Castables: Hardening After Heating, Brittleness & Pulverization — Full Analysis Of Core Mechanisms & Risk Avoidance Guidelines

Preface: Why do castables exhibit relatively low strength after construction before furnace baking, yet become highly rigid after high-temperature heating?

In high-temperature industrial sectors such as metallurgy, cement and petrochemical industries, refractory castables serve as the "core guardian" of kiln linings. Many construction personnel encounter two typical puzzles: freshly placed castables are soft, yet become rock-hard after furnace baking; meanwhile, certain castables tend to crack and powder both before and after baking with virtually no strength. This phenomenon is not caused by erratic material behavior, but governed by complex physical and chemical transformations. This article will elaborate on the essence of these two issues and clarify the key control parameters.

The relatively low strength of castables before furnace baking and sharp rise in hardness after high-temperature treatment stem fundamentally from a radical shift in bonding mechanism, coupled with densification of the microstructure. Together, these two factors bring about a dramatic improvement in performance.

 Castables are mixed with water for shaping during construction. Their strength during curing at ambient temperature mainly relies on hydraulic bonding, a process dominated by the binder (calcium aluminate cement, CAC, the mainstream type). The hydration reaction between calcium aluminate cement and water produces hydration products such as calcium aluminate decahydrate (CAH₁₀), dicalcium aluminate octahydrate (C₂AH₈) and tricalcium aluminate hexahydrate (C₃AH₆), alongside aluminium hydroxide gel (AH₃). These products intergrow between aggregate particles to form an initial three-dimensional network structure, enabling the castable to set from slurry and gain early strength sufficient for demoulding.

 However, the strength at this stage has obvious limitations. On the one hand, hydration products themselves possess low crystallinity and low density. A large number of pores exist in the network structure, and the bonding forces are mainly hydrogen bonds and van der Waals forces, making the whole structure fragile and vulnerable to damage. On the other hand, improper control of temperature and humidity during curing will aggravate structural defects. Low temperatures (5–10 °C) result in incomplete hydration reactions and insufficient generation of hydration products; a low-humidity environment causes rapid loss of surface moisture, creating a humidity difference between the interior and exterior and inducing microcracks, which further reduce strength. This is the core reason why castables appear weak and deformable before furnace baking.

The furnace baking process is a critical stage for performance upgrading of castables. As the temperature rises from ambient temperature to high temperature, three core reactions take place sequentially: dehydration, phase transformation and sintering, ultimately forming a robust "ceramic skeleton".

 First comes the dehydration and purification stage (100–400 °C): crystal water and free water contained in hydration products are gradually removed. The hydration network originally sustained by water temporarily suffers a "strength deficit", and fine cracks may even form due to excessive steam pressure. This explains why the strength of some castables drops temporarily at medium temperatures. Nevertheless, this process eliminates pore water inside the material and reserves space for subsequent densification.

 Then follows the phase transformation and reconstruction stage (400–1200 °C): hydration products decompose completely. Metastable phases such as CAH₁₀ and C₂AH₈ convert into stable calcium aluminate crystals (CA, CA₂). Meanwhile, ultrafine powders (e.g., silica fume) undergo pozzolanic reactions with hydration products to form high-strength calcium silicate hydrate (C‑S‑H) gel, which interweaves with calcium aluminate crystals to build a denser transitional network. For phosphate-bonded castables, polymerization of phosphates occurs at this stage; the material transforms from a colloidal state to a crystalline state, further improving structural stability.

 Lastly comes the ceramic bonding formation stage (above 1200 °C): At high temperatures, slight melting occurs on the surface of aggregate and powder particles within the castables, generating a small amount of liquid phase. Acting like a binder, this liquid phase fills the voids between particles and facilitates sintering densification. Meanwhile, strong chemical bonds (such as covalent bonds and ionic bonds) form among particles to construct a continuous ceramic bonding network. At this point, the strength of castables no longer relies on temporary hydration products, but is governed by a stable ceramic structure. Hardness and bulk density are greatly enhanced, completing the transformation into a rigid, solid mass.

Contrary to the normal phenomenon of hardening after baking, the tendency of castables to crack, powder and lack strength essentially stems from failure of the bonding system or irreversible defects in the microstructure. This problem is mostly induced by material formulation, construction technology or environmental factors, and the common causes can be classified into three categories.

