Cracking during the baking process is a common issue in the production of magnesia precast. This is primarily caused by the hydration process of magnesia, in which MgO mixes with water and results in hydration products like magnesium hydroxide and brucite crystals. The formation of these products can lead to significant increases in pH levels, followed by the production of H2CO3. This reaction can then lead to the formation of magnesium carbonate or magnesite, which can cause the volume expansion of the castable and lead to potential cracking.
To prevent cracking in magnesia castables, it is essential to formulate a reasonable curing and baking system that can reduce the hydration degree of magnesia. During the curing stage, the temperature should be strictly controlled to prevent the formation of too much brucite crystals in a short period. A temperature of around 70°C for 8 hours has been found to work best in this regard. In the baking stage, a rapid baking plan should be adopted to discharge the free water in the pouring body as quickly as possible. However, the baking rate should not be too fast so as not to evaporate the water inside the castable too quickly.
For thin plate prefabricated parts like wall panels, protection during product transfer should also be strengthened to prevent any microcracks from expanding during the curing to baking process. In on-site tests, a heating rate of 20℃·h-1 has been found to be sufficient to prevent serious cracks in slag retaining wall prefabricated parts. With these measures in place, manufacturers can reduce the risk of cracking and produce high-quality magnesia precast products.
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