Refractory ramming mixes are typically composed of 60% to 65% refractory aggregates, 35% to 40% powdered materials, and a very small proportion of binders and other additives. They are unshaped refractory materials applied through vigorous ramming.
Compared to refractory castables and plastic refractories, ramming mixes exhibit higher bulk density, and the density of the lined structure after ramming is also greater. Refractory ramming mixes can be compacted on-site using mechanical methods such as pneumatic hammers, with air pressure not less than 0.5 MPa. For smaller quantities or less critical areas, manual ramming may also be employed. Kiln linings constructed with refractory ramming mixes exhibit lower moisture content. After thorough compaction, their performance surpasses that of castables made from the same material.
Classification of refractory ramming mixes
Based on the type of refractory aggregate, rammed refractories can be classified into clay-based, high-alumina, mullite-corundum, silica, magnesia, and silicon carbide varieties. According to the type of binder, they can be categorized into phosphoric acid and phosphate, aluminum sulfate, water glass, cement, binding clay, and organic binders. High-alumina rammed refractories utilize bauxite clinker with 82% Al₂O₃ content as aggregate and powder, supplemented with a significant amount of fused alumina powder to enhance matrix properties. Additionally, clay must be added as a plasticizer.
Magnesium-aluminum-chromium rammed refractory is formulated using electrofused magnesium-chromium synthetic material and aluminum-magnesium spinel.
Aluminum-zirconium rammed refractories utilize bauxite clinker with 85% Al₂O₃ content as the refractory aggregate and powder, blended with zircon sand containing 64% ZrO₂.
High-strength magnesia refractory ramming mix primarily uses fused magnesia with 97% MgO content as aggregate and powder, with a critical particle size of 5mm. Additionally, it requires 2%–4% phosphate binder and composite metal powder.
Basic Characteristics of Refractory Ramming Mix
1. Refractory rammed materials exhibit thermal hardness, resistance to spalling, abrasion resistance, and corrosion resistance. Adding tar to dolomite rammed materials enhances resistance to hydration; graphite rammed materials using tar as a binder offer excellent workability.
2. Refractory ramming mixes operate at elevated temperatures, typically between 1450°C and 1700°C.
3. The particle size distribution of ramming mixes should be uniformly graded, with coarse, medium, and fine particles in an approximate ratio of 4:2:4. The maximum particle size ranges from 5 to 7 mm.
High-alumina rammed refractories are suitable for applications such as electric furnace roofs and rotary kiln discharge ports. Magnesium-based rammed refractories are suitable for the hearths of electric furnaces, open-hearth furnaces, and converters. Dolomite rammed refractories are suitable for the hearths of converters and ferroalloy electric furnaces. Graphite-based and silicon carbide rammed refractories are more commonly used in blast furnaces and molten iron channels.
Construction Requirements for Rammed Aggregate
Carbon ramming material may be applied using either cold or hot ramming methods. Prior to ramming the furnace bottom, the furnace base must be thoroughly dried and cleaned.
When cold-ramming carbon material, the material temperature should be approximately 10°C higher than the softening temperature of its binder.
For hot ramming of carbon ramming mix, finished mix is recommended. Prior to ramming, the carbon mix must be crushed and heated uniformly. The heating temperature should correspond to the mixing temperature of the finished mix. No hard lumps should remain in the heated carbon mix. Hot hammers should be used during ramming, with material temperature not falling below 70°C.
When using pneumatic hammers for ramming, the thickness of each layer laid should not exceed 100mm. The compacted density of each layer of carbon ramming material shall be verified against the specified bulk density or compression ratio. The compression ratio is calculated as: (Compression reduction / Loose layer thickness) × 100%, with values typically ranging from 40% to 45%.
When resuming ramming after an interruption, the compacted surface must be cleaned, roughened, and coated with coal tar.
For magnesia or dolomite ramming mixes using coal tar or coal pitch as binders, hot hammers should be used for ramming. Coal tar, coal pitch, and aggregates should be separately dehydrated and heated before mixing, ensuring thorough blending.
When applying magnesia rammed refractories using bituminous asphalt as a binder, the following points should also be noted:
① The magnesia sand used should be dried and heated, with the material temperature during ramming maintained between 80°C and 130°C.
② The surface temperature of the masonry should be heated to 50°C to 60°C.
③ Before ramming, the masonry surface should be thoroughly cleaned and coated with a layer of tar asphalt;
④ Lay materials in layers during tamping, with each layer 20–30 mm thick. Tamp until the surface is smooth, the tamper exhibits rebound force, and a metallic sound is produced.
Formwork should be installed as required for tamping operations. Formwork must possess sufficient strength and rigidity, with all connectors and reinforcements remaining secure during tamping. Wooden formwork is recommended for small-area, complex-shaped sections, while steel formwork is preferred for large-area, regular-surface sections. Wooden formwork should be supported by timber props, and steel formwork by structural steel props. Bolted connections are recommended for composite formwork.