Application of Isomorphic Ca-Si Rocks for the Synthesis of α-C2S Hydrate
Keywords:opoka, thermal treatment, α-C2S hydrate, X-ray diffraction, hydrothermal synthesis
AbstractThere are approximately 35 million t of carbonate opoka and 21 million t of chalk marl in Lithuania. These materials are suitable for the production of various hydraulic binders. In this work, the influence of burning temperature (700 °C – 950 °C) and duration (0.5 h – 2 h) on the calcination of isomorphic Ca-Si rocks was investigated. The goal was to prepare a mixture of opoka and chalk marl that would be reactive enough for the hydrothermal synthesis of various calcium silicate hydrates, as this would suggest another potential use of these stocks. It was determined that the largest amount of free CaO (59.09 %) is obtained after burning opoka for 2 h at 750 °C and after burning chalk marl for 2 h at 800 °C (84.22 %). The remaining CaO is bound to the structure of poorly crystalline pseudowollastonite, which transforms to wollastonite when the temperature of the thermal treatment is increased. Because the calcination temperature of both materials is similar, they can be burned simultaneously; this would significantly simplify the preparation procedure of these mixtures. The optimal temperature for burning the opoka – chalk marl mixture with a molar ratio of CaO/SiO2 = 2 (62.8 % chalk marl and 37.2 % opoka) was determined to be 825 °C for 2 h. In this case, the amount of free CaO is the largest and the crystals of formed wollastonite are poorly crystalline. This blend was treated hydrothermally at 200 °C for 24 h. The dominant product of the synthesis was α-C2S hydrate, which in literature was reported to be the main constituent in the production of new hydraulic binder.
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