What Types of Refractory Aggregates Are There?

In the field of refractory materials, refractory aggregate is a crucial component. Not only does it possess excellent refractory properties, but it also plays a key role in various high-temperature industrial environments. This article delves into the definition, classification, and applications of refractory aggregate in different fields.

PART 01

Refractory Aggregate

As a core component in the field of refractory materials, refractory aggregate plays a key role in providing structural support. These aggregates are made from various refractory raw materials, such as high alumina bauxite, through stringent high-temperature calcination, fine crushing processes, or meticulous artificial synthesis techniques. Their particle size is greater than 0.088mm, exhibiting granular material characteristics.

In monolithic refractories, refractory aggregates hold a significant position, typically comprising 60%-75% of the total material composition. Consequently, the names of monolithic refractories often reflect the type of aggregate used. For instance, if high alumina bauxite clinker is used as the aggregate, the resulting refractory castable or ramming mix is correspondingly named high alumina castable or high alumina ramming mix. This naming convention highlights the decisive role of refractory aggregates in determining material properties.

The specialized manufacturing process of refractory aggregates and their substantial proportion in monolithic refractories underscore their outstanding performance in refractory properties. Whether protecting high-temperature industrial environments or producing various refractory products, refractory aggregates are indispensable, ensuring the stability and durability of the entire refractory material system.

PART 02

Classification of Refractory Aggregates

Refractory aggregates, as core materials in high-temperature industries, have a professional and detailed classification system. First, from a material perspective, refractory aggregates include various types such as clay-based, high alumina, corundum, silica, magnesia, and magnesia-alumina spinel. Each type of material imparts unique high-temperature stability and chemical corrosion resistance to the aggregate, meeting the demands of different industrial environments.

Further classification by porosity divides refractory aggregates into dense aggregates and lightweight aggregates. Dense aggregates, with a porosity of no more than 30%, are known for their high density and high strength. These aggregates can be further subdivided into ultra-dense aggregates, high-density aggregates, and regular dense aggregates, each excelling in refractoriness, thermal stability, and mechanical strength.

In contrast, lightweight aggregates have a porosity exceeding 45%, characterized by low density and excellent insulation properties. Lightweight aggregates can also be subdivided into regular lightweight aggregates, ultra-lightweight aggregates, and special lightweight aggregates, such as alumina and zirconia hollow spheres. These lightweight aggregates not only exhibit excellent refractory properties in high-temperature environments but also effectively reduce thermal conductivity and improve energy efficiency.

PART 03

Particle Size of Refractory Aggregates

The particle size of refractory aggregates is a crucial aspect of refractory material preparation, directly affecting the material's structural performance and high-temperature stability. In the formulation of monolithic refractories, aggregates are precisely classified into coarse, medium, and fine particles based on their particle size.

This classification is not arbitrary but determined by the critical particle size, or maximum particle size. For instance, for aggregates with a maximum particle size of 8mm, coarse aggregates range from 8-3mm, medium aggregates from 3-1mm, and fine aggregates from 1-0.088mm. Notably, particles smaller than 0.088mm are no longer considered aggregates but are defined as fines or matrix materials, playing a filling and bonding role in refractory materials.

An ideal particle size distribution is key to achieving the densest packing. In this ideal state, the voids left by the contact of coarse aggregates are precisely filled by medium aggregates, and the voids left by medium aggregates are further filled by fine aggregates. This sequential filling method forms a solid aggregate skeleton, with the remaining voids filled by fines.

However, due to the complex and irregular shapes of refractory aggregate particles, achieving this ideal particle size distribution is challenging in actual production. Therefore, strict testing is usually required to determine the optimal aggregate size distribution. In practice, the distribution ratio of coarse, medium, and fine aggregates generally maintains a range of (35-45):(30-40):(15-25). This ratio ensures the stability of the aggregate skeleton while maximizing the density and high-temperature performance of the refractory materials.

PART 04

Shape of Refractory Aggregates

The shape of refractory aggregates is a complex and diverse research area. Most refractory aggregate particles are composed of multiphase polycrystalline materials, with their shape influenced by multiple factors. Firstly, the crystal structure, crystallization habits, and impurity content of each phase in the material affect the particle shape. These intrinsic factors, like the aggregate's "genes," determine its basic shape characteristics.

Furthermore, different processing methods significantly impact the shape of refractory aggregates. For instance, mullite produced by the electrofusion method typically forms columnar polycrystalline aggregates due to its crystallization habits. During crushing, these columnar polycrystalline aggregates tend to fracture along the weaker grain boundaries, producing many columnar polycrystalline particles.

In contrast, mullite produced by the sintering method may form needle-like, columnar, plate-like, or granular shapes due to environmental constraints during crystal growth. These diverse crystal shapes intertwine during growth, resulting in highly irregular particle shapes upon crushing, such as flake, needle-columnar, or spindle shapes.

Additionally, the final shape of refractory aggregate particles is closely related to the material's density and crushing method. For example, high alumina bauxite clinker with ultra-dense and high-density properties tends to produce flake or spindle-shaped particles when subjected to impact or extrusion crushing methods. In contrast, grinding crushing methods may result in irregular granular or near-spherical particles.

Therefore, selecting the appropriate crushing method is crucial to obtaining suitable aggregate particle shapes for monolithic refractory materials. This process requires comprehensive consideration of factors such as crystal structure, crystallization habits, impurity content, and processing methods to ensure the final product's performance and quality.

PART 05

Applications of Refractory Aggregates

The application of refractory aggregates is a complex field involving multiple factors, with particle shape having a particularly significant impact on construction performance. Different shapes of aggregate particles exhibit various characteristics when formulating monolithic refractory materials.

Specifically, particles with irregular shapes, such as flake, columnar, needle-columnar, and angular, exhibit relatively poor rheological properties when mixed into slurries. This is because these shapes do not allow for smooth sliding within the slurry, affecting the overall rheological properties. However, these irregular shapes offer unique advantages in certain applications. For instance, when formulating spray coatings and ramming materials, these irregular particles can interlock and form a pinning effect, enhancing bonding strength.

Conversely, near-spherical and spherical particles exhibit better rheological properties when mixed into slurries. These shapes allow for smoother sliding within the slurry, improving rheology and thixotropy, thus enhancing bulk density. Therefore, these rounded particles are ideal for formulating castables, coatings, and press-formed materials.

PART 06

As an important material in the high-temperature industrial field, refractory aggregates continually expand their performance and application range. With advancements in technology and industrial development, refractory aggregates will play an increasingly important role in future high-temperature industries. This article aims to help readers better understand the classification and applications of refractory aggregates, providing a reference for research and practice in related field.