Refractory materials are fabricated in two forms: shaped and unshaped (monolithic) refractories. Shaped refractories include fired and unfired materials with predetermined shapes, precast shapes and fusion cast refractories. Monolithic materials include plastic mixes, castables, ramming materials, dry vibratable, gunning materials, fettling materials, coatings and mortars. Monolithic materials differ from refractory bricks in that they are not shaped and fired before use. These materials do not have high energy requirements, are more readily available, take shorter times to install, can be repaired locally and require less manpower.
The greatest user of refractory materials is the iron and steel industry (~70% of total use), while other significant consumers are the cement and lime, ceramic, glass, chemical, nonferrous and foundry industries. Consequently, any developments in iron and steel production technology have knock-on effects on the refractories industry. A significant trend in refractories technology is the increased use of monolithic materials, which accounts for more than half of the total production in many countries, due to their quicker and cheaper installation, and to properties that approach those of formed products. Monolithic materials have evolved over the years into a widely used class of refractory materials that offer performance and cost-effectiveness.
History of Monolithic Refractories
The need for monolithic, joint-free refractories arose from the problems associated with traditional brick-mortar refractory constructions. Significant developments in monolithic materials have been driven by more severe demands placed on them by consumers. Advances took place in the type and quality of the binders, aggregates and additives, and to innovations in their design and installation. Castables and gunning materials are the most abundantly used monolithic materials.
Monolithic materials first appeared in the US as distinct refractory materials around 1914. The first plastic refractories were mixtures of plastic clay and crushed grog or calcined clay, supplied in a wet, mouldable (putty-like) form, installed chunk-by-chunk by workers using hammers.
By the early 1930s pneumatic ramming devices were first recommended for the installation of plastics, the use of anchors for installing plastic refractory linings was in practice, and refractory concretes were applied using the ‘cement gun’. Although Sainte-Claire Deville from France is considered the discoverer of the cementing properties of calcium aluminates, it was only in 1918 that the Lafarge Company in France commercially offered a calcium aluminate cement, produced from bauxite and lime. Around 1928 the commercial production of bagged mixes of refractory aggregates and calcium aluminate cement for monolithic refractory construction was started.
By 1934 there were at least 25 of these proprietary mixes (called castables by the manufacturers) on the market. The first refractory concretes were simple mixtures of hydraulic cements and aggregates.
By 1940 monolithic refractories only constituted 2–3% of the total refractory market, with products in the form of castables, plastic refractories and mortars. During the period 1940–1960 major breakthroughs were made in binders, with the introduction of intermediate and high-purity calcium aluminate cements (with CaO.Al2O3 as the principal anhydrous aluminate phase), and the development of phosphate bonding. The war years of 1941–1945 stimulated increased use of monolithic refractories due to their rapid rate of installation and therefore construction, which often removed the need for complete rebuilds. Graphitic plastics were first introduced in 1941. The first commercial phosphate-bonded plastic was introduced in the early 1950s, as were hot gunning repair.
By 1960 castables based on high-purity calcium aluminate cement and tabular alumina aggregates were fairly common, ramming mixes with Al2O3 contents exceeding 90% and useful at temperatures up to 1871°C were appearing, chrome ore-periclase ramming mixes were developed for the copper industry, graphitic and high alumina plastics and ramming mixtures (air-setting and phosphate bonded) were commonly available.
By 1960 monolithic materials constituted ~30% of the value of refractories in the US, and were competing with prefired refractories.
The 1970s also saw the concept of steel fibre reinforcement for monolithic materials being introduced, and the development of vibratable plastic refractories.
In the late 1970s low-cement castables were introduced in the US, and the gunning of conventional plastic refractories in the early 1980s.
A major trend in monolithic material development from 1960–1984 was product specialization through innovative changes in raw material grain sizing, reduced binder contents through innovative uses of additives, and improved mixing and installation procedures. From 1970–1980 monolithic materials, and mainly castables, proved their real value.
By the early 1980s there were mouldings, ramming mixes, castables and precast shapes, gunning mixes, and newly developed vibration materials. The introduction of low-cement castables and cement-free castables were some of the most impressive developments during those years.
Monolithic refractories have developed over the years to become what is being used today. They provide a very important contribution to the iron and steel industry and everyday new technologies are developing to overcome production challenges. We will keep an eye on new developments and will keep you informed!