What are the most important control points in the construction process of refractory castables?

Refractory castables have gained widespread adoption in industrial applications due to their simplified production processes and ease of construction. However, the effectiveness of refractory castable linings depends not only on their inherent properties and quality but also critically on the quality of installation. This article outlines key considerations during the construction process of refractory castables (anchoring components), specifically addressing the sequence: formwork installation → mixing → pouring and vibration → formwork removal → curing and drying.

1. Anchorage Construction

Anchors (claw pins) serve as support elements within castable refractory linings. Their material, shape, and arrangement depend on factors such as operating temperature, construction conditions, and castable thickness. Common shapes include Y-shaped and V-shaped anchors, typically fabricated from 6–12 mm round steel bars or 20 mm wide, 6–8 mm thick steel strips. The standard material is ICr18Ni9Ti. For operating temperatures below 500°C, ordinary steel may be used. Y-shaped anchors are primarily employed in thick or double-layer linings, while V-shaped anchors are used in thinner linings.

Anchors must be staggered with spacing and row spacing generally set at 200–300 mm, achieving a density of 10–15 anchors per square meter. Anchor height should typically be 85% of the lining thickness or 50 mm below the castable material level. During construction, welding between anchors and the shell must be fully fused with no cold spots. Only after passing inspection can subsequent processes proceed; otherwise, large-scale collapse of the castable may occur. Additionally, to mitigate adverse thermal expansion stresses from anchors on the lining, the anchor surfaces should be coated with asphalt paint, tar, or electrical tape. Our factory employs plastic sleeves with a wall thickness of 0.5–1 mm over the anchors, which facilitates construction and yields excellent results.

2. Formwork installation

Molds are typically made from hardwood boards. For curved pipelines, steel molds with a thickness of 2–3 mm may also be used. The side of the formwork in contact with the castable material must be coated with a release agent (such as machine oil or asphalt) to prevent moisture absorption during casting and facilitate demolding. If surface finish is critical, a layer of oil-coated paper may be applied to the formwork. Formwork stiffness and thickness should meet process requirements. During large-area construction, ensure uniform vibration of the base-layer castable to prevent honeycombing or voids. Formwork height should ideally not exceed 0.7m.

3. Stirring

Refractory castables are typically mixed using a mixer; manual mixing is only employed when the quantity of material is extremely small. The type of water or adhesive used for mixing depends on the specific castable variety. Taking water addition as an example: First dry-mix the castable for 1–3 minutes until uniform. Slowly add approximately 80% of the water from the edges, then mix for another 2–3 minutes. Add the remaining 20% of water and wet-mix for 2–3 minutes. Finally, test the consistency by squeezing a handful of castable—it should form a cohesive mass without water seeping between your fingers. The amount of water added during mixing directly impacts the castable's workability and performance. Typically, the effect of water addition becomes apparent only after 3–4 minutes of wet mixing. If the water content is appropriate, the initially dry material will suddenly become highly fluid during mixing, usually around the 2–3 minute mark. If the desired workability is achieved within 2 minutes of mixing, this indicates excessive water addition. During vibration, excessive moisture carries fine particles out of the mix, disrupting the material's optimal gradation. This impairs early strength development and creates internal voids that compromise quality.

4. Pouring and Vibrating

After the refractory castable is thoroughly mixed and the formwork is cleaned, proceed to the pouring and vibration stage. During vibration, ensure the vibrator does not contact the anchors to prevent separation between the anchors and the castable layer, which could create large voids. Second, avoid excessive vibration duration. Cease vibration once surface slurry appears, as prolonged vibration may cause segregation in the castable material. For large-area construction, expansion joints must be installed as specified. Typically, place a 5mm plywood strip or 10mm wooden strip vertically every 2m to serve as a reserved expansion joint. Additionally, during compaction, promptly adjust water content based on feedback to prevent uneven moisture distribution causing honeycombing and compromising construction quality. For composite linings, the castable material typically forms the working layer. The internal insulation layer (e.g., calcium silicate boards) should be pre-installed and waterproofed before pouring and compacting the castable material.

5. Formwork Removal

Formwork can be removed 5 to 8 hours after the castable material is poured. During formwork removal, carefully inspect the pour quality and take appropriate corrective actions. If spalling or pitting occurs (caused by excessively dirty formwork surfaces or inadequate surface preparation), perform necessary surface treatment for severe cases. For honeycombing, voids, color inconsistencies (resulting from poor pouring and vibration control), or surface sanding (due to high-alumina cement failure), rework critical areas. If deformation or bulging occurs (caused by insufficient formwork rigidity), address only in particularly severe cases.

Silica Ramming Mass