Silica-/calcium phosphate ceramics are of high interest in various aspects. On the one hand, they play an important role in medical applications due to their excellent biocompatibility. Therefore, detailed knowledge of the formation and stability properties of the high-temperature products ensures production under controlled conditions. On the other hand, they were identified as sinter deposits in industrial kilns, where it can indicate problems caused by too high combustion temperatures during the thermal combustion processes. Here, we report the results of two Raman heating studies to ~ 1300 °C in 10 °C-steps with nano-crystalline hydroxylapatite (HAp) and tricalcium phosphate (TCP), and a Raman heating study of natural silicocarnotite (to ~ 1200 °C, 50 °C-steps). The Raman experiments were complemented with thermal analyses. The Raman spectra of nano-crystalline HAp recorded at high temperatures revealed the stepwise loss of adsorbed water and surface-bound OH groups until ~ 570 °C. Significant loss of structural OH started at ~ 770 °C and was completed at ~ 850 °C, when HAp transformed to β-TCP. Between ~ 1220 and ~ 1270 °C, β-TCP was found to transform to α-TCP. The room temperature Raman spectrum of silicocarnotite is characterized by an intense v₁(PO₄) band at 951 ± 1 cm⁻¹ that shifts to ~ 930 cm⁻¹ at ~ 1200 °C. Using hyperspectral Raman imaging with a micrometer-scale spatial resolution, we were able to monitor in operando and in situ the solid-state reactions in the model system Ca₁₀(PO₄)₆(OH)₂-SiO₂-CaO, in particular, the formation of silicocarnotite. In these multi-phase experiments, silicocarnotite was identified at ~ 1150 °C. The results demonstrate that silicocarnotite can form by a reaction between β-TCP and α′_L-Ca₂SiO₄, but also between β-TCP and CaSiO₃ with additional formation of quartz.

Knowledge of the high-temperature properties of ternesite (Ca₅(SiO₄)₂SO₄) is becoming increasingly interesting for industry in different ways. On the one hand, the high-temperature product has recently been observed to have cementitious properties. Therefore, its formation and hydration characteristics have become an important field of research in the cement industry. On the other hand, it forms as sinter deposits in industrial kilns, where it can create serious problems during kiln operation. Here, we present two highlights of in situ Raman spectroscopic experiments that were designed to study the high-temperature stability of ternesite. First, the spectra of a natural ternesite crystal were recorded from 25 to 1230 °C, which revealed a phase transformation of ternesite to the high-temperature polymorph of dicalcium silicate (α’_L-Ca₂SiO₄), while the sulfur is degassed. With a heating rate of 10 °C/h, the transformation started at about 730 °C and was completed at 1120 °C. Using in situ hyperspectral Raman imaging with a micrometer-scale spatial resolution, we were able to monitor the solid-state reactions and, in particular, the formation properties of ternesite in the model system CaO-SiO₂-CaSO₄. In these multi-phase experiments, ternesite was found to be stable between 930 to 1020–1100 °C. Both ternesite and α’_L-Ca₂SiO₄ were found to co-exist at high temperatures. Furthermore, the results of the experiments indicate that whether or not ternesite or dicalcium silicate crystallizes during quenching to room temperature depends on the reaction progress and possibly on the gas fugacity and composition in the furnace.

In the last decades, Raman spectroscopy has become an important tool to identify and investigate minerals, gases, glasses, and organic material at room temperature. In combination with high-temperature and high-pressure devices, however, the in situ investigation of mineral transformation reactions and their kinetics is nowadays also possible. Here, we present a novel approach to in situ studies for the sintering process of silicate ceramics by hyperspectral Raman imaging. This imaging technique allows studying high-temperature solid-solid and/or solid-melt reactions spatially and temporally resolved, and opens up new avenues to study and visualize high-temperature sintering processes in multi-component systems. After describing in detail the methodology, the results of three application examples are presented and discussed. These experiments demonstrate the power of hyperspectral Raman imaging for in situ studies of the mechanism(s) of solid-solid or solid-melt reactions at high-temperature with a micrometer-scale resolution as well as to gain kinetic information from the temperature- and time-dependent growth and breakdown of minerals during isothermal or isochronal sintering.

The deposition of mineral phases on the heat transfer surfaces of brown coal power plants may have a negative effect on power plant boilers. The paragenesis of these deposits contains information about the actual temperature prevailed during the combustion of lignite, if the temperature-dependences of distinct mineral transformations or reactions are known. Here, we report results of a sintering study (to ~ 1100 °C) with samples containing anhydrite, quartz, and gehlenite, which are typical components of Rhenish lignite ashes. Thermal decompositions and solid-state reactions were analyzed (1) in situ and (2) both in situ and after quenching using confocal hyperspectral Raman imaging. This novel application of confocal Raman spectroscopy provides temperature- and time-resolved, 2-dimensional information about sintering processes with a micrometer-scale resolution. In the course of the sintering experiments with anhydrite and quartz with a weight ratio of 2:1 both polymorphs wollastonite and pseudowollastonite were identified in situ at about 920 and 1000 °C, respectively. The formation of pseudowollastonite was thus observed about 120 °C below the phase transition temperature, demonstrating that it can form metastably. In addition, \(\alpha_{L}^{\prime }\)-Ca₂SiO₄ was identified at about 1100 °C. In samples containing equal weight fractions of anhydrite and quartz that were quenched after firing for 9 h at about 1100 °C, β-Ca₂SiO₄ (larnite) crystallized as rims around anhydrite grains and in direct contact to wollastonite. We furthermore observed that, depending on the ratio between quartz and anhydrite, wollastonite replaced quartz grains between 920 and 1100 °C., i.e., the higher the quartz content, the lower the formation temperature of wollastonite.