlying the appearance of several compounds in dolomitic mortars (specifically hydromagnesite, Mg5(CO3)4(OH)2.4H2O), as well as the suitability of the X-ray diffraction (XRD) and thermogravimetric and thermodifferential simultaneous analysis (TGA-DTA) in their determination, different patterns from phases that could be present in mortars of these characteristics have been prepared and studied by these techniques. The standards were prepared from: hydromagnesite (HY) with calcite in weight/weight proportions 1:1 to 1:5; HY with calcite and quartz in proportions 1:1:1 and 1:6:12; HY with quartz, 1:1 and 1:2; HY with portlandite (calcium hydroxide), 1:1 and 1:2; HY with portlandite and quartz, in 1:6:12, and HY with magnesium oxide in 1:1, 1:2 and 2:1. The XRD results have shown that it is possible to detect HY and the other compounds (dolomite, calcite, magnesite, quartz, …), but when the HY is mixed, the intensity of its diffraction peaks is very weak, even not detectable in some cases. The poor crystallinity of the HY could be the reason of this drop in intensity. Therefore thermal studies were necessary to find HY phases in low weight percentages. TGA-DTA led us to establish the experimental conditions most suitable for thermal studies. A high CO2 pressure around the sample was required for the occurrence of an exothermic peak at 500ºC. This high pressure was guaranteed in the present work as follows: static air atmosphere, packed sample, high heating rate (20ºC.min-1), and alumina crucibles with holed lids in order to establish a selfgenerated atmosphere. The thermal behavior of hydromagnesite phases has been clearly established in contradiction to some references of the literature; specifically, the exothermic peak at 500ºC has been observed repeatedly. This result invalidates reports of the crystallization of magnesium carbonate from the amorphous phase. INTRODUCTION As Bruni et al. [1] and Dheilly et al. [2] report, the process of setting (by carbonation) of the dolomitic lime (that is, containing brucite Mg(OH)2) concludes frequently with the formation of hydromagnesite, HY (Mg5(CO3)4(OH)2.4H2O). This HY is a new stable phase coexistent with magnesite (MgCO3), that contributes to an increment in the mechanical strenght of the mortar. Vecchio et al. [3] indicate the presence of HY in Italian dolomitic mortars (11th and 14th centuries). Fiori and Macchiarola [4] point out that HY is the resulting phase of hydrolysis of the MgCO3 in lime mortars with dolomitic origin. Newton and Sharp [5] only find magnesite and brucite in some dolomitic mortars studied by XRD, although they observe another phase that was not identifiable. However surprisingly in a previous study of mortars of dolomitic origin of Santa María de Zamarce, the pres
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