The proportioning design of aggregate, fine powder and binder for refractory castables shall follow three core principles: dense particle packing, performance matching with working conditions, and controllable workability. Meanwhile, adjustments shall be made in combination with national standard requirements and actual application scenarios. The specific principles are as follows:
I. Principle of Dense Particle Packing (Core Physical Principle)
This principle serves as the basis for guaranteeing the bulk density, strength and wear resistance of castables. Its core objective is to enable the voids between aggregate particles to be fully filled by fine powder so as to reduce porosity.
1,Aggregate Gradation Design
It shall follow the theories of continuous gradation or gap‑graded gradation. Large‑size aggregate particles form the skeleton, while medium‑ and small‑sized particles fill the gaps among large particles. Reference shall be made to the aggregate particle‑size classification specified in GB/T 2994‑2018, Refractory Castables‑Terminology (coarse aggregate ≥5 mm; fine aggregate ＜5 mm).
2,Filling Ratio of Fine Powder
Fine powder (particle size ＜0.088 mm) shall sufficiently fill the residual voids of the aggregate skeleton, while providing sufficient reactants for hydration or sintering reactions.General proportion: fine powder accounts for 15%‑30%. For high‑alumina and corundum‑based castables, the fine‑powder proportion may be appropriately increased (25%‑35%) to enhance high‑temperature strength. For lightweight insulating castables, a lower fine‑powder proportion (10%‑15%) is adopted to retain partial porosity for heat‑insulation performance.
3,Maximization of Bulk Density
The proportioning shall be tested and verified for bulk density. Under ideal conditions, the ratio of actual bulk density to theoretical bulk density shall be ≥0.85, and the voidage shall be controlled within 15%‑25%. (Voidage of load‑bearing castables ＜20%; voidage of insulating castables ＞40%).
II. Principle of Performance Matching with Working Conditions (Function‑Oriented Principle)
The proportioning shall be adjusted targetedly for each component according to service conditions such as service temperature, corrosive medium and stress state of the castable, so as to ensure that the key performances meet the requirements.
1,High‑temperature Resistance Adaptation
High‑temperature service conditions (≥1400 ℃): Increase the proportion of aggregates / fine powders with high refractoriness (e.g. corundum, mullite, silicon carbide), and reduce low‑melting‑point impurities. For binders, phosphoric acid, phosphates or pure calcium aluminate cement shall be preferred; the cement dosage shall be reduced (≤8%) to avoid deterioration of high‑temperature strength.
Medium‑ and low‑temperature service conditions (≤1200 ℃): Clay‑based and high‑alumina aggregates can be adopted. The cement dosage of binders may be appropriately increased (10%‑15%) to guarantee room‑temperature strength.
2,Erosion‑resistance Matching
Acidic media (e.g., glass kilns, sulfuric‑acid kilns): Siliceous and pyrophyllite‑based aggregates shall be selected, and alkaline components shall be avoided. Quartz powder may be added into fine powder fraction to improve acid resistance.
Alkaline media (e.g., steel‑making converters, cement rotary kilns): Magnesia‑based and magnesia‑spinel aggregates shall be adopted. Fine powder shall be matched with magnesia powder, and alkaline cement or composite binders shall be used.
3,Mechanical‑property Matching
For load‑bearing sections (e.g., kiln bottoms and kiln walls): Increase the aggregate proportion (60%‑70%) and reduce the dosage of fine powder and binder to boost compressive strength, which shall comply with the requirements of GB/T 3001‑2017, Test Method for Cold Modulus of Rupture of Refractory Products.
For repairing / thin‑layer sections: Reduce the coarse aggregate proportion (＜20%) and raise the proportions of fine powder and binder to improve bonding strength and flexibility.
III. Principle of Controllable Workability (Process‑Implementation Principle)
The proportioning shall take both physical performances and construction operations into account, so as to avoid problems such as difficult mixing, water bleeding and delamination caused by improper proportioning.
1,Control of Binder Dosage
For cement‑based binders: The dosage shall be adjusted according to the water absorption rate of aggregates, with a general proportion of 5%‑15%. Excessively high dosage will increase the drying shrinkage rate of castables and cause easy cracking; excessively low dosage results in poor flowability, making it difficult to achieve dense compaction by vibration.
For chemical binders (e.g., phenolic resin, phosphates): The addition proportion shall be strictly controlled (normally 2%‑8%). Meanwhile, the dosage of curing agent shall be matched to ensure that the initial‑setting time meets construction requirements (generally 2‑6 hours).
2,Balance of Water‑to‑Solid Ratio
The water‑to‑solid ratio (by mass) of cement‑based castables is generally controlled within 0.18‑0.25：
Excessively high water‑to‑solid ratio: Good flowability, yet increased porosity and deteriorated strength as well as high‑temperature resistance. Excessively low water‑to‑solid ratio: Poor flowability and difficult vibration compaction, with internal voids readily formed.
3,IV. Principle of Additive Synergy
Water reducer (e.g., polycarboxylate‑based, naphthalene‑based) can be added to cut water consumption; retarder / accelerator can be used to adjust the setting time; anti‑crack fibers (e.g., polypropylene fibers) can be incorporated to reduce drying shrinkage. The dosage of additives shall be determined by tests (generally ＜1%) to avoid adverse reactions with binders.
IV. Principle of Compliance (Standard‑Constraint Principle)
All proportioning designs shall comply with the national standards for corresponding products, for example:
High‑alumina refractory castables: GB/T 5072.2‑2004, Test Method for Cold Crushing Strength of Dense Shaped Refractory Products
Refractory castables for cement kilns: JC/T 707‑2020, Refractory Castables for Cement Kilns