Synthesis, characterization and formation mechanism of friedel’s salt (FS: 3CaO·Al2O3·CaCl2·10H2O) by the reaction of calcium chloride with sodium aluminate

Abstract

The synthesis of Friedel's salt (FS: 3CaO center dot Al2O3 center dot CaCl2 center dot 10H(2)O) by the reaction of calcium chloride with sodium aluminate was investigated. Factors affecting the preparation of Friedel's salt, such as reaction temperature, initial concentration, titration speed, aging time and molar Ca/Al ratio were studied in detail. XRD, SEM images and particle size distribution show that the reaction temperature, aging time and molar Ca/Al ratio have significant effect on the composition, crystal morphology, and average particle size of the obtained samples. In addition, the initial CaCl2 concentration and NaAlO2 titration speed do not significantly influence the morphology and particle sue distribution of Friedel's salt. With the optimization of the operating conditions, the crystals can grow up to a average size of about 28 mu m, showing flat hexagonal (or pseudohexagonal) crystal morphology. Moreover, two potential mechanisms of Friedel's salt formation including adsorption mechanism and anion-exchange mechanism were discussed. In the 'adsorption mechanism, Frieders salt fonus due to the adsorption of the bulk Cl- ions present in the solution into the interlayers of the principal layers, [Ca2Al(OH-)(6)center dot 2H(2)O](+), in order to balance the charge. In the anion-exchange mechanism, the freechloride ions bind with the AFm (a family of hydrated compounds found in cement) hydrates to form Friedel's salt by anion-exchange with the ions present in the interlayers of the principal layer, [Ca2Al(OH-)(6)center dot 2H(2)O](+)-OH-.