Solution to Surface Damage of Low Cement Castable Lining
When we use low cement castables, we sometimes find surface damage. At least it will cause the surface to be powdered and peeled off, at worst it will directly cause the green body to lose its bonding strength and cause it to collapse. Especially for large castable prefabricated blocks, it will cause great losses. So, why is the surface of low cement castables damaged? What are the specific solutions?
Reasons for Surface Damage of Low-Cement Castables
"Alkali impurities" cause surface powdering
In the formula of refractory castables, the main refractory raw materials, cement and sodium salt additives, etc., all contain soluble sodium. In low-grade refractory raw materials, the content of alkali metal impurities is often relatively high, and admixtures will also introduce sodium ions. The addition of cement will increase the alkalinity of the system. At the same time, relatively more hydrated mineral phases are generated, and a series of reactions occur in the presence of these soluble alkali.
The soluble alkali in the castable dissociates when it encounters water and reacts with carbon dioxide in the air to produce carbonates. At the same time, the cement is hydrated, and the two will continue to react and continue to decompose calcification. As long as cement hydration products exist, the above reactions will proceed cyclically. As the product continues to decompose, the castable body will be damaged from the outside to the inside.
Maintain ambient temperature and humidity
After the castable is poured and formed, the curing temperature is usually between 15 and 20°C. In order to increase the curing strength, large prefabricated blocks will be cured in a low-temperature kiln at 30 to 35°C. After observation, increasing the curing temperature can enhance the strength and service life of the green body, and the phenomenon of powdering on the surface of the green body is also reduced accordingly. It can be seen that the temperature and humidity of the green body curing environment are a major factor in the occurrence of damage. Generally speaking, the greater the humidity, the easier it is to wet the pores in the castable body. Dissociation of soluble bases in the castable is easier under humid conditions.
Effect of Green Body Density
The density of the green body is also an important factor causing the powdering of the castable surface. When the density of the green body is low, the porosity increases and hydrolyzed carbon dioxide in the air can more easily enter the green body through diffusion. This causes a damage reaction to occur, causing the body to decompose and pulverize from the surface to the inside.
The impact of water addition during construction
The initial strength and construction performance of castables is closely related to the amount of water added during construction. Adding more water will improve the construction performance, but it will have certain side effects on the initial strength and density of the green body. At the same time, adding more water will inevitably increase the hydration reaction and make the surface of the green body more likely to be powdered. It is difficult to control the amount of water added to meet the performance requirements of construction, so the amount of water added during construction is also a factor that affects damage.
Solution to Surface Damage of Low Cement Castables Cement
1. Use high-purity raw materials.
High-purity and high-density raw materials can reduce the content of soluble alkali metals. For example, refractory alumina aggregate should be calcined in a rotary kiln as much as possible, with less impurities and high bulk density. Or use dense corundum as aggregate, strictly control the impurity content of raw materials that have not been calcined, and control the addition amount as little as possible.
2. Reasonably select the types of admixtures.
Traditional castable admixtures usually use lignin calcium salts and sodium salts, which increase the alkali metal content of the castables (https://refractorycastablecement.com/low-cement-castables) and accelerate the hydration and decomposition reaction. After adjusting and testing the composite water-reducing agent, the addition of calcium salt was reduced. And strictly control the addition amount, change the type of admixture, and achieve the appropriate addition amount. The powdering phenomenon on the surface of the green body has been improved, extending from 3 to 5 days to about a week, and the strength of the green body has also been improved. In this way, combined with the control of drying time to dry as early as possible and enhance early strength, powdering on the surface of the green body rarely occurs now.
3. Control construction water use
We have strengthened the control of construction water quality and tried to precipitate the water in large containers before production. Avoid water turbidity and impurities to reduce the entry of other admixtures that can increase the hydration rate of cement.
4. Control the construction environment
Enhance early strength In order to reduce and reduce the contact degree between the surface of the castable and the air, the surface covering method is adopted to close the surface pores. Try to isolate the diffusion of carbon dioxide and water vapor into the castable body to prevent damage reactions. At the same time, in order to dry the green body as soon as possible, the thermal insulation of the factory building was strengthened. And if necessary, the green body will be dried in a low-temperature kiln and then de-moulded, so that the green body will harden within a 36-hour curing period to ensure the strength of the cast body.
Rongsheng Unshaped Refractory Materials Manufacturer has an advanced fully automatic unshaped refractory material production line. Our unshaped refractory products are available in a complete range and can be customized according to the actual use needs of industrial furnaces. Contact us to get free samples and quotes.
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