According to the content of AL2O3, fire cement can be divided into CA-50, CA-70, CA-80 type and so on. The raw materials of CA-50 cement are natural bauxite and limestone, that is, high alumina cement (alumina cement). CA-70 type fire cement is made of industrial AL2O3 and CaO as raw materials, with few impurities, good setting properties and refractoriness, and is suitable for the preparation of high-performance cement-bonded castables. The AL2O3 content of CA-80 type fire cement is as high as 80%, because it contains a considerable amount of α-AL2O3 micropowder, which can act as a binder for castables when no (or less) other micropowder is used, and also improves the medium temperature strength of the cement. Low deficiency.
Generally speaking, the higher the alumina content, the higher the refractoriness, and its application is biased towards refractory materials; the lower the alumina content, the higher the calcium oxide content, the better the hydration performance of cement, and its application will be biased towards Concrete and chemical building materials.
fire cement can provide sufficient demoulding strength for castables within 24h. It is a commonly used hydration binder in casting refractory materials. The hydration behavior of fire cement after encountering water will affect the fluidity, working time and demoulding of castables. time, which in turn affects the normal construction of the castable.
The hydration process of fire cement is essentially a reaction process of low-solubility solids and water to produce lower-solubility solid products, which can be divided into three stages: dissolution, nucleation and precipitation. When fire cement is in contact with water, the surface of fire cement particles is first hydroxylated to release Ca2+ and AL(OH)4- ions (tetrahydroxyaluminate) until the ion concentration in the solution reaches saturation; then hydration products are formed in the saturated solution. The nuclei grow and the number of nuclei increases, but the deposition process does not begin, which is called the nucleation period or induction period; when the nuclei of the hydration products grow to a critical size, the fire cement hydration products are deposited and accompanied by release of heat.
In addition, the hydration process of fire cement has another feature, that is, fire cement can form Ca2+ and AL(OH)4- ions during the hydration process, and AL(OH)4- ions can continue to ionize in the solution. Thus, AL3+ and OH- ions are formed, so the solution after hydration of fire cement is usually alkaline.
After the fire cement is in contact with water, the mineral phases CA and CA2 contained in it will react with water and form hydration products during the hydration reaction. The types of hydration products will vary with the curing temperature of the fire cement. different.
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