Refractory castables

Refractory castables

Refractory castables comprise a large and diverse group of materials in the family of monolithic (shapeless) refractories. Their use has grown significantly in the past 30 years. Progressing from simple mixes, refractory castables today comprise some very complex and technical formulations, finding use in a variety of very  demanding and severe applications.

Refractory castables are premixed combinations of refractory aggregates, matrix components or modifiers, bonding agents, and admixtures (Fig 1). They are mixed with a liquid (usually water) at the point of installation and vibrated, poured, pumped, or pneumatically shot into place to form a refractory shape or structure that becomes rigid because of hydraulic or chemical setting. The majority of refractory castables use calcium aluminate cement as the bonding agent though in recent years other bonding agents have also been developed. All castables have refractory aggregates and matrix components which allow their use to temperatures up to 1850 deg C.

Components of refractory castables

Fig 1 Components of refractory castables

 Refractory castables are classified in several ways. The primary classification is based on chemistry which separates the refractory castables based on alumina and alumino-silicate aggregates from the castables based on basic refractory oxides such as magnesite and dolomite. Also this division is fundamental in that different bonding agents are utilized in each category.

Alumina and alumino-silicate refractory castables are further classified in several ways based on the attributes such as chemistry and/or mineralogy, alumina content/refractoriness ( super duty, 60 % alumina etc.), mineral base ( mullite, fused silica etc.), density/thermal insulating value (dense, medium weight, light weight/insulating etc.),  cement content ( conventional, low cement, and ultra low cement etc.), and flow/placement characteristics (vibrating, casting, free flow, shotcrete etc.).

The proportions of each components (refractory aggregates, matrix components, bonding agents, and admixtures) used in the refractory castable vary in each castable composition to achieve the desired physical and chemical properties and characteristics for the intended castable application. The general ranges of component quantities in a refractory castable are (i) aggregate, 40 % to 80 %, (ii) modifiers, 5 % to 30 %, (iii) bonding agents,  2 % to 50 %, and (iv) admixtures ,up to 1 %.

The refractory aggregates constitute the basic skeleton of the castable and account for the largest amount of the formulation. The sizes of the aggregates can range from 20 mm to 300 mm. Aggregates are sized and proportioned for achieving the desired packing and particle size distribution. A wide variety of refractory aggregates is available, and castables can be formulated based on one or a combination of aggregates to achieve the desired chemistry, mineralogy, and physical properties.

In order to ‘fill out’ the particle sizing and to impart other desired attributes such as expansion control, chemistry/mineralogy modification, bond enhancement, etc., refractory fillers and/or modifiers are added to the castable composition. In many cases multiple fillers are used and in various particle size distributions. Fillers and modifiers can be finer sized fractions of the same minerals used as aggregates or other minerals chosen for compositional enhancement. There are some exotic modifiers which are routinely used in refractory castables for specific applications or operating conditions such as slag resistance, and thermal shock resistance etc.

The types of bonding agents used in refractory castables have increased in number over the years though, in alumino-silicate castables, calcium aluminate cement is still the principal bonding agent. Specialized castables have been developed in the past 20 years using non cement bonds such as hydratable alumina, clay, silica and alumina gels, and chemical bonds such as mono aluminum phosphate, phosphoric acid, and alkali silicates. Basic castables rely on chemical or organic bonds, the most common of which are alkali silicates, sodium phosphates, mineral or organic acids, and resins.

Different types of additives or admixtures are also used to modify the flow/rheology characteristics of the castable, control setting behaviour (retarding or accelerating), reduce casting water,  stabilize and control the pH, or stabilize storage behaviour. In many cases, multiple additives are used in a single formulation. Admixtures are used in very small amounts (up to 1 %), and may possibly change in function by varying the quantity used. The proper use and control of admixture combinations is an essential aspect of advanced castable mix design.

The aggregates and fillers/modifiers used in a refractory castable formulation are sized and proportioned to achieve a desired packing and particle size distribution. The packing and sizing of the components of the refractory castable affects the rheology of the castable mix during installation and the ultimate density and strength of the placed castable when in service.

