What are the effects of refractory aggregates on castables?

Refractory aggregate is the core component of refractory castables, and its characteristics directly affect the mechanical, thermal, and construction properties of the castables. The following are the various effects and mechanisms of refractory aggregates on the performance of castables:

1、 Skeleton action and mechanical properties

Strength and Wear Resistance

Aggregates account for 60% -70% of the total mass of the casting material, and their high hardness and strength (such as corundum aggregates) provide skeleton support for the material, significantly improving its compressive strength and wear resistance.

Particle shape influence: Irregular or triangular particles have stronger mechanical bite force and better wear resistance than spherical particles; But spherical particles have better fluidity.

Thermal shock resistance

Coarse aggregates (such as 20-25mm corundum) can change the crack propagation path, suppress the propagation of long cracks through crack displacement and bifurcation, and improve the anti peeling performance. However, excessive coarse aggregate (>7%) can actually reduce performance due to stress concentration.

Micro porous lightweight aggregate (pore size<10 μ m) reduces thermal stress concentration and high temperature shrinkage rate by uniformly distributing pores (such as a shrinkage rate of only 0.3% -0.5% at 1400 ℃).

2、 Thermal performance control

Thermal conductivity and insulation properties

Lightweight aggregates such as ceramic particles and floating beads can reduce bulk density and thermal conductivity, but traditional lightweight aggregates have low strength (such as ordinary lightweight high aluminum aggregates with compressive strength<20MPa) and need to be combined with high-strength microporous aggregates.

At high temperatures (>1000 ℃), the difference in thermal conductivity between microporous lightweight aggregates and ordinary aggregates decreases, but the former can still maintain a lower thermal expansion rate.

Fire resistance and high temperature stability

Aggregates with high Al ₂ O ∝ content, such as brown corundum, can enhance the fire resistance (>1790 ℃) and load softening temperature (>1500 ℃) of castables.

Aggregates with low impurity content, such as white corundum, reduce the formation of low melting point phases and avoid high-temperature volume expansion or contraction.

3、 Construction performance and microstructure

Particle size distribution and compactness

The principle of three-level grading: Coarse (10-5mm), medium (5-1.2mm), and fine (1.2-0.15mm) particles are mixed in proportions of 40-60%, 10-30%, and 20-40% to reduce the apparent porosity (<15%).

Fill the gaps between aggregates with fine powder (<0.045mm) to reduce water demand (e.g. spherical aggregates require 1% less water than triangular aggregates).

Liquidity control

Self flowing castables require spherical aggregates and continuous particle size distribution (q value 0.21-0.26), while vibration construction castables are suitable for intermittent gradation (q value 0.26-0.35).

4、 Special function optimization

Corrosion resistance

The slag erosion resistance of microporous corundum aggregate is comparable to that of plate-like corundum, but the former can further reduce slag infiltration due to its closed pore structure.

Economy and Environmental Protection

Partially replacing brown corundum with low-cost aggregates (such as high-grade alumina) can reduce the cost of pouring materials for cement kilns while maintaining performance (such as compressive strength>50MPa).

summary

Refractory aggregates have a comprehensive impact on the performance of castables through their chemical composition (Al ₂ O ∝ purity), particle characteristics (shape, gradation), and pore structure. Future trends include:

High purity microporous aggregate: balancing lightweight and high strength;

Composite aggregate design: such as corundum spinel system to enhance thermal shock resistance;

Intelligent grading: Optimizing particle distribution through numerical simulation.