The key to improving the performance of low-cement refractory castables (cement content ≤8%) lies in optimizing density, strength, volume stability, and corrosion resistance. This is primarily achieved through the following approaches:

Raw Material Optimization: Laying the Foundation for Performance

1. Cementitious Material Selection

1.1 Prefer pure calcium aluminate cement (CA-50/CA-70) to reduce cement content (typically 4%-6%) to minimize hydration shrinkage and high-temperature liquid phase formation;

1.2 Adding active micropowders (such as α-alumina micropowder, silica fume, and spinel micropowder) utilizes the pozzolanic effect to enhance cementitious strength while filling aggregate gaps and improving density.

2. Aggregate Grading Design

2◦1 Use continuously graded aggregate (coarse aggregate 5-15mm, medium aggregate 1-5mm, fine powder ≤ 0.088mm) to ensure dense particle packing and minimize voids (target void ratio ≤ 20%).

2◦2 Use high-purity aggregates (such as high-alumina aggregate, corundum aggregate, and spinel aggregate) to reduce impurity content (such as Fe₂O₃ and Na₂O) and avoid the formation of low-melting-point phases at high temperatures.

Additive Control: Optimizing Construction and Performance

3. Dispersants

Adding dispersants such as polycarboxylic acids, sodium tripolyphosphate, and sodium hexametaphosphate reduces water addition (typically controlled to 6%-8%), reduces porosity caused by water evaporation, and improves castable fluidity for easier construction and compaction.

4. Volume Stabilizers

4◦1 Kyanite, sillimanite, and andalusite (expansion coefficient 3%-5%) can be added to utilize the micro-expansion caused by mullite formation at high temperatures to compensate for high-temperature shrinkage and prevent cracking.

4◦2 For applications requiring low expansion, non-oxide aggregates such as silicon carbide and silicon nitride can be added to suppress high-temperature volume changes.

5. Setting Accelerators/Retarder

5◦1 When rapid demolding is required, add setting accelerators such as calcium chloride and lithium carbonate to shorten the setting time (24-hour strength ≥ 15 MPa).

5◦2 For long-distance transportation or large-volume construction, add retarders such as sodium citrate and tartaric acid to extend the workable period (initial setting time ≥ 4 hours).

Construction and Curing Process Control: Ensure Performance

6. Strictly Control Water Addition

The water addition amount must be precisely matched to the formula (error ≤ 0.5%): Excessive addition will increase porosity, while insufficient addition will result in poor fluidity and low density.

7. Vibration Compaction

Use a high-frequency, plug-in vibrator (frequency ≥ 15,000 vibrations/min) to ensure a tight bond between the aggregate and the matrix, paying particular attention to corners and around anchors to prevent trapped air bubbles.

8. Scientific Curing

8◦1 Room Temperature Curing: After pouring, cover and moisturize the cement (curing at 20-25°C for 72 hours) to ensure full hydration of the cement and enhance early strength.

8◦2 Drying: Use a step-by-step drying process (50°C → 110°C, heating rate ≤ 10°C/h) to slowly drain free water and prevent cracking caused by rapid evaporation.

Microstructural Analysis

Heat treatment modification (such as pre-calcination at 1000-1200°C) promotes the formation of high-temperature phases such as mullite and spinel at the matrix-aggregate interface, strengthening interfacial bonding. For special applications (such as corrosive conditions), nanopowders (such as nano-Al₂O₃) can be added to fill microscopic pores and enhance penetration and corrosion resistance.