Behavior of silicate anions on the formation processes of gyrolite followed by ²⁹Si NMR

Using lime and amorphous silica as the starting materials, gyrolite (Ca₈(Si₁₂O₃₀) (OH)₄·7H₂O) was prepared hydrothermally with the Ca/Si molar ratio of 0.66 and 0.50 at 200 °C for 0.5-128 h. The formation process was studied mainly by ²⁹Si NMR. The initially formed C-S-H consisted mainly of long chains of silicate anions. For Ca/Si = 0.66, the chain which had been formed was broken, and C-S-H changed into gyrolite and xonotlite. For Ca/Si = 0.50, only gyrolite was formed from a long chain of the silicate anions. It is thought that the differences of the C-S-H structures, which were formed in the initial stage, greatly influenced the final products. In addition, for Ca/Si = 0.50, the Z-phase was also formed as a precursor of gyrolite and its silicate anion structure was similar to that of the gyrolite.