Analytical Electron Microscopic Studies of Doped Dicalcium Silicates

Dicalcium silicates having CaO/SiO₂ molar ratios of 1.8 to 2.2 were sintered at 1450°C for 90 min with or without small quantities of dopants (K₂O or Al₂O₃) and were air quenched. The microstructures of the fired samples were characterized using electron microscopy (SEM and TEM) and associated microanalytical techniques. There was no evidence for the existence of Ca_1.8SiO_3.8 or Ca_2.2SiO_4.2. Amorphous grain‐boundary phases were observed between grains and as inclusions within the grains; the amounts decreased as CaO/SiO₂ ratios increased. The compositions of the amorphous phases were always rich in dopants and had a CaO/SiO₂ ratio close to that of wollastonite. High levels of Al₂O₃ were observed to enter the β‐Ca₂SiO₄ grains under lime‐rich conditions (CaO/SiO₂= 2.2) up to a saturation level of about 3.0 wt%. Some additional crystalline phases were observed to form depending on stoichiometry and dopant level.