When purchasing high-temperature gunning mixes, four core groups of indicators shall be prioritized: high temperature resistance,construction adaptability, physical and chemical stability, and service safety. These indicators directly determine the service life and protective performance of the gunning mix under high-temperature operating conditions. Below are the specific key indexes to watch out for and the corresponding selection criteria.
Ⅰ. Core High-Temperature Resistance Indexes
Maximum service temperature is the core index of high-temperature gunning mixes, which shall match the actual operating temperature of equipment (it is recommended to be 50~100℃ higher than the actual working temperature).
Classified by material: Aluminum silicate gunning mixes withstand temperatures of 1000~1400℃; Corundum-mullite gunning mixes withstand 1400~1800℃; Silicon carbide and silicon nitride based varieties can resist temperatures up to 1800~2200℃.
Note: Do not blindly pursue ultra-high temperature resistance. Excess performance beyond working condition requirements will raise costs and may reduce room-temperature adhesion strength.
Refractoriness: It refers to the material's capacity to resist melting at high temperatures under load-free conditions, and it must be higher than the maximum operating temperature of the equipment.
Softening temperature under load: It reflects the material's deformation resistance under high temperature and load, and is a critical index to judge whether the gunning mix can be stably applied on load-bearing parts (such as kiln bottom and load-bearing sections of flues). The higher this value, the better its high-temperature structural stability.
Thermal shock resistance: it refers to the capacity of the material to resist cracking and spalling during repeated heating and cooling cycles, which is measured by water cooling cycles or strength retention rate after rapid heating and quenching.
Application scenarios: For frequently started and stopped equipment such as metallurgical blast furnaces, glass kilns and industrial boilers, select gunning mixes with water cooling cycles ≥ 30 times.
Test Standard: Refer to GB/T 30873-2014 Refractory products - Test method for thermal shock resistance.
II. Construction and Forming Performance Indexes
Spray workability
Slurry Viscosity and Thixotropy：The viscosity shall match spraying equipment (mechanical spraying / manual spraying). Gunning mixes with favorable thixotropy are less prone to sagging during spraying and can adhere rapidly to substrate surfaces.
Rebound Rate：It stands for the proportion of material loss caused by rebound during spraying. Premium gunning mixes feature a rebound rate ≤5%. Excessively high rebound rate leads to material waste and impairs coating uniformity.
Setting Time：It is divided into initial setting time and final setting time, which shall coordinate with construction schedule. Normally, the initial setting time is controlled within 10~30 min, and the final setting time shall not exceed 24 h. Too fast setting prevents subsequent repairing work, while excessively slow setting delays follow-up procedures.
Adhesion and Bonding Strength
Cold bonding strength: Ensures the coating will not peel off during equipment startup, shutdown and vibration; the required value is ≥1.5 MPa.
High-temperature bonding strength: Bonding performance under operating temperature, which shall be verified via high-temperature tensile test. This index determines the long-term service stability of the coating at high temperatures.
III. Physical and Chemical Stability Indexes
Bulk Density：It reflects the compactness of the material. Excessively high density increases equipment load, while excessively low density results in poor erosion resistance. Selection shall be based on working conditions (1.8~2.6 g/cm³ is generally adopted for kiln linings).
Apparent Porosity：Gunning mixes with moderate apparent porosity balance thermal insulation and erosion resistance, normally controlled at 15%~30%. Excessively high porosity allows easy penetration and erosion by molten slag and flue gas; excessively low porosity impairs thermal shock resistance.
Erosion ResistanceFor working conditions involving contact with molten metal, furnace slag, acidic or alkaline flue gas, the slag resistance and acid-alkali corrosion resistance of gunning mixes shall be prioritized:
Metallurgy and aluminum smelting industries: Select SiC- or corundum-containing gunning mixes with superior resistance to molten metal erosion;
Chemical kilns: Choose zircon-based acid and alkali resistant gunning mixes to prevent damage from corrosive flue gas.
Linear Thermal Expansion Coefficient：This parameter represents the expansion degree of materials under heating. Its value shall be close to that of substrates (steel, ceramics, etc.). Otherwise, excessive expansion mismatch at high temperatures will cause cracking and spalling of the coating.
IV. Safety and Environmental Protection Indexes
Content of Hazardous Substances：It shall comply with GB 18582-2020 Limit of harmful substances in wall coatings for buildings. Special attention shall be paid to the contents of VOC and heavy metals (lead, cadmium, mercury). For gunning mixes applied in confined spaces, low VOC products feature higher safety and environmental friendliness.
Flame Retardancy：Gunning mixes for industrial high-temperature equipment shall possess flame retardancy to eliminate fire hazards, and it is recommended that the flame retardant grade reach Class B1 or higher.
Purchasing Tips
Clarify working conditions: Inform suppliers of the equipment's operating temperature, medium (flue gas/molten slag), start-stop frequency and substrate material, so that they can recommend matching products.
Small-batch trial: Carry out small-area spraying tests before mass procurement to observe the coating's drying rate, adhesion performance and whether cracking occurs after high-temperature exposure.
