Properties and functions of retarders
The substance that can delay the setting and hardening time of refractory castable is called retarder. The action mechanism of retarders varies with the properties of binders and retarders used. Generally speaking, it has the following two functions:
1. To form a complex. The retarder forms a complex with the positive ions dissociated from the binder, which inhibits the crystallization of hydrates or reaction products, or inhibits grain growth.
2. To form a thin film. The retarder is adsorbed on the surface of the cement particles, which prevents the cement particles from contacting with water and restrains the hydration reaction rate. Retarders are mainly used in calcium aluminate cement bonded castables, such as gluconic acid, ethylene glycol, glycerol, starch, phosphate, lignosulfonate, etc.
Properties and functions of delayed coagulant
The substance in which water is mixed for a certain period of time to promote the setting of the binder is called delayed coagulant. They are a kind of counterionic substances which are mixed with water and need to be hydrolyzed to provide counter-ionic substances which are different from those carried on the surface of the fine powder in the mixture. For example, the charge on the surface of ultrafine silica powder is negative, so the delayed coagulant used must be a substance that can provide positive ions. For example, if the castable with ultra-fine silica powder as the main binding phase, calcium aluminate cement can be used as delayed setting accelerator, because calcium aluminate cement can slowly release Al3+ and Ca2+ ions during hydrolysis, which can promote the coagulation of silica ultra-fine powder slurry.
Properties and functions of foaming agent (air entraining agent)
The substance that can reduce the surface tension of the liquid and produce a large number of uniform and stable foam is called foaming agent (air entraining agent). The principle of bubbles produced by foaming agent and air entraining agent is similar, that is, when it dissolves in water, it is easy to be adsorbed on the gas-liquid interface, which reduces the surface tension of barren liquid and increases the contact area between liquid and air. Because of the protective effect of the adsorbed foaming agent on the liquid film, the liquid film is relatively firm and the bubbles are not easy to burst. Foaming agents are rosin size, resin saponin grease, aluminum petroleum sulfonate, etc. Air entraining agents include rosin thermopolymers, Nonionic surfactants, alkylbenzene sulfonates, carboxylic acids and their salts, etc. In addition, there is a kind of aerating agent whose aerating effect is that the castable produces bubbles due to chemical and physical action during mixing and after pouring, which is uniformly distributed in the slurry. The aerating agent usually uses metal aluminum powder, which is highly active and reacts with water to replace hydrogen. The escaped H2 gas forms bubbles in the castable. However, the addition of aluminum powder should not be too large, otherwise the strength of castables will be greatly reduced. This kind of admixture is used to prepare lightweight porous castables.
Properties and functions of defoamers
The substance that can make the bubbles disappear quickly when the refractory castable is mixed and shaken is called defoamer. Defoamer is also a surface active material, its ability to reduce liquid surface tension is much stronger than foaming agent, so that the strength of the liquid film formed is very low, which can greatly reduce the stability of foam. And the spreading speed on the liquid surface is also faster, and the faster the spreading speed is, the stronger the defoaming effect is. The types of defoamers include alcohols, fatty acids and fatty acid salts, amides, organosilicon compounds, etc.
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