Low Cement, Ultra-Low Cement, and Cement-Free Castables

In the early 1970s, Lafarge in France successfully developed low cement castables, followed by the development of ultra-low cement castables, which gained global application in the 1980s. In China, there is no unified standard for categorizing these castables. According to ASTM standards in the United States, they are defined based on the CaO content in the product.

Unlike conventional refractory castables, low cement and ultra-low cement castables partially or largely replace calcium aluminate cement with ultrafine powders of the same or similar chemical composition as the castable's main material. Additionally, small amounts of dispersants (water reducers) and delayed-setting accelerators are added. Cement-free castables use ultrafine powders or oxide sol entirely as binders.

The setting and hardening mechanisms of low cement, ultra-low cement, and cement-free castables differ from those of conventional calcium aluminate cement. While traditional cement relies primarily on hydration bonding, low cement castables exhibit both hydration and coagulation bonding, ultra-low cement castables are predominantly coagulation-bonded, and cement-free castables rely entirely on coagulation bonding.

The principle behind coagulation bonding is as follows: in silica-alumina castables containing SiO₂ ultrafine powder, mixing the powder with water forms colloidal particles due to the high activity of SiO₂ ultrafine powder. The surface of these colloidal particles dissociates Si-OH groups into Si-O⁻ and H⁺, giving the particles a negative charge. These negatively charged particles adsorb Al³⁺ and Ca²⁺ ions slowly released during the hydrolysis of calcium aluminate, leading to a decrease in the zeta potential of the colloidal particles. When adsorption reaches the isoelectric point (where the colloidal particles are neutral), coagulation occurs, forming bonds that harden through drying.

Advantages of Low Cement, Ultra-Low Cement, and Cement-Free Castables:

1. Reduced CaO Content: Lower CaO content reduces the formation of low-melting phases, enhancing refractoriness, high-temperature strength, and slag resistance, with cement-free castables offering superior performance.
2. Lower Mixing Water Requirement: The water needed for mixing is only 1/2 to 1/3 of that for conventional refractory castables (about 4%–6%), resulting in lower porosity and higher bulk density.
3. Improved Strength: After forming and curing, minimal or no cement hydration products are generated. This avoids significant strength reduction due to the breakdown of hydration bonds during heating. Instead, strength gradually increases with sintering at higher temperatures.

Aggregate and Powder Materials

Low cement, ultra-low cement, and cement-free castables can use aggregates and powders made of clay, high alumina, mullite, corundum, carbon-containing materials, or silicon carbide. The choice of binders such as ultrafine powder or silica/alumina sol depends on the chemical composition of the aggregate. For example:

• Corundum-based castables should use alumina ultrafine powder or a combination of alumina and silica ultrafine powder.
• Silica-alumina castables can use silica ultrafine powder alone, a combination of silica and alumina ultrafine powder, or silica sol as the binder.

These innovative castables represent a significant advancement in refractory material technology, providing enhanced performance and broader applications across high-temperature industries.