Hardening mechanism and correct storage of phosphate refractory castables
Phosphate castable refers to a castable combined with phosphoric acid or phosphate, and its hardening mechanism is related to the type of binder used and the hardening method.
The binder of the phosphate castable can be phosphoric acid or a mixed solution of aluminum dihydrogen phosphate produced by the reaction of phosphoric acid and aluminum hydroxide. Generally, the binder and aluminum silicate do not react at room temperature (except for iron). Heating is required to dehydrate and condense the binder and bond the aggregate powder together to obtain the strength at room temperature.
When using a coagulant, heating is not required, and fine magnesia powder or high alumina cement can be added for coagulation. When magnesium oxide fine powder is added, it reacts quickly with phosphoric acid to form, causing refractory materials to set and harden. When aluminate cement is added, phosphates with good gelling properties, water-containing phosphates such as monocalcium phosphate or diphosphate are formed. Hydrogen calcium, etc., cause the material to condense and harden.
Phosphoric acid combined with refractory castable samples are stored indoors, outdoors, and in the standard curing room for castables. With the increase of storage time, the compressive strength at room temperature increases, but indoor storage is better than outdoor. Standard curing after 3 months The intensity tends to decrease slowly. When stored indoors, the compressive strength after burning is more than 10MPa higher than that in the open air. The compressive strength after burning stored in the standard curing room is also better than that in the open air. Samples stored in the open air may have low performance of refractory castables due to changes in cold and heat and cycles of dry and wet.
It should be pointed out that although the storage environment has some influence on the room temperature performance of phosphoric acid and phosphate refractory castables, it has little effect on the high temperature performance and does not affect the use. This is because the cemented phosphoric acid produced by adding coagulant before 200℃ Salt plays a leading role, while at high temperatures the ceramic bond formed plays a leading role. Therefore, after the phosphoric acid and phosphate refractory castables with coagulant are hardened, the requirements for the storage location are not very strict. Phosphoric acid and phosphate refractory castables without accelerator can be stored only after heat treatment. Without heat treatment, there is no strength, and the heat treatment temperature is low, and it is easy to produce "deliquescent" phenomenon.
Although the binder in the phosphate castable does not react with the material itself at room temperature, it will slowly condense and harden due to temperature changes during storage or transportation or other environmental changes. If you need to store it for a long time, you can add it Retarders such as citric acid, tartaric acid, etc. to extend the storage time.
When phosphoric acid and phosphate refractory castables are stored or under low-temperature use conditions, white crystals will precipitate on the surface of the refractory castables. After chemical and X-ray analysis, "white hair" is the crystallization of aluminum salt and calcium salt. In the case of high temperature or long-term heating, the aluminum salt and calcium salt are transformed into insoluble salts, and the "white hair" also disappears. This phenomenon has no effect on the performance of refractory castables under normal circumstances. However, excessive precipitation will loosen the internal structure of the castable, reduce its strength, or even collapse, so care should be taken during storage or use.