Testing refractory materials is a crucial process to ensure their durability, thermal stability, and mechanical strength in high-temperature industries. This article provides a detailed overview of refractory materials testing, including refractory bricks, castables, and other refractories.
1. Testing of Refractory Physicochemical Properties
The evaluation of refractory materials involves a series of critical tests designed to assess their performance under various conditions. These tests are vital for quality control and ensuring the material’s suitability for its intended application.
Testing Items for Refractory Bricks
The primary tests for refractory bricks include:
Chemical Composition: Determination of the main chemical components, such as Al₂O₃, Fe₂O₃, CaO, MgO, and SiO₂.
Cold Crushing Strength (CCS): The maximum compressive force a brick can withstand at room temperature.
Thermal Shock Stability: The ability of the material to resist damage when subjected to rapid temperature changes.
Refractoriness Under Load (RUL): The temperature at which the material begins to deform under a specified load.
Testing Items for Refractory Castables
For refractory castables, the key test items are:
Chemical Composition: Analysis of the main chemical components.
Compressive and Flexural Strength: Measurement of the mechanical strength of the cast material after hardening.
Permanent Linear Change (PLC) After Firing: Assessment of any permanent dimensional change (shrinkage or expansion) after being heated to a specific temperature.
Bulk Density: A measure of the material’s density, including its pore volume.
It is mandatory that all of the aforementioned physicochemical property tests are conducted at a state-certified Refractory Products Quality Supervision and Inspection Testing Center to ensure accuracy and impartiality.
2. Sampling Rules
For the testing of refractory bricks, including dimensional checks, visual inspection, cross-section examination, and performance testing, a random sampling method is employed. The sampling rules are as follows:
For visual quality inspection, the sampling is limited to a maximum of ten tons per batch.
For physicochemical property testing, the limit is a maximum of sixty tons per batch.
For dimensional, visual, and individual weight inspection, twenty pieces are sampled.
For physicochemical property testing, six pieces are sampled.
The minimum number of samples for each model or type of refractory brick shall not be less than three pieces.
3. Physical Testing Methods for Refractories
(I) Apparent Porosity, Water Absorption, and Bulk Density
Apparent porosity, water absorption, and bulk density are fundamental properties that describe the macroscopic pore structure of refractory materials. These properties are closely correlated with the degree of sintering, compressive strength, RUL, thermal shock stability, thermal conductivity, and slag resistance. Their testing is of great practical importance for controlling manufacturing processes, evaluating material quality, and predicting in-service performance. These three indicators are routinely used to characterize the compactness of a material.
Definitions
Apparent Porosity:Apparent Porosity = (Volume of open pores / Total volume) × 100%
Water Absorption:Water Absorption = (Mass of water absorbed in open pores / Mass of dry sample) × 100%
Bulk Density:Bulk Density = (Mass of dry sample / Total volume) × 100%
Testing Precautions
Sample Dimensions: The test sample must have a volume between 50 and 200 cm³ and the longest edge shall not exceed 80 mm.
Sample Appearance: The sample surface should be flat and clean (free from fine powder or particles from cutting, as well as cracks), without chipped edges, corners, or pockmarks visible to the naked eye.
Drying: Samples must be fully dried at 110℃ to a constant weight to completely eliminate all absorbed water.
Weighing: The same balance must be used for all weighings of a single sample to prevent measurement errors.
Vacuum Saturation: The specified vacuum level and evacuation time must be strictly adhered to, and the liquid must completely submerge the sample.
Weighing in Liquid: Care must be taken to ensure no air bubbles are attached to the sample or the sample holder during the apparent mass measurement.
Surface Wiping: When wiping excess liquid from the saturated sample’s surface, prevent the liquid from being drawn out of the pores. The accuracy of this test method is primarily determined by the consistency of the wiping procedure and the precision of the weighing.
(II) Cold Crushing Strength (CCS)
The cold crushing strength of a refractory material represents the maximum pressure per unit area that the product can withstand at room temperature before failure. It is a crucial mechanical indicator for assessing product quality and is directly related to porosity, bulk density, and thermal shock stability.
Sample Shape and Dimensions
Cube: For products with a thickness less than or equal to 100 mm, a cube sample of the same thickness is used. For thicknesses greater than 100 mm, a cube with a side length of 100 mm is required.
Cylinder: A cylinder with a diameter of 50 ± 1 mm and a height of 50 ± 1 mm is used. For products where these dimensions cannot be obtained, the largest possible cylinder (with equal diameter and height) is used.
Sample Preparation
The bearing surfaces of the test sample must be flat. The direction of compression should align with the direction of the forming pressure of the product, and this direction should be marked on the sample.
The sample must not have any defects such as chipped edges, corners, or cracks caused during sample preparation. If such defects are present, a new sample must be prepared.
Typically, samples of fired products can be tested after air-drying. If they have absorbed moisture, they must be dried at 110 ± 5℃ for two hours before testing, and then cooled naturally to room temperature.