 This is the primary inducement for powdering during curing, and it tends to occur more readily under high-temperature and high-humidity conditions. Hydration products of calcium aluminate cement (such as CAH₁₀ and C-S-H gel) are alkaline. They react with acidic gases in the atmosphere including CO₂ and SO₂ to form loose and porous products such as carbonates and sulfites, bringing about decomposition and collapse of the original three-dimensional network structure. Such carbonation reaction causes progressive surface powdering of castables. In severe cases, the internal structure also becomes porous and loses load-bearing capacity.

 In addition, if raw materials contain soluble salts, water evaporation during curing will lead to salt crystallization and precipitation. Volume changes accompanied by crystal transformation aggravate internal cracks, further reducing strength and resulting in a fragile structure.

Most strength-related problems are directly associated with non-standard construction operations, mainly revolving around three key parameters: water dosage, temperature and humidity.

 Excessive water addition is a common misconception. Extra water is added to improve flowability, which leads to a substantial rise in internal porosity after curing, sparse distribution of hydration products and weakened bonding force. Rapid evaporation of water during furnace baking also generates numerous microcracks, ultimately forming a porous and loose body that is fragile with low hardness.

 Unbalanced temperature and humidity control can also be fatal. Curing at low temperatures (<5 °C) slows down hydration reactions, resulting in insufficient hydration products and a loose structure. In low-humidity environments, surface water evaporates too rapidly, causing drying shrinkage cracks and incomplete cement hydration, which yields extremely low surface strength. Curing at high temperatures (>40 °C) accelerates cement hydration excessively with fast water consumption, triggering false set. This leads to an incomplete internal structure prone to cracking and powdering in later service.

 Besides, improper operations including insufficient mixing (separation of aggregates from powders and uneven dispersion of admixtures), inadequate vibration during shaping (hollowness and voids inside the material), and premature demoulding (before hydration strength reaches the required level) will lay hidden risks for cracking and powdering.

 Inherent flaws in material formulation constitute fundamental drawbacks. For instance, conventional high-cement castables with excessive cement content leave abundant pores after decomposition of cement hydration products at high temperatures, leading to a sharp drop in strength within the medium-temperature range (900–1200 °C) and susceptibility to cracking and powdering. In contrast, low-cement or ultra-low-cement castables lacking sufficient ultrafine powder or equipped with improperly selected dispersants fail to form a dense cohesive bonding network, so the strength cannot meet requirements naturally.

 In addition, unreasonable aggregate grading (excessively large interstices between particles without fine powder filling), poor binder quality (e.g., low-purity calcium aluminate cement), and excessive impurities (which generate excessive low-melting liquid phase at high temperatures and result in loose structures after cooling) all lead to inherently low strength of castables and render them prone to cracking and powdering.

Once the core mechanisms are understood, targeted control measures can be adopted to avoid problems. The key points are as follows:

 Strictly control mixing water dosage: minimize water addition while ensuring flowability, comply with the water dosage range recommended by the material manufacturer (typically 6%–8% by mass), and avoid arbitrary water addition.

 Optimize the curing environment: For curing at ambient temperature, the optimal conditions are 20–25 °C and relative humidity ≥70%. Thermal insulation measures (such as covering with thermal insulation films) shall be adopted for construction under low temperatures, and accelerators may be added when necessary. In high-temperature and high-humidity environments, adequate rain and moisture protection should be implemented to prevent erosion by acidic gases.

 Standardize construction procedures: During mixing, dry-mix aggregates and powders first, then add water with pre-dissolved admixtures, with a mixing duration of no less than 3 minutes. Ensure sufficient vibration during shaping to eliminate internal hollowness and voids. Determine the demoulding time according to the achieved hydration strength and avoid premature demoulding.

 Adopt scientific furnace heating procedures: Establish a reasonable heating curve. Heat slowly within the range of 100–400 °C (heating rate ≤50 °C/h) to prevent steam spalling caused by rapid water evaporation, creating favorable conditions for the subsequent formation of ceramic bonding.

 Select appropriate material formulations: Choose a suitable bonding system according to the operating temperature and working conditions of the kiln. Low-cement or ultra-low-cement castables are preferred for medium and high-temperature zones. Ensure reasonable aggregate grading and qualified impurity content.

Conclusion

 

Strength variation of castables is not accidental, but results from the synergistic effect of physical and chemical processes. The hardness transformation before and after furnace heating represents a positive evolution from temporary bonding to stable ceramic bonding. In contrast, cracking and powdering indicate structural failure induced by chemical erosion, improper construction control or formulation defects. Understanding these mechanisms and precisely controlling the whole process including materials, construction and curing enables castables to deliver full performance and extend the service life of kilns.

 

What other difficult problems have you encountered during castable construction? Feel free to consult us!