There are two primary approaches in aggregate sizing and particle packing used in refractory castables. These are called as random and ordered systems. In the random system, the aggregates are sized in broad mesh distributions and blended with the fillers/modifiers to obtain an ‘adequate’ particle size distribution yielding acceptable rheology/flow on mixing and good physical properties. Normally conventional dense and lightweight castables are formulated on this approach.

Ordered systems are employed primarily in advanced castables to reduce water demand, impart desired rheological characteristics (such as vibratory/thixotropic flow or free flow/self leveling behaviour), minimize porosity, and maximize particle contact for enhanced bonding and optimum strength development and fracture resistance. In ordered systems, there are two distinct packing methods namely (i)gap sizing, and (ii) continuous sizing.

Gap sizing relies on the mixing of two, three, four, or more tightly graded aggregate mixtures to achieve good packing density. The main disadvantage of this system is poor flow, and castables based on a gap sized aggregate distribution require intense vibration for consolidation and placement.

In continuous particle sized systems, the aggregate fractions are added in a relatively large number (or as a packaged blend) of closely sized screened fractions. Continuous packing distributions have the advantage of very good rheologies at relatively low water contents, and good compaction with low shrinkage and high strength.

In recent years, various continuous packing theories and approaches have been advanced resulting in castables with extremely fluid behavior. These free flow or self leveling castables have their rheology and flow improved, mainly by paying more attention to continuous packing in the sub sieve and submicron particle fractions.

Refractory castables must set or harden at room temperature. Either water or a catalyzing agent must activate the bonds used in castables. After setting, a controlled heat up procedure is normally needed to dewater or dehydrate the bond as the refractory castable lined vessel or furnace is put into service.

The majority of alumina and alumino-silicate castables produced have historically been bonded with calcium aluminate cement. In the different grades of commercially available calcium aluminate cement, there are many mineralogical phases present and in various percentages. However, the principal and most important hydrating phase, common to all of them, is calcium mono-aluminate (CaO.Al2O3 or CA). Upon the addition of water in optimum ambient conditions, CA hydrates per the following equation: CA + H = C3AH6 + AH3 + heat, where C  is CaO, A  is Al2O3, and H is H2O.

While calcium aluminate cement is still the dominant bond used, some alumina and alumino-silicate castables systems and basic castables utilize other bonding agents. In recent years, many cement less castables have been developed around a variety of bond systems. One such bond is a hydratable alumina binder. This binder is a hydraulically setting, reactive, transitional alumina and is finely ground for maximum reactivity and sometimes modified with an organic polymer to provide added low temperature strength.

A colloidal silica solution is sometimes used to bond the castable. Hardening is achieved by the use of a small amount of a setting agent ( less than 1%) such as magnesia, lime, or calcium aluminate cement.

Gel bonding is another method of cement less bonding. In this case, setting and low temperature bonding occurs because of a polymerization of organic monomers suspended in the casting liquid.

Polymerization takes place either by a heat setting or by a chemical catalyst. Strength development in gel cast systems is dependent on temperature treatment and sintering rather than the initial bond. This results in a weak castable after polymer burnout ( around 250 deg C) until the ceramic bonding processes begin.

There are other bonding agents used in castables which are known as chemical binders. A system that has seen an increasing use in recent years utilizes phosphoric acid or mono aluminum phosphate (MAP). Setting occurs by the use of an additive (in the dry portion of the castable) that reacts exothermically with the acid. Two commonly used setting additives that react with the phosphoric acid or MAP are powdered MgO (or MgO aggregate in basic castables), and calcium aluminate cement.

Similar to the use of phosphoric acid for bonding, is the use of alkali phosphates (sodium phosphate) or alkaline phosphates (magnesium phosphate and calcium phosphate). These phosphates, especially sodium phosphate, find extensive use in bonding basic refractory castables since they can react with magnesite or dolomite aggregate and fines and stiffen the castable by coagulation.

Another bond used in basic castables and for producing acid resistant alumino-silicate castables involves the use of alkali silicates, either sodium silicate or potassium silicate. Alkali silicates react with acids, salts, and metal hydroxides and stiffen or set by formation of a silica hydro gel. This gel dewaters continuously as temperature increases with complete dehydration at 350 deg C. Setting agents used to set alkali silicate bonded castables include sodium silico fluoride, aluminum poly chloride, sodium phosphate, aluminum polyphosphate, magnesium polyphosphate, and calcium and magnesium hydroxides.

