Structure of the hydrogen bonds and silica defects in the tetrahedral double chain of xonotlite

We use a combined experimental/theoretical approach to investigate the structure and stability of the silicate tetrahedral double chain in xonotlite. The M2a2bc polytype of xonotlite was found to be dominant in the synthetic samples. This polytype was used to predict the relative stability of silica defects in the tetrahedral double chain of xonotlite. The defects in Q3 sites were found to be substantially more abundant compared to Q2 tetrahedra. Moreover, the paired substitutions in the silicate double chain at neighbouring Q3 sites are energetically more stable then isolated Q3 defects. The defects in the silicate chains of xonotlite offer structural channels accessible for diffusion of ions and water. The defect sites are potential candidates for incorporation of foreign ions in the xonotlite structure. Based on the crystal-chemical similarities with xonotlite the structure of silicate chains in tobermorite and C–S–H phases is discussed.