The Dissolution of Ca12Al14O33 Crystal in CaO–Al2O3–SiO2-Based Melt and Its Structural Evolution with SiO2 Additions

Mayenite (Ca12Al14O33) is one of the most important crystalline phases in CaO–Al2O3–SiO2 based system, which is the common system in natural geological and artificial materials. However, due to the unstable structure of the CaO–Al2O3–SiO2 based melt, Ca12Al14O33 crystal seems to be difficult to precipitate as the main crystalline phase in its melt. Therefore, the dissolution of Ca12Al14O33 crystal and the structural evolution of the CaO–Al2O3–SiO2 based melt were studied in this work. The amount of Ca12Al14O33 crystals in the CaO–Al2O3–SiO2 based melt significantly decreased with SiO2 additions. Structural analysis showed that with the addition of SiO2, the released Q0(Si) would link with the non-bridging oxygen of Al–O−(Ca2+) in the Q3(Al) to form Q1(Si) and Q4(Al). Then, the Q3(Al) and Q4(Al) were broken into Q2(Al) with the help of free oxygen in accordance with Al avoidance rule. Due to a lack of Q3(Al) and Q4(Al) units, the precipitation of Ca12Al14O33 crystals was completely suppressed. When SiO2 was added to 9 mass pct, the depolymerized Q2(Al) was repolymerized to the complex and stable aluminosilicate structure composed of Q4(Al), Q3(Al) and Q2(Si) and Q1(Si) units, which assisted in increasing the structural polymerization degree and stabilizing the aluminate structure, and thus weakened the crystallization tendency of CaO–Al2O3–SiO2 based melt.