Economics of gunned plastic refractories gaining interest.

Recent developments in gunning equipment are widening applications, particularly among smaller installations

Gunned plastic refractories, introduced in Europe in 1980, promise to make their economic benefits and new application technologies available to service a broader range of foundry furnaces.

Often criticized in the past as costly to use and cumbersome to apply, gunned plastic refractory installations were noted chiefly for excessive material rebound losses and premature lining failures. These concerns and the lack of industry standards made many potential users uses cautious even when benefits were obvious.

Though there are many instances of successful refractory applications, few numbers exist. Estimates put worldwide installation of gunnable plastics at over 100,000 tons and growing as a percentage of refractory output. Recent equipment developments are extending applications, especially for smaller installations previously excluded due to high costs for low volume gunning.

Resistance remains, however, mainly because of the lack of a better understanding of current application techniques.

Early monolithic refractory industrial furnaces were constructed of refractory brick and mortar that served both heat-containment and structural purposes, but there were two main problems:

* installation was slow, labor intensive and costly;

* thermal expansion/contraction made brick and mortar joints vulnerable to cracking or spalling.

The use of structural steel for furnace support made nonstructural refractory linings possible. The material, originally termed pliable or "plastic" firebrick, was applied in semibaked, putty-like chunks. It was hammered into a monolithic mass around anchors affixed to the steel structure or applied against a structural brick wall built with a pattern of recessed pockets to key and hold the plastic in place. Trimmed to a uniform surface and baked out with a slow fire, the material produced a unified lining with no joints.

Another monolithic alternative, castable refractory concretes, were mixed on-site, poured into mold forms and allowed to cure before heating.

In either form, monolithic refractories were considered specialties and, until World War II, represented no more than 2-3% of the refractory market. In the last 30 years, however, monolithic refractories steadily improved.

New aggregates and binders, alternatives in raw material grain sizing and innovative uses of additives and admixtures have enhanced their performance. This growing diversity of monolithic materials tailored to specific applications has extended even to refractory repair. Modern monolithic refractories have become materials of choice for many applications today.

Through this period of growth, plastics had the advantages of broader bonding alternatives, better thermal shock and corrosion-resistance properties and less stringent bakeout requirements.

Castables held the edge in mechanical strength and installation speed. Even though carpenters were required to build (and remove) casting forms and the cast materials needed a 24-hour curing period before a bakeout sequence could begin, the process was generally quicker than the manual hammering or ramming techniques required for plastic materials. Casting also eliminated the rammed material's variability caused by differing installer skills and the tedious ramming process.

Gunning Castables

Castable refractory appeal increased with the introduction of a gunning technique adapted from civil concrete applications that pneumatically blows a predampened castable mix in a continuous stream through a hose onto the application site, adding water as necessary at the nozzle.

Gunning made castable installation less labor-intensive by eliminating the need to build and remove forms, and it allowed multisurface jobs to be completed in a single, continuous operation rather than by casting separate sections. Anchors could be set in advance rather than as the work progressed and there was no need to set up and remove "whalers" (support planks) essential when ramming ceiling installation.

Gunning castable mixes have some problems. Because the castable material moves through the hose while relatively dry, a second hose is needed to deliver water to the nozzle. This requires the nozzle operator to determine how much moisture to add during application. Accordingly, consistency can vary widely with the skill and attention of the nozzle operator.

The short dwell time in the nozzle allows little time for the water to disperse throughout the material, sometimes resulting in laminations in the lining, material waste (rebound) and a potential for dusting.

Dusting can become a major problem requiring continuous air quality monitoring and mandatory use of cumbersome, self-contained breathing equipment that slows production considerably.

Gunning Plastic Refractories

Solutions to the problems intrinsic to gunning castables began to emerge in 1979, when a plastic refractory was first gun-applied in Germany to repair a blast furnace runner lining. Subsequent trials on soaking pits and reheat furnaces were successful in Japan in 1981, and in the U.S. and Canada in 1982.

Gunned plastic refractories demonstrated physical properties comparable to those of properly rammed plastics. Both air-bonded and alumina phosphate-bonded types are contrasted in Tables 1 and 2, respectively.

Further tests indicated that gunned plastic refractory had half the creep of conventional rammed materials and could be installed up to three times faster than ramming a comparably rated plastic. Dusting is eliminated because gunned plastics contain no dry components.

Rebound ranges from 15-25%, similar to most conventional gunned castable mixes. But unlike castables, gunned plastic rebound material can be recycled through the system, ultimately reducing rebound waste to about 5%.

This technique is steadily growing as a primary choice for installing monolithic linings because of the advantages of plastic refractories and the economic benefits of gunning.

Three Alternatives

The choice among monolithic refractories for a specific application, often dictated by personal preference, usually is determined by the service requirements, configurations and locations of the linings to be installed. Yet, in the growing number of cases where gunned plastics promise a fast, low-cost, efficient alternative, specifiers must decide among three different installation techniques and select a specific refractory composition.

