Method for improving the strength of aluminate cement-bonded castable at medium temperature
Aluminate cement binding castable is mainly made of calcium aluminate cement as binder, which is proportionally mixed with refractory aggregate and powder or admixture, and is made after water mixing, molding and curing. Calcium aluminate cement is a hydraulic cementitious material made of calcium aluminate cement clinker produced by sintering or melting method. The main mineral phases are CA and CA2, which have the characteristics of fast hardening, high strength, fire resistance and sulfate resistance. Aluminate cement binding castables are clay, high alumina and corundum castables, which are mainly used in hot blast stove bottom, lower manhole, flue port, cold air inlet, hot air outlet, hot air pipe, burner and other parts in blast furnace hot blast stove. The overall pouring of complex parts can improve the structural strength and construction efficiency, and improve the service life of hot blast stove.
The compressive strength of aluminate cement refractory castable at room temperature increases with the extension of curing time, and can reach higher strength after 1d, and the subsequent strength increases slowly, mainly because the main mineral phase CA of aluminate cement hydrates quickly, and the heat release can reach more than 80% within 12h after hydration.
The compressive strength of calcium aluminate cement-bonded castable decreases gradually with the increase of heating temperature, and decreases to the lowest at 900~1200 ℃, and then continues to increase the temperature. Due to the formation of ceramic bonding, the strength is greatly improved. Medium temperature strength refers to the strength at 900~1200 ℃. The medium temperature strength decline of aluminate cement combined with castable is a general rule. Generally, the medium temperature strength decline rate is 20 ~ 50%, because the dehydration temperature of refractory castable is 10% at 150 ℃, 20 ~ 35% at 290~330 ℃, and about 80% of crystal water and free water at 510 ℃ to 600 ℃, at 900 ℃, about 90% of free water and part of bound water can be removed. When the remaining bound water is removed between 900 and 1200 ℃, the original crystal lattice is destroyed, new mineral structure and secondary CA and CA2 are formed, resulting in an increase in apparent porosity. At the same time, due to low temperature and insignificant sintering effect, its structure is crisp and loose, resulting in a decrease in the strength of refractory castables.
There are two methods to improve the medium temperature strength, one is to add α-Al2O3 fine powder to make the chemical reaction of the expansion effect of the castable at 900~1200 ℃, so as to make up for the defects such as pore enlargement and structural looseness caused by the dehydration reaction of the castable. The reaction is as follows:
CaO Al2O3→CaO·Al2O3 (1).
CA Al2O3→CA2 (2)
The volume expansion effect of Reaction 1 is 19.6% and the volume expansion effect of Reaction 2 is 12.86%.
Second, in aluminate cement castable, adding sintering promoting agent can also improve the medium temperature strength. The sintering promoting agent is mainly soft clay, micropowder, etc. The decrease rate of medium temperature strength can be controlled within 10% by adding about 5%, because the sintering promoting agent can make the castable sinter at a lower temperature and realize ceramic combination in advance to increase the strength.