Generally, during the curing process of low cement castables during production, the natural reaction exhaust stage is within 24 hours. The blank is slightly hot and the surface hardens slowly. After 3 to 5 days of placement, the surface will peel off, and pure white fine particles will appear around the bubble pores. Gently press with your hand and find that the surface 3 to 5 mm has already softened and gradually powdered and peeled off, and some even reach 10 to 15 mm. This is bound to affect the structural strength of the new product, resulting in a significant reduction in the service life of the equipment, so that it cannot be used. Take the samples that have just been poured and solidified and the samples after powdering for physical index testing.
1. Analysis of the causes of surface damage
a. Surface powdering caused by "alkali impurities"
In the formula of refractory castables, the main refractory raw materials, cement and sodium salt admixtures contain soluble sodium. In low-end refractory raw materials, the content of alkali metal impurities is generally very high. Admixtures will also introduce sodium ions, and the increase of cement will increase the alkalinity of the system.
The soluble alkali in low cement castable will dissociate when it encounters water, and react with carbon dioxide in the air to produce carbonates. At the same time, the cement will hydrate, and the two will continue to react. Continuously decompose and calcify.
As long as there are cement hydration products, the above reaction will be circulated, and the continuous decomposition of the products will cause damage to the castable compact from the outside to the inside.
b. Curing ambient temperature and humidity
When the castable is cast, the general curing temperature is 15-20℃. Large prefabricated blocks will enter the low-temperature kiln for curing at 30-35℃ in order to increase the curing strength. Careful observation shows that the increase in curing temperature can enhance the strength and service life of the green body, and the powdering phenomenon on the green body surface is further reduced. It can be seen that the environmental temperature and humidity of green body curing are a major factor in the damage.
c. Influence 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 the water and carbon dioxide in the air can more easily diffuse into the green body, causing a damage reaction, resulting in the decomposition and powdering of the green body from shallow to deep.
d. Influence of water addition during construction
The initial strength and construction performance of castables are closely related to the amount of water added during construction. More water addition can improve construction performance, but it has certain side effects on the initial strength and density of the green body. At the same time, more water addition will inevitably increase the hydration reaction, making it easier for the surface of the green body to become powdery. It is difficult to meet the performance requirements of construction by controlling the amount of water added, so the amount of water added during construction is also a factor affecting damage.
2. Solutions to castable surface damage
Based on the existing analysis of the causes of damage to the surface of low cement castables, corresponding measures are taken to prevent or reduce the degree of surface damage.
a. Use high-purity raw materials
High-purity and high-density raw materials can reduce the content of soluble alkali metals. For example, refractory alumina aggregates should be calcined in rotary kilns as much as possible, with fewer impurities and higher bulk density, or dense corundum should be used as aggregates. For raw materials that have not been calcined, the impurity content should be strictly controlled, and the amount added should be controlled as little as possible.
b. Reasonable selection of admixture types
Traditional castable admixtures usually use lignin calcium salt and sodium salt, which increases the alkali metal content of the castable and accelerates the hydration and decomposition reaction. After adjustment, a new type of composite water reducer was tested to reduce the addition of calcium salt and strictly control the addition amount. After laboratory verification, the test was carried out in production to change the type of admixture and achieve the optimal addition amount. The phenomenon of powdering on the surface of the green body was improved, extending from 3 to 5 days to about one week, and the strength of the green body was also improved. The effects of different admixtures on the performance of castables are shown in Table 2. In this way, combined with the control of drying time, drying as soon as possible, and enhancing early strength, the surface powdering of the green body has rarely occurred.
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c. Control of construction water
Strengthen the control of construction water quality, try to precipitate water in a large container before production, avoid turbidity and impurities in the water, etc., to reduce the entry of other admixtures that can increase the hydration rate of cement.
d. Control of construction environment
In order to reduce and lower the contact between the surface of low cement castable and the air, the surface covering method is adopted to close the surface pores, and try to isolate the diffusion of carbon dioxide and water vapor into the castable body, thereby preventing damage reactions. At the same time, in order to dry the body as soon as possible, the insulation of the factory is strengthened, and when necessary, the body is put into a low-temperature kiln for drying and demoulding, so that the body hardens within the optimal curing period of 36 hours to ensure the strength of the cast body.
