Potentially any aggregate may be used in a monolithic refractory formulation. They are selected based upon their stability at the temperature of application, corrosion resistance and mechanical strength.
Temperature Limits

As a guide, Table 1 shows the temperature limits of operation for castables with various aggregates.

Table 1. Upper limits of service temperatures for various aggregates (CAC=Calcium aluminate cement).

Cement type


% Al2O3


Aggregate


Approx. temperature limit (°C)

Heat resistant concretes

Grey CAC


40


Granite/basalt


700-800

Grey CAC


40


Emery


1000

Grey CAC


40


AlagTM


1100

Brown CAC


50


Olivine


1200

Dense refractory concrete

Grey CAC


40


Chamotte


1300

Brown CAC


50-55


Molochite


1400

White CAC


70


Molochite


1450

Grey CAC


40


Sillimanite or gibbsite


1350

Brown CAC


50-55


“ “


1450

White CAC


70


“ “


1550

Grey CAC


40


Brown fused alumina


1400

Brown CAC


50-55


“ “


1550

White CAC


70


“ “


1650

White CAC


80


“ “


1750

White CAC


70


White fused alumina


1800

White CAC


80


“ “


1850

White CAC


70


Tabular alumina


1800

White CAC


80


“ “


1900

Thermally insulating concretes







Grey CAC


40


Pumice, diatomite


900

Grey CAC


40


Vermiculite, perlite


1000

Grey CAC


40


LytagTM, LecaTM


1100

Brown CAC


50


Expanded chamotte


1300

White CAC


70


Bubble alumina


1700

White CAC


80


Bubble alumina


1800

Table 2. Pyrometric cone equivalent of calcium aluminate cement.

Types of CAC


Al2O3/CaO


PCE (°C)

Grey CAC


1.15


1270-1290

Brown CAC


1.40


1430-1450

White CAC


2.50


1590-1620

White CAC


4.70


1770-1810
Refractory Aggregates
Bauxite

Bauxite is an ore which principally consists of either Boehmite (a monohydrate of alumina, Al2O3.H2O) or Gibbsite (alumina trihydrate, Al2O3.3H2O.). Raw bauxite may contain other impurities such as titania, silica and ferrous oxide. Refractory grade bauxite has a high alumina and low iron content. Most of the material mined in Europe has a higher proportion of Boehmite whereas bauxites mined in Asia and South America have high proportions of Gibbsite. The Bayer is used to process bauxite to produce higher grades of alumina (which can be used in higher temperature applications).

Bauxite used in refractory applications is generally calcined in a rotary kiln producing a material mainly consisting of corundum (alpha-Al2O3), mullite (3Al2O3.2SiO2) and a small amount of a glassy phase.
Calcined and Sintered Alumina

Calcined aluminas are produced from bauxite processed via the Bayer process. The resulting material has very low levels of impurities. Calcined alumina is made from heating bayerite (Al(OH)3) in a rotary kiln. Calcined aluminas are stable to very high temperatures.

Sintered alumina is manufactured by sintering calcined alumina at 1,800°C in a rotary kiln. Subsequently to this it is crushed and classified according to grain size.
Fused Alumina

Fusing aluminous raw materials electrically in an electric arc furnace can produce fused alumina. The fused product is then cooled into ingots, crushed and then classified. There are two types of fused alumina - brown and white. Brown fused alumina is manufactured from bauxite. Its impurities are reduced to precipitate as Fe-Si-Ti system iron alloys, but some of the titanium forms a solid solution with alumina. This causes the brown colour of the material. Fused alumina has a very high degree of toughness and is frequently used as a grinding material. White fused alumina is formed from calcined alumina. The only impurity in fused alumina is sodium, which is present in the form of alpha-alumina. Fused alumina, with its perfect crystallisation is difficult to sinter. It is inactive and does not readily react with other raw materials. Fused aluminas tend to be used in refractories which are exposed to very severe conditions.
Fused Bubble Alumina

Fused bubble alumina is used in high temperature insulating monolithic refractories. Bubble alumina is made by blowing a stream of high-pressure air into molten alumina. This forms bubbles in the material. The incomplete bubbles are then separated.
Spinel

Spinels which are used in refractories are generally synthesised from bivalent and trivalent oxides, mixed at equal mole ratios forming materials of the general formula XY2O4. A typical refractory spinel product is the double oxide of magnesia and alumina (MgAl2O4). Further examples of spinels are provided in table 3.

Table 3. Examples of spinels

Aluminate


Chromite


Ferrite

MgO.Al2O3


ZnO.Cr2O3


ZnO.Fe2O3

FeO.Al2O3


MgO.Cr2O3


MgO.Fe2O3

MnO.Al2O3


FeO.Cr2O3


FeO.Fe2O3

ZnO.Al2O3


MnO.Cr2O3


MnO.Fe2O3

NiO.Al2O3


NiO.Cr2O3


NiO.Fe2O3

Both spinel (Mg.Al2O3) and dichromite (MgO.Cr2O3) are used in refractory castable formulations. Their respective melting points are 2135°C and 2350°C. Spinel is more neutral at high temperature than alumina and its corrosion resistance against basic slags is high. Against Fe2O3 the Al3+ may be substituted by Fe3+ causing corrosion. Spinel’s thermal conductivity and coefficient of thermal expansion is smaller than that of magnesia and it has good resistance to spalling.
Magnesia

Magnesia clinker is basic, and it has high corrosion resistance to basic slags. It has a high thermal conductivity and a large coefficient of thermal expansion. Calcining magnesite (MgCO3) in a rotary kiln produces magnesia clinker. Iron oxide forms a solid solution with periclase (MgO) which acts as a sintering aid. Another magnesia raw material is fused magnesia, which is obtained by electrically fusing seawater magnesia clinker. Magnesia has a tendency to react with water to form magnesium hydroxide (Mg(OH)2), therefore magnesia based castables must be stored carefully and the contact with steam avoided.
Dolomite

Dolomite is a double salt of CaCO3 and MgCO3. Mixtures within the CaO.MgO system do not melt below 2300°C and are highly refractory. Dolomite refractories are used in steel making. Dolomite shows excellent corrosion resistance to basic slags.
Silicon Carbide

Silicon carbide is made by heating silica sand and petroleum coke packed around electrodes in an electric resistance furnace to above 2200°C. This material is very resistant to abrasion and to corrosion with a molten slag. It also has excellent resistance to thermal spalling. However as it is a carbide, it will oxidise readily. With respect to other refractory aggregates silicon carbide has a fairly high conductivity.
Chamotte

Is made from clay, which is sintered, to the point of no further shrinkage. Typically to make a chamotte, raw clay is extruded then fired in either a tunnel or rotary kiln. The main phase in chamotte at high temperatures is mullite. Some chamottes are also made from shale clays, which are calcined in either a shaft or rotary kiln.
Vermiculite

Vermiculite is clay mineral not too dissimilar to mica. It has a three layered structure with a MgO.6H2O layer between sheets. When vermiculite is heated to a temperature of around 350°C it begins to shrink. At above 400°C the combined water in the material is released and it exfoliates. In this process vermiculite will swell to 10-20 times its original volume. Exfoliated vermiculite is fairly poor in strength but it has a very low thermal conductivity making it an excellent insulating material. Vermiculite is commonly used in insulating castables.