The crystal structure of Na2CaAl4O8 and its hydration behaviour

Na2CaAl4O8 represents a crystalline phase occurring in metal-slag and calcium aluminate cements. Despite the industrial implications of these materials, its crystal structure had not been yet solved. We have solved the crystal structure of Na2CaAl4O8 by means of Single Crystal X-ray Diffraction from grains obtained from synthesis. From single Crystal analysis we demonstrate that Na2CaAl4O8 is orthorhombic, with P212121 space group, a ​= ​7.2541(1), b ​= ​10.4301(2), and c ​= ​10.4348(2) Å and V ​= ​1287.92 ​Å3. The structure is characterized by a framework of corner-sharing aluminium centred tetrahedra, described as a stacking along [101] of (101) layers of composed of six-membered tetrahedral distorted rings; layers are linked by sharing some AlO4 tetrahedra apices with Al tetrahedra forming the adjacent layers along [010], along with the Ca-centred octahedra. Na atoms occupy extra-framework spaces in two types of coordination, one being asymmetric, representing a hydrophilic point, explaining the water reactivity.