Investigação estrutural e espectroscópica de aluminatos de cálcio: uma abordagem teórico/experimental

Abstract

The main component of Portland cement is clinker, which is a mixture of several products, the main one being calcium aluminate (CA), which turns out to be the precursor of the cement setting/hardening process. This precursor may have more than one phase, where each of these ends up changing its physical and chemical properties, one of the present phases is tricalcium aluminate (C3A), being the most reactive among aluminates, reacting instantly with the water within seconds, showing rapid hardening. In addition to being used in clinker, C3A is a very refined material and has been widely studied as a basis for biomaterials, dentistry and orthopedic fields applications. Despite these numerous studies there is still much to discover about this compound, especially in photoelectric areas, studies on its Band gap and variations of it. Despite the studies with this compound, there is a lack of microstructural analyzes in the literature that analyze what happens when the temperature and proportions of the precursors are varied, thus, this work analyzes what happens with this material in X-ray Diffraction analyzes, micro-Raman spectroscopy, UV-vis and Band gap spectroscopy, and from these analyzes you can get a discussion of the use of this material in future applications. The main synthesis methods described in the literature are defined by three types: solid state, wet chemistry and hydrothermal. The use of unusual methods for the synthesis of CAs is justified by the fact that they improve the qualities and properties of the final product, obtain lower production costs and especially the reduction of the formation temperature of the compound. The polymeric precursor method, known as a variant of the Pechini method, created in 1967, allows a significant decrease in temperature by approximately 40 %. Therefore, the use of this technique has been widely used for CA compounds, ceramic materials preparations. among others as the Perovskites. From this fact, nine samples of C3A nanoparticulate powders with three different proportions of the base compound were obtained for three calcination temperatures, obtained by the polymeric precursor method. To know what happens when there are changes in proportions, we studied the variation in the proportion of aluminum nitrate and calcium nitrate with different calcination temperatures for the nine samples. The X-ray diffraction analyzes showed that the samples have the presence of C3A and other secondary phases such as mayenite (C12A7) as expected, besides the presence of organic and analogous materials. to mixing materials which are characteristic of the process of obtaining the powders. As the temperature increases, the concentration of C3A also increases, becoming the majority phase, as expected. From this fact, the presence of these secondary materials was confirmed by micro-Raman microscopy analysis, showing characteristic vibrations of these secondary compounds and mainly of the primordial phase at C3A. For the UV-vis spectroscopy analysis, the energy gaps characteristic of the two main phases of the samples were identified, showing that the secondary phase C12A7 containing a Band gap around 5 eV, ends up modifying the C3A phase energy, but as the temperature increases, C3A turns out to be the only phase and thus has its power Band gap around 5,9 eV. To complement this study, an "ab initio" calculation was made with the aid of the CRYSTAL09 structure calculation program. The presence of indirect type Bands gap was determined, where in the valence band and driving band they go from the ’k points’ X to R, respectively. Due to the lack of answers in the literature, it has not been possible to identify all the vibrations present in our micro-Raman spectroscopies. However, it is clear that there are severe changes in spectra as a function of calcium and aluminum concentration and temperatures. It is observed that some vibrational modes cease to exist in detriment of the emergence of others indicating local structural changes for the compounds. This indicates that there is still much to research and find out about this compound. This justifies the more detailed investigation of this compound using advanced ab initio calculation techniques.