Cement refractory blocks have for long been used by small and medium enterprises dealing with iron and steel -- direct reduction furnace, rotary kilns, reheating furnaces, etc. These refractory blocks also find extensive use in units in the aluminium, cement, ceramic, petrochem, fertiliser and glass sectors, as well as by engineering units that deal with heat treatment and annealing furnaces.

The use of low cement castables – otherwise known as monolithic refractories or refractory castables – could prove cost-effective and advantageous to these SMEs. Hydraulic calcium aluminate cement-based castable refractories offer many advantages over conventional refractory bricks. Technically, refractory castables are mixtures of calcium aluminate cement and suitably graded refractory aggregates. The cement in refractory castables reacts with water at room temperature to form a strong solid mass, which is used for quick lining and repairs of furnaces.

The main advantage for SMEs is that pre-fired shaped bricks need not be stocked. The other reasons why refractory castables could replace the conventional bricks are: 1. Readily available material 2. Quick and relatively cheap installation 3. Scarce and skilled brick layers are eliminated 4. Jointless construction leads to better resistance to chemical corrosion and abrasion 5. Good thermal shock resistance 6. Thermal conductivity is about half to one third of the bricks. Hence, thermal efficiency is better. 7. Savings in energy during manufacture

Improving technology: When a traditional monolithic refractory is heated, the free water and ultimately combined water is driven off so that the hydraulic bond, which is responsible for the high cold strength, is weakened. The loss in strength with rising temperature is associated with de-hydration of the cement hydrates and of the alumina gel. The inadequate performance of conventional high alumina cement castables, especially in contact with molten metal and slag, led to intensive research and development of superior products such as low cement, ultra low cement, and cement free castables/ ramming masses. These new generation monolithics have performed very well in the metallurgical industry, particularly in the iron and steel industry. In India, the manufacture of low cement castables started in 1986.

Low cement castables: The expense resulting from the reduction in strength of the hydraulic bond could prove crippling, especially to SMEs. It was found that reducing the water content in castables led to greater control over the reduction in strength. Low moisture content was achieved by lowering the percentage of bonding material (cement) to about 5-8 percent. This led to the development of low cement castables (LCC).

These castables contain a range of ingredients, all of which play an important role in determining the properties of the final product. The main constituents in LCC are cement, deflocculant and very finely grained (1-10 microns) refractory powders -- calcined alumina, amorphous silica, silicon carbide, chrome oxide or alumino silicate minerals. These ultra fine particles play three important roles that ultimately improve the installation and service performance:

• In the presence of appropriate dispersant, these particles fill voids between cement particles, which would otherwise be filled with water. This helps to reduce water content, decrease porosity and increase bulk density of the installed material.
• These particles affect castable fluidity and working/setting time.
• The nature of these particles determines the phase composition matrix at elevated temperature

• No reduction in strength at intermediate temperatures (300-12000C)
• Higher hot modulus of rupture
• Lower permanent linear change
• Low lime content
• Better thermal shock resistance
• Improved corrosion resistance.

The different types of LCC manufactured are LCC-45, LCC-60, LCC-70, LCC-80, and LCC-90, the numbers indicating the percentage of Al2O3. The specific raw materials used for LCC formulations from 45-90 percent Al2O3 content are:

• Calcined clay
• Sintered/fused bauxite
• Sintered/Tabular/Fused alumina
• Sillimanite sand
• Calcium aluminate cement
• Micro fine silica/Alumina
• Additives: water

The development of LCC has been further refined, as it was found that in LCCs made of alumino silicate aggregates, if the CaO content exceeded 0.5 percent in the matrix bond, the castables showed very poor high temperature strength at 15000C. This led to a further reduction in the use of cement, and the development of ultra low cement castables or ULCC. In the ULCC, the cement content is reduced to less than 4 percent and silica fumes increased to about 10 percent.. The ULCC has higher hot MOR values than LCC.

In an effort to improve upon these castables, it was found that the use of calcium aluminate binders (cement) can be reduced and even totally eliminated. This has resulted in the development of no cement castable (NCC). Apart from high strength, these castables exhibit higher abrasion and erosion resistance, and do not require any vibration during installation.

Installation of castables: Due to better understanding of the behaviour of castables, it has become possible to pre-cast and pre-fire huge blocks, which can be readily installed at the site. This development eliminates the difficulty of long curing and firing cycles and helps faster commissioning. This speed is especially valuable to SMEs, where too much time spent in installation and testing could mean loss of revenue. The use of pre-cast shapes started in the steel industries and later covered petrochemical, fertilisers and glass industries.

All castables, except NCC, contain calcium aluminate cement, which deteriorates with ageing or on storage. Proper storage of castables is important and necessary to prevent consolidation and caking during storage. Here are some of the points to be kept in mind while installing castables: · Surface preparation prior to refractory placement. · Suitability of the form work · Anchoring · Quality of water used for mixing, preferably cold. · Ambient temperature while casting · Mixing methods and equipment. · Construction and expansion joints. · Finishing of the lining surface. · Curing and firing.

End users: LCC can be used extensively in most industries that use furnaces. Obviously, it is used most in the iron and steel sectors in the following industries: · Direct reduction furnace, rotary kiln. · Steel ladle side wall and bottom Tundish safety lining · Rinsing and powder injection nozzles · Reheating furnaces - hearth and wall · Walking beam furnace hearth. · Pelletising kiln hood

Apart from iron and steel, there are other industries which use furnaces and which can use LCC. Some of these industries are: · Foundry - Siphon, taphole, cupola · Aluminium - Anode production, electrolyte kiln, holding furnace, transfer ladle · Cement - Nose ring, burner pipe, clinker outlet, grate cooler · Ceramic - Kiln car, furnace wall, burner block · Petrochemical - Secondary reformer, transfer lines, carbon black reactors, fluidised catalytic cracking units, CO-boilers, regenerators · Fertiliser - Primary reformers, secondary reformer, waste heat boiler, process gas transfer lines. · Glass - Bottom paving, crown of glass tank · Power - Boiler · Engineering - Heat treatment furnaces, annealing furnaces

Bottomline: Consumption of monolithics in advanced countries constitutes more than 50 per cent of all the refractories consumed, whereas in India it is about 25-30 percent only. With the emergence of user-friendly refractories such as LCC, ULCC, NCC and pumpables or free flowing castables, it is expected the gap will be filled in the years to come.