Historically, refractory castables have been mixed and placed in small quantities (less than 225 kg), by mixing manually in a mortar box or mechanically in paddle or drum mixers. Flow and placement consistencies are similar to that of a ‘stiff’ mix portland cement concrete. Many conventional refractory castables are still placed this way. With the improvement in refractory castable technology, specifically starting with low-cement castable development, other placement methods and techniques have come into use. Early low cement systems were either gap sized, requiring extensive vibration during placement (vibration cast), or ‘sticky’ thixotropic materials, which use vibration assistance for flow and consolidation. Mixing of these early advanced castables are still done in small quantities.

Castable placement technology improved with the large-scale application of low and ultralow cement castables. Second generation advanced castables has improved flow with most of the mix ‘stickiness’ removed. Mixer and batch placement size are increased with paddle type and turbine type mixers being introduced that are capable of correctly mixing up to 1350 kg castable.

The need for faster placement of larger batches has resulted into the development of two technological approaches and solutions. One is the equipment approach resulting in the successful development of specially designed refractory pumps that are able to place up to 10 cum/hr of advanced castable at distances of more than 100 m. The other is a refractory castable approach resulting in free flow or self flowing refractory castables that can be used with readily available concrete pumps. Free flow castable systems are available today in both conventional and advanced types and are widely used. Pumping technology also exists, allowing for the placement of castables with extremely stiff flow consistencies.

In the 1990s, another placement method was developed to speed up placement of castables and eliminate expensive forming. It is called refractory shotcreting or shotcasting. It combines many of the benefits of a correctly mixed castables with the speed of gunning placement. In this method, a large batch of castable is mixed with the correct amount of water and charged into a pump. The castable is pumped similar to the method used in pump casting applications. However, at the end of the hose, a special nozzle assembly is employed. In this assembly, the castable is mixed with compressed air (at around 0.4–0.5 MPa) and a small amount of an activator or accelerator that begins a stiffening of the castable. The mixture is the pneumatically shot onto the installation surface similar to the dry refractory gunning process. Because of the activator, the castable mix begins to stiffen and allows the surface to be built up without forms. Overhead applications are even feasible with the proper anchoring and support systems.

Since the refractory castables are a large and diverse group of materials, their physical properties and characteristics vary between castable types and application purpose, i.e., insulating, conventional dense, low-cement, etc., and because of mineralogical and formulation differences among castables within a group. Formulation and composition differences (raw material sources) can produce significant variance in physical properties in seemingly identical materials (similar densities and reported chemistry).

The primary purpose of insulating refractory castables is to provide thermal insulation either as a backup lining or as a primary lining in non severe applications. As such, insulating castables are formulated for low relative thermal conductivity and not for strength or abrasion resistance. The vast majority of insulating castables are alumino-silicate based though high alumina, high purity insulating castables are produced for applications in very high temperature environments (up to 1800 deg C) or in combustion atmospheres containing high concentrations of hydrogen.

 The category of refractory castables classified as dense castables represents a large and diverse group consisting of conventional type castables and the various categories of advanced products. The differences in the types of advanced castable are important to understand since dense refractory castables are used as structural components in furnaces and kilns, as primary linings in vessels containing liquid metal and slag, in corrosive atmospheres, and in furnace areas experiencing physical abuse and abrasive conditions. Based on the intended application, different physical characteristics come into play when choosing the appropriate castable material.

Density of dense castables increases as the alumina content of the base aggregates increases, since higher alumina raw materials are denser. Particle sizing and packing also influence density with castables based on ordered sizing being denser than materials based on random sizing. Porosity, on the other hand, does not depend as much on aggregate as it does on sizing, with gap sized systems having the lowest porosity and random systems having the higher porosities.

A property that is related to porosity and that very much depends on particle sizing and packing is the permeability of the castable. Permeability is the capacity of a refractory for transmitting a fluid or gas. This property is important since the ability to safely dehydrate or bake out a refractory castable depends on its permeability. Permeability can also play a role in gaseous corrosion reactions.

