The inhibition effect of fly ash on the deformation of magnesium oxychloride cement is not obvious. With the increase of fly ash, the deformation of magnesium oxychloride cement decreased at first, and then increased. The smallest deformation is the proportion of FA-35. The fly ash can promote the formation of the 5·1·8 phase crystal and slow the speed of 5·1·8 phase changing into Mg (OH)2. The growing crystals were disordered, like the scattered tree branches. The causes of FA-35 specimen expansion deformation can be explained by the configuration of the crystal.
