Long-term observation of yeelimite clinker hydration in environment of saturated water vapour

Due to ecological overloading decrease, industrial by-products are utilized. Fly ash also belongs among these by-products. In Portland-composite cements, the high-temperature fly ash is applied nowadays, but fluidized bed combustion fly ash (FBC fly ash) is not used. The reason is that the FBC fly ash hydrates to form an undesirable ettringite, C(3)A center dot 3CaSO(4)center dot 32H(2)O. Decomposition temperature of ettringite in ambient conditions is most often mentioned in a range of 50 degrees C to 110 degrees C. In order to assess the ettringite long-term stability, ettringite was synthesized by hydration of mineral yeelimite, 3CaO center dot 3Al(2)O(3)center dot CaSO4. The aim of this paper is long-term observation of the hydration of yeelimite in environment of saturated water vapour and examination of how stable the products of hydration are under given conditions. Raw materials for preparation of yeelimite were gypsum, limestone and corundum, weighed according to stoichiometric ratio of yeelimite. The mixture was heated at temperature of 1200 degrees C, soak 3 hours. After quenching and grinding, yeelimite was hydrated with a water-yeelimite ratio of 0.3. The system was cured as pastes and stored in conditions at laboratory temperature and in settings of saturated water vapour (100% R.H.). In order to follow the progress of synthesis regularly, namely up to age of 180 days of hydration, X-ray diffraction (XRD) was chosen using diffractometer Empyrean PANalytical (CuK-alpha). Based on the XRD patterns, only minor part of ettringite was formed, whilst a majority of yeelimite was preserved. It has been found that ettringite was not the only one product of yeelimite hydration. From an age of 20 days of hydration, a small amount of monosulfate was identified, whose quantity rose gradually. Especially at later ages of hydration, transformation of ettringite into monosulfate was observed. (C) 2016 Published by Elsevier Ltd.