Cement Manufacturers Association (CMA)

35 impact on the stability of a structure. Ensuring precise measurements and appropriate sizing is critical to achieve a proper fit for the refractory shape and reducing the need for excessive thickness and joints during construction. Porosity: Porosity is a crucial metric that gauges the available open pore space within a refractory material, allowing substances like molten metal, slag, and vapors to permeate and potentially weaken its structure. The porosity level of refractory is quantified as the average percentage of open pore space within its total volume. Materials with high porosity are recognised for their superior insulation properties due to the significant amount of air they retain, which acts as an inefficient thermal conductor. Consequently, materials with low porosity are typically selected for areas exposed to high temperatures, while more porous variants are employed for additional thermal protection. However, highly porous materials are inadequate for applications involving elevated temperatures and direct exposure to flames, as they are prone to contracting under such circumstances. Refractory materials with elevated porosity are not recommended for use where they will encounter molten slag, as they are more difficult to penetrate in such scenarios. Bulk Density: Bulk density is commonly analyzed alongside apparent porosity and is defined as the weight of a specific volume of the refractory material. Generally, a higher bulk density is indicative of better quality in most refractories, as it signifies lower porosity. Furthermore, increasing bulk density enhances volume stability, heat capacity, and resistance to abrasion and slag infiltration. Cold Crushing Strength: Some argue that the cold crushing strength may not be highly relevant beyond simply showing the material’s ability to withstand transportation stresses. However, it can still serve as a valuable measure of firing quality and resistance to abrasion, when considered alongside factors like bulk density and porosity. Pyrometric Cone Equivalent (PCE): Because of their intricate chemical composition, refractories gradually melt at varying temperatures. It is best to determine their refractoriness or fusion point using the cone fusion method. The pyrometric cone equivalent (PCE) refers to the standard cone that melts to the same degree as the test cone. As per ASTM C24 - 01, the PCE is gauged by forming a cone from the material and subjecting it to heat until it bends at 3 O’clock. Notable PCE values for specific refractories are cones 33-34 for super duty fireclay, cones 29-31 for medium duty fire clay, and cones 36-37 for a 60% alumina product. Reported cone values for refractories adhere to a specified time – temperature correlation, meaning that different PCE values will arise from varied heating rates. The monitoring of PCE can be beneficial for quality control, enabling the identification of fluctuating batch chemistry originating from alterations or mistakes in the raw material composition. Refractoriness under load: Refractoriness refers to the ability to withstand extreme heat conditions (temperature > 1800 °F) and corrosion while containing and processing hot, molten materials during transportation and processing. The capacity to resist exposure to high temperatures without significant deformation is assessed by refractoriness. The Refractoriness Under Load test (RUL test) indicates the temperature at which bricks will fail under similar load conditions, although in real service scenarios, most of the load is actually borne by the cooler part of the bricks. Consequently, the RUL test serves as a gauge of refractory quality rather than a precise design parameter. In situations where the refractory material is heated on all sides, like in checkers or partition walls, the RUL test data holds more importance. Creep at high temperature: In materials science, creep refers to how a material deforms over time and at a specific temperature when subjected to stress. For refractory materials, it is crucial to maintain their shape stability under extreme conditions such as high temperatures and constant exposure to corrosive elements like hot liquids and gases. There is a standard criterion— where the compressive creep (the material’s deformation under stress at a given time and temperature) must not exceed 0.3% within the first 50 hours of testing under normal working conditions of load and temperature. This limit ensures that the rate of creep stabilizes by the end of the initial period, ensuring the refractory’s long- term safety for prolonged use. Volume stability, expansion and shrinkage at high temperature: In-service, refractories can contract or expand, leading to permanent