Table 1. Air-Bonded Superduty Plastic Refractory
                               Gunned           Field-Rammed
Weight in service
(km/m3)                          2050                2019
Linear change (%)
815C                             -0.5                -0.7
10900C                           -0.6                -0.7
1340C                            +0.5                +0.2
Cold MOR (M Pa)
110C                              3.2                 2.3
8150C                             2.9                 1.9
1090C                             4.5                 3.5
1340C                             6.3                 5.2
Hot MOR (MPa)
815C                              4.3                 3.2
1340C                             1.1                 0.8
Table 2. 70% Alumina Phosphate-Bonded Plastic Refractory
                                Gunned         Field-Rammed
Weight in service
(kg/m3)                           2211                2242
Linear change (%)
815C                              -0.2                -0.4
1090C                             -0.3                -0.4
1340C                             +0.2                +0.1
Cold MOR (MPa)
110C                               4.0                 4.4
815C                               5.1                 5.5
1090C                              6.5                 7.8
1340C                             12.8                13.5
Hot MOR (MPa)
815C                               6.3                 6.2
1340C                              4.1                 4.3

Of the three techniques, only two are considered true plastic applications; the third is a low-cement, high-clay version of a gunned castable mix. The castable mix eliminates the need for forms and provides the logistic advantages of gunning, but falls short of the bonding and physical service properties of a true plastic. It still requires the water supply line for water addition at the nozzle and does not solve the problems of dusting, laminating, high rebound waste and water mix inconsistency at the gun.

These "pseudo" plastic gunning mixes contain up to 25-30% clay (vs. 5% or less in conventional castable mixes) and still rely on binder content to stiffen and hold the lining in place while wet. They have complex curing and bakeout requirements (similar to conventional castables) and require a day or two before heating can begin. Where users have reported disappointment with the performance of so-called gunned plastic refractories, most often one of these "pseudo" plastic mixes is the source of their discontent.

True Plastics Differ

Significant differences exist between the two true gunnable plastic refractories in both the material and the application process.

The original plastic gunning process is characterized by on-site shredding of "slugs" of a soft, pliable mixture of clay, aggregates and binders, with no cement content.

The mixture is produced with a controlled moisture content, extruded into slugs and wrapped in protective plastic to prevent evaporative moisture loss during transfer to the jobsite.

Just prior to application, the slugs are unwrapped and fed through a rotary shredder that converts them into a loose, granular consistency that passes directly into the gun feed system. Propelled by a high-volume compressor at 65-85 psi, the shredded material moves through a 2-in. diameter hose (up to 250 ft) and is blown onto the host surface for uniform buildup around prefired anchors, as shown in Fig. 3.

The second true plastic alternative uses no shredding equipment. The refractory material is granulated at the factory and mixed with proprietary organic additives to prevent the granules from clumping in transit. The material is propelled to the gun at lower velocity, typically 45-65 psi. Hose runs are limited to 50-100 ft.

Both methods require less downtime than rammed installations, allow easier refractory delivery into confined spaces typical of foundries and are far less labor-intensive.

The primary contrast between the two methods of plastic gunning (slugs and granular) is in the density of the linings they produce. The lower pressure used in the second method allows somewhat smaller compressors and eliminates the cost of an on-site shredder, but tests indicate that lower pressures tend to reduce material compaction.

This, and the aerating effect of the organic additives needed to keep the factory-granulated material loose, tend to result in a less dense lining.

In the original method, more consistent granular moisture and density resulting from on-site shredding and higher delivery pressures and velocities, give the gunned material more predictable adhesion and uniform compaction to maximum density. This adds mechanical strength and improves the lining's ability to withstand cracking, shrinking and deterioration.

The original method also uses a gunning system specially designed to accommodate plastic refractories.

The second method does the opposite, relying on reformulating the plastic refractory for use with a conventional gunning system much like those used to gun castables.

Extending Applications

The two methods noted above have restricted their resulting advantages to relatively high-volume refractory installations. Recently, however, "mini" systems are beginning to make true gunned plastic refractories accessible to smaller scale refractory users traditionally reliant on brick-and-mortar or rammed plastic linings.

The mini system emulates the original method with on-site shredding and high-pressure delivery. As a downsized system, however, it permits use of a much smaller compressor (350 cfm), commonly available for rent anywhere. Hose length is less than for larger systems, but its more compact support machinery can, in many cases, move closer to the application site.

Most important, its size/cost relationship will make gunning feasible in places where plastics have had to be rammed for economic reasons, usually in installations of fewer than 15 tons.

The mini systems will make gunning a competitive alternative for larger scale users such as foundries and where refractory repairs have traditionally required masons to hand-ram small quantities of plastic in selective locations.