1. The physical and chemical changes of silica bricks during the firing process. The silica bricks undergo the following changes in temperature sequence during the firing process: residual moisture is discharged from the bricks below 150°C, and Ca(OH)2 begins to evaporate at 450°C. Decompose; at 450~500℃, Ca(OH)2 dehydration is completed, the combination of silica particles and lime is destroyed, and the strength of the green body is greatly reduced.
In the range of 550~650℃, β-quartz transforms into α-quartz. Since the transformation process is accompanied by a volume expansion of 0.82%, the quartz crystal will have microscopic cracks with varying densities. Between 600 and 700°C, the solid phase reaction between CaO and SiO2 begins, and the strength of the brick increases.
Starting from 1100°C, the transformation speed of quartz increases greatly and the specific gravity of the bricks also decreases significantly. At this time, the volume of the bricks increases greatly due to the transformation of quartz into low specific gravity variants. Although the liquid phase is also increasing at this time, cracks can still occur in the range of 1100~1200.
2. The volume of siliceous bricks changes during the heating process, which is different at different temperature stages. According to the changing characteristics of various types of silica green bodies, they can be divided into stages below 600℃, 600~1100℃, 1100~1250℃ and above 1250℃. There is significant expansion in the β→α quartz transition between 500 and 600°C.
At 700~800℃, there is a solid phase reaction between CaO and SiO2, which causes microscopic shrinkage. At 1000~1100℃, the shrinkage is more significant due to the generation of liquid phase. From about 1200℃, due to the acceleration of quartz transformation, the green body begins to expand violently. The starting temperature and rising trend vary greatly depending on the type of raw material.
