| Abstract [eng] |
Calcium aluminate as a biomaterial has been studied since the beginning of the 1990s in relation to its physical and mechanical properties, as well as its biocompatibility [1]. Mayenite (C12A7), one of calcium aluminate minerals with CaO/Al2O3 =1.7, has stimulated the research interest because of its better accelerator effect-rapid hardening activity, oxygen mobility, ionic conductivity and catalytic properties in comparison with the other calcium aluminates [2]. C12A7 hydration exhibits an induction period of a few minutes during which pastes have suitable rheological properties and remain workable. Besides, the setting can be controlled by adding additives to the blend and the resulting hydrated cements have shown to be biocompatible in vivo and in vitro [3]. Mayenite can be synthesized through solid state reaction, however, the main disadvantages is a very high calcination temperature (1300–1500 °C), low surface area, low porosity. Sol–gel procedure increases the operation cost due its complexity, the use of energy at different synthesis steps and the required precursor, which has to be calcined in a 600–900°C temperature range. Another method of C12A7 preparation is a hydrothermal synthesis of precursors, which is followed by the thermal treatment at temperatures ranging from 25 to 1000 °C. It was determined that this method allows to synthesize C12A7 powders with relatively high specific surface area [4]. For this reason, the main objective of the present work is to determine the formation and thermal stability of mayenite by using two steps synthesis in a 25 – 1150 °C temperature range. In this paper, the following reagents were used: calcium oxide from Ca(OH)2 was additionally burned at 550 °C temperature for 1.5 h, the quantity of free CaO is equal to 92 wt. %. γ-Al2O3 was produced by burning aluminium hydroxide at 475 °C for 4 hours. Dry primary mixture with molar ratio of CaO/Al2O3 = 2.8 was mixed with water to reach the water/solid ratio (W/S) of the suspension equal to 10.0. The hydrothermal synthesis was carried out in unstirred suspensions, in “Parr instruments” (Germany) autoclave, under saturated steam pressure at 130 °C temperature for 1-72 hours (the temperature was reached within 2 h). After hydrothermal treatments, the autoclave was quenched to a room temperature. The suspensions after synthesis were filtered, products rinsed with acetone to prevent carbonization of materials, dried at 50 °C ± 5 temperature for 24 h, and sieved through a sieve with a size width of 60 μm. In-situ XRD analysis was made with a high-temperature camera MTC-hightemp (Bruker AXS, Karlsruhe, Germany). It was found that, in unstirred CaO–Al2O3 suspensions, when CaO/Al2O3 molar ratio of primary mixture was equal to 2.8, within 1 hour of isothermal curing at 130 °C, katoite, was formed. Also, the basic reflections of partially unreacted gibbsite were observed in X-ray diffraction patterns. Moreover, the carbonation appeared, when the products and/or partially unreacted primary components were dried in an air conditioned chamber (50 °C, 24 h), because the diffraction peaks characteristic to calcium monocarboaluminate. After 4 h of hydrothermal treatment, XRD analysis data showed that gibbsite was fully reacted and more intensive diffraction maximums characteristic to katoite were observed. The thermal stability of products obtained after 4 h of isothermal curing was investigated in a hightemperature camera MTC-hightemp in a 25–1150 °C temperature range. It was evidenced that katoite is stable till 275 °C. At higher temperature (350 °C) katoite is fully transformed in to mayenite. It should be underlined that due to different CaO/Al2O3 molar ratio between katoite (3) and mayenite (1.72), reflection patterns of calcium oxide were identified in the products. It was determined that the crystallinity of mayenite increased by increasing temperature till 1150 °C. |
