A refractory castable is a mixture of heat-resistant aggregate and heat-resistant hydraulic cement; when used, it is mixed with water and tamped or poured into place. The use of refractory castables continued to expand, and the chemical and physical properties continued to improve over time. At first, this was only possible through the expansion and improvement of calcium aluminum cement (CAC). But the real push for refractory castable innovation occurred in the mid-80s when castables were produced with complex formulations, which were even better than refractory bricks in many applications.
The availability of ultra-fine fillers allows continuously graded formulations as low as sub-micron levels. Therefore, refractory molds with very low aluminum cement content can be formulated, leading to ultra-low cement content (ULCC) and Low cement castable(LCC). The latest technology focuses on optimizing the rheological properties of the castable, enabling the applicator to install the refractory without mechanical vibration. These are known as self-forming fluids (SFC) and have promoted the remarkable development of monolithic refractories. Basically, in terms of shape types, there are two types of refractory materials, namely brick (with the shape) and monolithic(without shape). Calculated by shape type, bricks were the world's largest refractory material segment in 2013.
In general, the global refractory market continues to grow, maintaining huge commercial potential, rapid development, innovation, and higher acceptance. Without the latest development of newly developed additives based on polycarboxylate ether (PCE) technology, the growth and wider acceptance of refractory materials would have been impossible. These additives contribute essentially to the further penetration for the above-mentioned types of self-flow castable.
The success factors of this development are, for example, faster installation and less mixed water to produce castables flow, thereby reducing the porosity of the casting material while increasing density and strength. All these features provide higher durability and cost savings for the fire-resistant applicator.
The main role of additives in the dispersed castable system is to provide sufficient fluidity to cast refractory concrete with low water addition, which is necessary to achieve low porosity, high strength, and long durability.
Colloidal (sol) bonding system Among different non-lime-free bonding materials, colloidal silica or silica sol bonding has always been the most important and has been widely popular for a long time, mainly because the sol is even in the high solid content. Colloidal silica is a stable dispersion of silica particles mainly in water. The silica particles present are within the colloidal size range (10-6 to 10-9 m) and are suspended in such a way that gravity will not precipitate them. When used in moldable compositions, these nano-sized sol particles surround and encapsulate the refractory aggregates by forming a three-dimensional framework network.
HA produces hydraulic bonding by reacting with water and forming hydrated phases (i.e. bayerite and boehmite and alumina gel), filling pores and interface defects, and being fire-resistant when used in moldable compositions that provide strength. A honeycomb structure is formed in the aggregate. HA as a binder provides a non-reactive protective edge of nano-sized particles that surround alumina aggregates and create a complete alumina system (for high-aluminum moldable materials). The crystallization of HA is conducive to the formation of crystals on the surface of the aggregate,
Silica fume (micro silica fume) acts as an effective additive for the formation of nanostructures in the process of cement hydration. It is a by-product of the silicon and ferrosilicon industry, containing a polymorph of amorphous silica with spherical particles with an average size of less than 150 nm. Since this silicon powder contains at least 90% silicon dioxide, which is a good refractory material, it was planned to use silicon powder in refractory materials from the beginning. Silicon fume is mainly used for refractory castings to improve its fluidity because it has a spherical shape and a finer size. Therefore, it helps to reduce the water content, thereby improving various high temperatures and environmental performance. Due to its finer particle size, silicon powder particles can easily enter the gaps between coarse and medium particles, even very small particles, and fill them.
Nanoparticles are used as additives in castables, mainly to improve the sintering process (as a sintering additive), increase densification at lower temperatures, reduce energy consumption, and impart various required properties to the castable composition. The dry mix and its positive effects can be obtained from the mold during the mixing process. These nanoparticles react with the composition of the castable to form a transient liquid phase at a temperature lower than the conventional sintering temperature and help the castable to diffuse and break faster.