Calculation of Results
P = F / S
P – Crushing strength, MPa
F – Maximum load at which the sample fractures, N
S – Area of the bearing surface, mm²
4. Chemical Testing Methods for Refractories
(I) Chemical Analysis Components
Chemical analysis involves the determination of key components, including: Al₂O₃, Fe₂O₃, CaO, MgO, SiO₂, K₂O, Na₂O, SiC, loss on ignition (LOI), ash content, volatile matter, and fixed carbon.
(II) Analytical Methods
The two main analytical methods are Gravimetric Analysis and Volumetric Analysis.
1. Gravimetric Analysis
This method involves converting the component to be analyzed into a stable compound, which is then weighed to calculate the component’s content. Based on the treatment method, gravimetric analysis can be categorized into precipitation, volatilization, and electrogravimetry.
Precipitation Method: The analyte is precipitated as a sparingly soluble compound, which is then filtered, dried, and ignited before being weighed. This weight is used to calculate the analyte’s content (e.g., BaSO₄).
Volatilization Method: The sample is heated or otherwise treated to cause a volatile component to escape. The content is calculated either from the resulting weight loss of the sample, or by absorbing the volatile component with an absorbent and calculating the content from the increase in the absorbent’s weight.
Electrogravimetry: This method uses the principle of electrolysis to deposit metal ions onto an electrode, which is then weighed to calculate the content.
2. Volumetric Analysis (Titration)
This method involves the controlled addition of a standard solution (a reagent with a known, accurate concentration) into the analyte solution until the reaction reaches its equivalence point. The analyte content is then calculated from the volume and concentration of the standard solution used. Volumetric analysis is known for its high accuracy, wide applicability, and speed, but it has low sensitivity and is not suitable for trace component analysis. It is further classified into acid-base, complexometric, redox, and precipitation titrations.
Acid-Base Titration: Also known as neutralization titration, it commonly uses HCl and NaOH as titrants. This method is used, for example, in the determination of calcium carbonate in cement clinker.
Complexometric Titration: The common complexing agent is EDTA, which can quantitatively complex with most metal ions in a 1:1 ratio.
Direct Titration: For ions like Ca²⁺ and Mg²⁺.
Back Titration: For ions like Al³⁺, which form slow-reacting polyhydroxy complexes. An excess of EDTA is added, and the mixture is boiled to ensure quantitative complexation. The remaining excess EDTA is then back-titrated with a standard Cu²⁺ or Zn²⁺ solution.
Replacement Titration: Utilizes a displacement reaction to replace an equal amount of another metal ion or EDTA from a complex, which is then titrated.
Indirect Titration: For metal ions that do not complex quantitatively with EDTA, they can be separated, and the concentration of another ion is determined via complexometric titration to indirectly calculate the analyte’s concentration.
5. Refractory Brick Routine Inspection and Out-of-Spec Handling
This section outlines the standard procedures for inspecting refractory bricks and the actions to be taken when defects are identified.
Note: A separate record sheet should be filled out for each material type.
1. Tolerances for Dimensions
The allowed tolerances for the large and small ends are 1 mm, and for length, 2 mm. The height tolerance is specified as 1%. For the group’s kilns, three different brick heights are used: 200 mm, 220 mm, and 250 mm. The tolerance for different heights varies but should not exceed 2-2.5 mm; generally, we aim to control it within 1-1.5 mm. These dimensional variations can be compensated for using mortar during installation.
2. Flatness
Flatness should be controlled to a maximum of 0.5 mm. A feeler gauge is used to measure the flatness of the main brick surface. Common gauge sizes are 0.1, 0.2, 0.25, 0.3, 0.4, and 0.5 mm. A brick is considered to have poor flatness if it allows a 0.5 mm feeler gauge to pass through. In such cases, additional boxes should be opened and more bricks tested. Operationally, it is best to control flatness to 0.2-0.3 mm. If the standard value of 0.5 mm is exceeded, the bricks are more susceptible to crushing during sealing and during kiln rotation. If the issue persists, contact the supplier to jointly determine a suitable masonry plan, and obtain a commitment from the supplier regarding the in-service quality of that batch.
3. Individual Weight
The theoretical weight of a brick can be calculated or provided by the manufacturer. The actual weight is compared to the theoretical weight to estimate the material’s density. A weight deviation of ±0.5 kg is considered ideal. A brick that is heavier than the theoretical weight may indicate that its dimensions are larger than specified or that it has absorbed moisture. A lighter brick suggests a higher porosity and a looser texture, indicating it may not meet the standard density. Bricks that fall outside this range and are not accompanied by an explanation or guarantee from the supplier can be rejected.
4. Edge and Corner Damage, Cracks, Pits, Fusion Marks, and Bulges
These defects are typical damages that occur during manufacturing, firing, packaging, and transportation. As long as these defects are within the specified numerical range on the inspection form, the bricks are considered usable. Generally, none of these issues should be present in a six-brick sample. If one or two bricks in the sample exhibit these defects, an additional box should be opened. If the defects are still present, contact the supplier for an explanation and document the communication in a formal memo.
Ensuring the highest standards of refractory material quality is essential for operational safety and efficiency. At Firebird Refractories, we specialize in providing high-performance, meticulously tested refractory products that meet and exceed these rigorous standards. Our commitment to quality control and technical excellence ensures you receive materials you can trust. For reliable, durable refractory solutions, explore our full range of products and expertise on our website.
