The proportion of refractory castables in the overall refractory material is increasing, mainly due to the development of low cement and ultra low cement combined castable technology. The low-cement refractory castable with aluminate cement as the binder is usually used. In the alumina-cement-bonded refractory castable, a considerable part of the product is susceptible to some damage during the curing process after the construction. The phenomenon causes the surface to be pulverized and peeled off, and the weight directly causes the green body to lose the bonding strength and pulverize and collapse. Especially for large-scale on-site production of castable prefabricated blocks, it is a big loss. Therefore, in view of this phenomenon, the damage mechanism is analyzed, and practical methods and countermeasures to avoid or reduce the surface damage of castables are formulated.
1 Refractory castable surface damage phenomenon
Accurately speaking, the surface damage of refractory castables should refer to the surface anomalies that occur during the curing of refractory castables after the end of on-site casting in actual production and use, which in turn may affect the performance of the entire castable. There are many forms of expression, but in summary, it should be summarized into the following aspects.
(1) The vast majority of surface damage occurs during the curing period after the construction of castables, and is often considered to be closely related to environmental conditions (such as temperature, humidity, etc.) during curing.
(2) This phenomenon often occurs in aluminate cement combined castables, and similarly occurs in some magnesia castables. The main research in this paper is the aluminate cement combined castable system.
(3) It is generally characterized by first forming a white spot on the surface of the cast body, or growing a white fluffy precipitate: then the white spot or the fluffy precipitate continues to spread to the entire surface of the blank: as the curing proceeds, the surface is dried. Fish scale-like peeling, or powdery attachments, resulting in a low strength of the surface of the blank. People often refer to this phenomenon as "back to alkali phenomenon"
(4) Some of the more serious surface damage will develop along the surface of the blank to the depth of the cast body, resulting in a decrease in the overall strength of the cast body, which cannot be used later. When it is particularly serious, the cast body loses its strength and the body is self-pulverized and disintegrated.
2 Analysis of the causes of surface damage of refractory castables
2.1 Surface pulverization caused by "alkali impurities"
In the formulation of refractory castables, the main refractory raw materials, cement and sodium salt admixtures all contain soluble sodium, and the admixture also introduces sodium ions, while the cement increases, the alkalinity of the system increases, and the There are also relatively many hydrated mineral phases, and a series of reactions occur in the presence of these soluble bases.
The soluble alkali in the castable is hydrolyzed, reacts with carbon dioxide in the air to produce carbonate, and the cement hydrates, and the two will continue to react. Continuous decomposition of calcification.
As long as there is cement hydration product, the above reaction will be cycled, the product will continue to decompose, and the castable body will be damaged from the inside and outside.
The presence of soluble alkali in the castable increases the solubility of CO2 and is an important prerequisite for rapid reaction. The greater the alkalinity of the system, the more hydrated mineral phases, the more favorable the reaction is.
2.2 Environmental temperature and humidity for curing
After the castable is cast, the curing temperature is usually 15-20 °C. The large precast block will enter the low temperature kiln for curing at 30-35 °C for the increase of curing strength. After observation, the curing temperature can enhance the blank. The strength and service life, as well as the phenomenon of chalking on the surface of the blank, is correspondingly reduced. It can be seen that the temperature and humidity of the green body are a major factor in the damage. In general, the greater the humidity, the easier it is to wet the pores in the castable body. Under wet conditions, the dissociation of soluble alkali in the castable is easier, and the reaction described in 2.1 above proceeds more smoothly.
2.3 Influence of the density of the green body
The density of the green body is also an important factor causing the surface of the castable. When the density of the green body is low, the porosity increases, and the carbon dioxide in the air can diffuse into the blank more easily, causing damage. The reaction occurs, causing the green body to be decomposed and pulverized by the surface and the inside.
2.4 Impact of construction water and water addition
The initial strength and construction performance of the castables are closely related to the quality of the construction water and the amount of water added. The construction water should be used to prevent the use of industrial water, alkali water and other chemical components of sewage and wastewater, should use civil domestic water, increase the amount of water The construction performance can be better, but it has certain side effects on the initial strength and density of the green body. At the same time, the addition of water will increase the hydration reaction, and it is easier to make the surface of the green body pulverize. Controlling the amount of water added, it is difficult to achieve the performance requirements of the construction, so the amount of water added to the construction is also a factor that causes damage.