Dense conventional castables are much stronger than insulating castables with strengths generally equaling those of normal firebrick of the same class. In dense conventional castables, hot strength behaviour with increasing temperature is principally dependant on the cement type (low, medium-purity, or high purity calcium aluminate cement), and the amount used in the formulation. With the exception of high purity, high alumina conventional castable formulations, the hot MOR (modulus of rupture) of dense castables begins to decrease above 1100 deg C.

Low cement, ultralow cement, and cement free castables derive their enhanced strength from a combination of improved particle size distribution, reduced water demand, improved density, optimum cement/bond hydration, and the use of micro sized additions, most notably silica fume and reactive alumina. Silica fume is used in the vast majority of these materials produced, contributing to particle packing, rheology, and, most importantly, to strength development. While silica fume causes a significant strength increase especially in the sub 1300 deg C temperature range, it does react with the calcium aluminate cement in low and ultralow cement castables and contribute to anorthite formation at elevated temperatures, similar to what occurs in conventional alumino-silicate castables.

Normally alumino-silicate based low cement castables have the best strength development below 1100 deg C, while ultralow cement castables and cement free castables develop and retain relatively higher strength at elevated temperatures.

The fracture resistance of refractory castables is one of the important property to be considered in the refractory selection process but it is usually the non reported property due to lack of available test equipment and a standard test method. It is a fact that a refractory lining develops cracks during service, so it is important to know how well the refractory will resist crack growth propagation. Work of fracture (WOF) is a measure of the energy needed to propagate a crack through a material. This property is influenced very strongly by aggregate type, particle size distribution, and bond phase development. In many cases a relatively weaker castable or refractory as evaluated by modulus of rupture may be a more crack resistant material and a better choice for an application. Subtle changes to a formulation or size distribution may not affect MOR as much as WOF, especially in advanced castable systems.

For dense conventional castables, the hot WOF trend is easy predicted, as cement and aggregate type have little effect on work of fracture. WOF for most alumino-silicate castables, regardless of the cement  used, increases to a maximum at 1100 deg C and then decreases rapidly. The higher strength, high-purity conventional castables have slightly higher WOF values though not in proportion to their hot strength.

One can readily see the advantages of the advanced castable system over dense conventional castables and brick by comparing hot WOF values. The low cement castables have the best fracture resistance below 1100 deg C, and the ultralow cement and cement free products yield better values at higher temperatures. The main advantage of these castables for the user is the fact that they are much tougher than other comparable refractories such as brick, plastic refractories, or conventional castables. The resistance to crack propagation also plays an important role in resistance to thermal shock and impact.

Many applications where refractory castables are used require that the castable be resistant to abrasion or erosion. Most of these applications are in the moderate to low temperature range (less than 1100 deg C). Dense conventional castables are average abrasion resistant materials. For abrasion resistant applications, advanced castable systems such as low cement, ultralow cement, high cement, low moisture, and chemical bonded castables are used. Abrasion loss also varies with prefiring temperature and can reflect strength (MOR) variances. It should be noted that the higher alumina compositions in each class generally, but not always, have the lowest measured abrasion losses.

The thermal conductivity trend of dense alumino-silicate and high alumina refractory castables is similar to that of refractory brick and is easy to predict (as alumina content and density increase, so does thermal conductivity).

As with insulating castables, very high-alumina castables have a much higher thermal conductivity due to the higher amount of crystalline alumina. Thermal conductivity for many advanced castables is significantly higher than their comparable conventional dense castable counterparts.  This is partly due to their increased density and improved particle packing, but primarily because of ultrafine powder additions, most notably silica fume. The ultrafine powders help reduce casting water and porosity, improve matrix continuity, and increase the crystalline content of the matrix. This increase in thermal conductivity (20 % to 30 %) caused by a silica fume addition is visible in both low cement, and high cement low moisture castables as compared to a conventional dense castable.

For the past 30 years, refractory technologists have significantly improved refractory castable formulations, widening both the types of castables and the installation methods available. The improvement in their physical properties and characteristics have enabled castables to gain in application at the expense of other types of refractories.