Binders for Refractory Castables: An Overview
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Binders for Refractory Castables: An Overview R. Sarkar1 Abstract: 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. Keywords: castable, binder, alumina cement, colloidal system, hydratable alumina, chemical binder
1. Introduction Unshaped refractories, especially castables, are replacing conventionally shaped refractories in most application areas in the user industries due to various advantages, like the ease of production and easy installation technique, no firing, fewer joints resulting in lower corrosion, etc. [1]. The advantages have inspired refractory scientists and manufacturers to continuously investigate these materials in depth and to improve their quality and performance. Since the early 1960s, the commercial availability of pure calcium aluminate cements (CACs) with higher alumina content has made castables purer and has made them applicable for various high-temperature applications [2]. Refractory castables are a blended mixture of different fractions of refractory aggregates and fines with bonding agents and additives. They are supplied by manufacturers as loose dry powders, mixed with a liquid (usually water). In the user industry, they are cast or poured into the application area to get the desired shape and dimensions, become rigid by hydraulic or chemical setting and are
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then fired for densification and strength development [3]. The proportions of each of the components used in the castable composition are varied to achieve the desired properties and to suit the intended application. The bonding agents or binders are a must in a castable composition, as they contribute to the development of strength both at ambient and elevated temperatures. Binders also help to retain the shape at green conditions, as no compaction is involved during the shaping process of the castables [4]. Binders also contribute to the sintering of the castable compositions. The mass transfer in the castable for sintering is le
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