accelerating agent/set retarder/ water reducer/castable binder

Accelerating agent

Substances that promote the setting and hardening of refractory castables are called accelerators. The mechanism of action of accelerators is relatively complex and varies depending on the properties of the binder and the accelerator used. Different binders require different types of accelerators. Accelerators used with calcium aluminate cement binders are mostly alkaline compounds: NaOH, KOH, Ca(OH)2, Na2CO3, Na2SiO3, etc.; accelerators used in combination with phosphoric acid and aluminum dihydrogen phosphate binders include: activated aluminum hydroxide, talc, magnesium oxide, calcium aluminate cement, etc.; accelerators used in combination with water glass (sodium silicate) binders include sodium fluorosilicate, aluminum phosphate, sodium phosphate, lime, dicalcium silicate, glyoxal, etc.

set retarder

Substances that delay the setting and hardening time of refractory castables are called retarders. The mechanism of action of retarders varies depending on the properties of the binder and the retarder used. Retarders are mainly used in monolithic refractory materials bonded with calcium aluminate cement containing fast-hardening minerals. Commonly used retarders include: low concentrations of NaCl, citric acid, tartaric acid, ethylene glycol, glycerol, starch, phosphates, and lignosulfonates.

Water reducer

Water-reducing agents (also known as water-reducing agents or dispersants) can significantly reduce the amount of mixing water required for refractory castables while maintaining their flowability essentially unchanged. High efficiency water reducer / Water-reducing agents themselves do not chemically react with the material components to form new compounds; they only play a surface physicochemical role. Therefore, High efficiency water reducer are either surface-active substances or electrolytes. After dissolving in water, High efficiency water reducer adsorb onto the particle surface, increasing the zeta potential of the particle surface in the solution, increasing the repulsive force between particles, and releasing free water trapped in the aggregated structure composed of microparticles, thus playing a wetting and dispersing role; hence, High efficiency water reducer are also called dispersants. Because they can release free water from the aggregated structure, they can reduce the unit water consumption while maintaining the rheological properties (workability) of the castable, or increase the material's rheological properties without changing the unit water consumption, improving workability and making the material easier to construct and mold. For refractory castables using calcium aluminate cement, binding clay, and oxide powder as binders, the commonly used High efficiency water reducer(dispersants) include inorganic ones such as sodium pyrophosphate, sodium tripolyphosphate, sodium tetrapolyphosphate, sodium hexametaphosphate, sodium hyperpolyphosphate, and sodium silicate; and organic ones such as sodium lignosulfonate, calcium lignosulfonate, naphthalene-based water-reducing agents, and water-soluble resin-based High efficiency water reducer agents.

Castable Binder

A binder for refractory castables is a bonding material that bonds a loose powdery mix, retains its shape after casting and allows the cast shape to set and harden for handling and further processing. A binder also improves sintering, as the castables are not compacted during shaping, causing poor densification and strength. Initially, plastic clay was used as a binder for castables, but soon it was found to have more disadvantages than benefits. Next came the alumina cements with an alumina content below 50 %, but this resulted in degraded hot properties due to low temperature liquid phase formation. The performance of the castables was improved with an increase in the alumina content of the cement. However, the presence of lime continued to be a concern with high temperature uses. Then came the binders without lime, like sol-gel, hydrated alumina, phosphates, etc. Different bonding systems have their own advantages and limitations and are being used as per the requirements of the application site. The present paper reviews the different types of binders available for castables and gives an elaborate idea for their functioning and performance.