What factors are affected by the intensity of fire -resistant pouring materials?

Refractory castables, as an indispensable refractory material, have strength characteristics that are directly related to their quality and performance. In many industrial fields, such as metallurgy, cement production, glass manufacturing and chemical industry, castables play a pivotal role. Therefore, the research and development of high-performance castables is particularly important.

Aluminum-magnesium castables are particularly worth mentioning, which are widely favored for their excellent high-temperature stability and corrosion resistance. We often see them in high-temperature equipment such as ladles, RH refining furnaces, cement kilns and glass kilns. It is worth mentioning that this type of castable can also effectively dissolve harmful elements in steel slag, such as iron (Fe) and manganese (Mn), without damaging its own structure. When activated alumina and magnesia are used as raw materials, the hydration reaction of the two can promote each other to form a material with a microporous structure. This structure can eliminate expansion caused by spinel formation and other reasons under high temperature conditions, thereby ensuring its volume stability.

In addition, the particle size distribution, particle morphology and physical properties of the raw materials have a crucial impact on the slurry fluidity, green body forming and post-firing performance of the castable. In essence, refractory castables are a composite material consisting of large aggregate particles wrapped in a fine powder matrix. In order to ensure its smooth forming, the material needs to exhibit good fluidity and thixotropy during the pouring process. After curing and calcination, it must also meet the low porosity and high strength requirements of specific application environments.

However, the strength of refractory castables is not constant and is affected by many factors. This article will explore these factors in depth to help you better understand and apply refractory castables.

Conditions of Use

For most castables, adding water to make them fluid is a crucial step, which ensures that the castable can be smoothly formed on site. Before heating and curing, the initial strength of the castable mainly depends on the complexity of the hydration and dehydration process. Therefore, water plays a dual role here: it is a necessary condition for the binder to generate strength, and it is also a key carrier of the castable.

Castables are usually made of a fine mixture of multiple components. This complexity gives it a unique microstructure. It is worth noting that there is a close relationship between its physical and chemical properties and the use environment and temperature.

The amount of water added is a parameter that needs to be precisely controlled. It directly affects the construction performance, rheological properties and mechanical properties of the castable after firing. If the amount of water added is insufficient, the binder may not be fully hydrated, which in turn affects the fluidity of the slurry. On the contrary, excessive water will reduce the viscosity of the castable, cause aggregate segregation, and even cause serious stratification, ultimately leading to uneven internal structure.

In addition, during the baking and calcining process, excessive moisture will produce a large number of pores inside the castable as free water evaporates and bound water separates. These pores will not only reduce the strength of the castable, but also weaken its ability to resist slag erosion. Therefore, under the premise of ensuring that the castable meets the operating requirements, the amount of water added should be minimized to avoid explosions or excessive pores during the heating process, thereby ensuring the quality and performance of the castable.

Raw materials and particle size distribution

In the preparation of castables, the addition of water is a crucial step. It mainly occupies the gaps between particles to ensure the uniformity of mixing and the fluidity of construction. In order to optimize the use efficiency of water and meet the requirements of operational fluidity at the same time, we use fine powder in the matrix to wrap large aggregate particles, thereby increasing the spacing between coarse particles and achieving a lubricating effect. This measure can also effectively adjust the packing density of particles, thereby significantly improving the fluidity of the castable and effectively preventing the occurrence of segregation.

When water is added to the castable, we must overcome the van der Waals and capillary forces between the fine powders to ensure uniform mixing. This process has a decisive influence on the construction performance of the castable, so it can be said that the operation performance of the castable depends largely on the particle size distribution of its dry powder. Although large-particle aggregates are not as good as fine powders in terms of lubrication and fluidity optimization, their cost advantages, improvement of the mechanical properties of the fired castable, and reduction of linear expansion at high temperatures cannot be ignored. In addition, the presence of aggregates also provides the necessary channels for the evaporation of internal moisture, effectively avoiding the risk of cracking and damage of the product during heating and temperature rise. Through these comprehensive measures, we can ensure that the castables maximize cost-effectiveness and construction efficiency while maintaining excellent performance.

Usage Process

During the construction of refractory castables, self-pulverization is a problem that needs attention. This phenomenon is mainly caused by the reaction between the hydration products of calcium aluminate cement and the acidic gases in the atmosphere, such as CO2, SO2, H2S, etc. Specifically, these acidic gases will react chemically with the hydration products, causing them to decompose, thereby significantly reducing the strength of the wear-resistant castable.

The main chemical reactions involved in the self-pulverization phenomenon include the carbonation reaction of calcium aluminate cement, the carbonate reaction of calcium aluminate cement and alkali, and the reaction of aluminate cement and sulfite. These reactions will gradually appear as the water evaporates during the curing or natural drying process of the castable.

In addition, during the curing or natural drying stage, as the water gradually evaporates, some soluble carbonates, phosphates, sulfites and other salts will crystallize out. Especially those salts containing crystal water will change their crystal form during weathering or dehydration, accompanied by changes in volume. These factors will further aggravate the strength loss of refractory castables.

It is worth noting that there is also a class of salts that do not contain water of crystallization, which will not cause changes in crystal form and volume during the salting out process. Therefore, they have little effect on the surface strength of wear-resistant castables, and mainly affect the appearance quality of the castables.

Sintering process

In coarse-grained castables, microcracks are very likely to form due to the difference in shrinkage between coarse and fine particles during sintering. This microcrack formation is particularly common in castables, and it becomes more serious when we replace medium-grained particles with coarse and fine particles, resulting in a further increase in the number of microcracks.

Taking the refractory castables used in kiln mouth as an example, the challenges they face are particularly severe. Considering that the temperature of the clinker outlet is as high as about 1400 degrees Celsius, and the temperature of the three blasts entering the kiln fluctuates frequently due to the constant changes in the kiln conditions, coupled with the gradual increase in the diameter of modern kilns and the continuous increase in kiln speed, the thermal and mechanical stresses on the refractory castables at the kiln mouth have increased significantly.

In view of the above factors, the kiln mouth refractory castable must have excellent refractoriness, mechanical strength, thermal shock stability and alkali resistance to adapt to such a harsh working environment. Combined with the specific operating conditions and requirements of the kiln mouth, we recommend the use of high-performance refractory castables designed for the kiln mouth or improved and optimized refractory castables for the kiln mouth. Such a choice can ensure the long-term stable operation of the kiln mouth and also help improve the overall production efficiency.

The strength of refractory castables is affected by many factors, including raw material quality and proportion, construction technology and maintenance conditions, sintering process and other factors. In order to obtain high-strength refractory castables, comprehensive consideration and optimization must be made from all aspects. I hope this article can help you better understand and apply refractory castables, and provide protection for the safety and stability of industrial production!