The chemical composition of refractory materials is one of the most basic characteristics of refractory materials. Generally, the chemical composition of refractory materials is divided into two parts according to the content of the components and their functions:
- (1) The basic components that account for the absolute majority and play a decisive role in their performance – the main components.
- (2) The subordinate components that account for a small amount – the secondary components. The secondary components are impurities accompanying the raw materials or additives (additives) specially added during the production process to achieve a certain purpose.
Main Component
The main component is the component that constitutes the refractory matrix in the refractory material and is the basis of the characteristics of the refractory material. Its nature and quantity play a decisive role in the properties of the material. The main component can be an oxide or a non-oxide. Therefore, the refractory material can be composed of refractory oxides, or a refractory oxide and carbon or other non-oxides, or it can be composed entirely of refractory non-oxides. Oxide refractory materials can be divided into three categories according to the chemical properties of their main component oxides: acidic, neutral and alkaline.
- (1) Acidic refractory materials. This type of material contains a considerable amount of free SiO₂. The most acidic refractory material is siliceous refractory material, which is composed of almost 94% to 97% free SiO₂. Clayey refractory materials have a relatively low content of free SiO₂ and are weakly acidic. Semi-siliceous refractory materials are in between.
- (2) Neutral refractory materials. High-alumina refractories (with a mass fraction of Al2O3 above 45%) are acidic and tend to be neutral, while chromium refractories are alkaline and tend to be neutral.
- (3) Alkaline refractories contain a considerable amount of MgO and CaO. Magnesia and dolomite refractories are strongly alkaline, while chromium-magnesium and forsterite refractories and spinel refractories are weakly alkaline.
This classification is of great significance for understanding the chemical properties of refractories and judging the chemical reactions between refractories and between refractories and contact materials during use.
Impurities
The raw materials for refractory materials are mostly natural minerals, so they often contain a certain amount of impurities. These impurities can reduce certain properties of the refractory. For example, the main component of magnesia refractories is MgO, while other oxides such as silicon oxide and iron oxide are impurities. The higher the impurity content, the greater the amount of liquid phase formed at high temperatures.
Impurities in refractory materials directly affect the material’s high-temperature properties, such as refractoriness, load deflection temperature, corrosion resistance, and high-temperature strength. On the other hand, impurities can lower the firing temperature of the product, promoting sintering.
At high temperatures, andalusite transforms into mullite and a free SiO2 glass phase. In the Al2O3-SiO2 system, mullite is chemically stable, so refractories containing andalusite are also chemically stable.
Additives
In the production or use of refractory materials, especially amorphous refractory materials, a small amount of additives is added to improve the physical properties, molding or construction performance (operation performance) and use performance of refractory materials. The amount of additives added varies with their properties and functions, ranging from a few ten-thousandths to a few percent of the total amount of refractory materials.
Additives are divided into the following categories according to their purpose and function:
- (1) Changing rheological properties: including water reducers (dispersants), plasticizers, gelling agents, degumming agents, etc.
- (2) Adjusting the setting and hardening speed: including accelerators, retarders, etc.
- (3) Adjusting the internal structure: including foaming agents (air entraining agents), defoamers, shrinkage inhibitors, expansion agents, etc.
- (4) Maintaining the construction performance of materials: including inhibitors (anti-swelling agents), preservatives, antifreeze agents, etc.
- (5) Improving the use performance: including sintering aids, mineralizers, quick-drying agents, stabilizers, etc.
These added components, except those that can be burned off, remain in the material’s chemical composition.
Chemical composition analysis allows the purity and properties of a product or raw material to be determined based on the types and quantities of the components present. Phase diagrams can also be used to roughly estimate the product’s mineralogical composition and other relevant properties.
