Crystal structure, structural disorder, and hydration behavior of calcium zirconium aluminate, Ca7ZrAl6O18

Crystal structure, structural disorder, and hydration behavior of Ca7ZrAl6O18 were investigated by laboratory X-ray powder diffraction ( Cu K alpha(1)) and conduction calorimetry. The initial structural model with 21 independent atoms in the unit cell was determined using direct methods, and it was further modified to a split-atom model, in which the five types of Ca atoms and four types of A1O(4) tetrahedra were, respectively, positionally and orientationally disordered. The crystal structure is orthorhombic ( space group Pmn2(1), Z = 2) with lattice dimensions a) 1.08486( 1) nm, b) 1.05913( 1) nm, c) 0.766945(8) nm, and V= 0.88123( 1) nm(3). The electron density distribution was determined by the maximum-entropy methods-based pattern fitting method, which resulted in the final reliability indices of R-wp= 9.59% (S) 1.22), Rp= 7.16%, RB= 2.38%, and RF= 1.87%. This compound showed the hydration behavior comparable to Ca3Al2O6. The reaction of Ca7ZrAl6O18 with water gave hydrogarnet as the main crystalline product. Addition of gypsum effectively retarded the hydration reaction, with the major hydration products of calcium monosulfo-aluminate hydrate and calcium aluminate hydrate.