In OPC clinker, C3A forms solid solutions with some minor components such as alkali oxides, Fe2O3 etc., present in OPC clinker. In the cubic crystal structure of C3A, Fe3+ can substitute Al3+ up to a certain concentration. For this investigation syntheses of C3A1-xFx in the range of 0≤ x ≤0.20 were performed at 1250°C in a laboratory furnace. By Rietveld refinement of the C3A1-xFx lattice parameters a limit of Fe3+ ! Al3+ substitution of 6 ± 0.2 mole% has been determined for the used synthesis parameter employed. Influence of Fe3+-doping of C3A on its hydration behaviour was investigated with and without gypsum as retarding agent (w/c= 0.7; t= 72h; 23°C) with a heat flow calorimeter. The investigation has shown that overall heat flow and the ferric content of C3A1-xFx are correlated. Irrespective of the addition of gypsum as retarding agent, the heat of hydration declines with increasing ferric content. Reduction of overall heat flow is notably stronger when gypsum is added to C3A1-xFx, whereas the maximum heat flow and its point of time do not depend on the ferric content of C3A1-xFx solid solutions. Similarly, the ferric part does not influence the beginning of the transformation from Ettringite to Monosulfate.
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