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Why Do Refractory Castables Show Efflorescence, and How Can It Be Prevented?
Causes and chemistry of efflorescence in refractory castables, and remedies such as prompt dry-out and protective coatings.
White bloom appearing on the surface of a refractory castable after installation is called efflorescence (alkali bloom). Why does it happen and how can it be solved? Several factors are involved; the causes and solutions are detailed below.
Why refractory castables develop efflorescence
Hydraulic refractory castables are unshaped products made of refractory aggregate, fines and various additives that develop strength after being mixed with water. If a castable is not dried out promptly after demoulding, white whiskers may appear on its surface during standing, followed by gradual dusting and powdering—sand can clearly be rubbed off by hand. This is called efflorescence.
There are many causes, mainly the following:
1. Alkaline substances in the raw materials
Castable raw materials contain Fe2O3 and alkali metal oxides such as Na2O and K2O. When mixed with water, these set and harden, and the free water floating on the surface carries a loose, gelatinous white precipitate. Unhydrated cement particles on the surface form a saturated solution with water, precipitating alumina gel and hydrated calcium aluminate, which react with alkali carbonates to cause efflorescence. The raw materials, the calcium aluminate cement binder, and additives such as water reducers, accelerators and anti-explosion agents often contain small amounts of alkali metal oxides. Part of this alkali dissolves in water—the alkali in dispersants almost entirely—and migrates through capillary pores to the surface of the hardened body, where it reacts with atmospheric CO2 to form carbonates, which then react with cement hydration products to cause efflorescence. The surface then shows white whiskers and frost. Although efflorescence does not affect the overall performance of the castable, it leaves the surface rough and uneven and spoils its appearance.
The main chemical reactions of efflorescence are:
Diffusion and reaction of Na+ ions within the castable:
Na2O + H2O → 2Na+ + 2OH-
2Na+ + 2OH- + CO2 + H2O → Na2CO3·xH2O
The carbonate then reacts with the hydration products of calcium aluminate cement:
Na2CO3 + CaO·Al2O3·10H2O → CaCO3 + Na2O·Al2O3 + 10H2O
2Na2CO3 + 2CaO·Al2O3·8H2O → 2CaCO3 + Na2O·Al2O3 + 2NaOH + 7H2O
3Na2CO3 + 3CaO·Al2O3·6H2O → 3CaCO3 + Na2O·Al2O3 + 4NaOH + 4H2O
The NaOH produced reacts with atmospheric CO2 to form Na2CO3 again, which again reacts with hydrated calcium aluminate to form NaOH. This cycle repeats continuously, steadily decomposing the hydrated calcium aluminate and gradually breaking down the hardened cement. In other words, alkali acts as a medium that promotes attack by atmospheric CO2 on the hydration products of calcium aluminate cement.
2. Porosity and pore size
Extensive research by Lafarge on aluminate-cement-bonded castables found that porosity is one of the main factors affecting efflorescence. The amount of porosity and the pore size influence the migration of alkali components to the surface and have a major effect on efflorescence.
3. Ambient humidity
Studies of powdering in aluminate-cement-bonded lightweight castables under different humidity levels show that the higher the ambient humidity, the more pronounced the efflorescence.
How to solve efflorescence in refractory castables
1. Dry out the castable promptly after demoulding and curing.
The key to efflorescence is the presence of free water. The solution is to remove free water promptly, so that alkali and alkaline earth metal compounds form stable compounds before they convert to hydroxides and migrate, preventing efflorescence.
If efflorescence is found after demoulding, dry out the castable as soon as possible to remove moisture and eliminate sodium salts inside, so the alkali stays within the body and no longer migrates to the surface. If prompt drying is not possible, seal the castable's environment by covering its surface with plastic film to stop moisture carrying alkali to the surface.
2. Apply an anti-penetration organic coating to the surface of the newly installed lining.
If newly installed castable cannot be dried promptly, cover it with plastic film or apply an anti-penetration organic coating to the new lining surface, then apply a second coat after 24 hours of curing, to stop moisture carrying alkali to the surface. The coating should be colored to distinguish coated from uncoated areas; its ignition point is generally around 150°C, so it burns off completely when the equipment is in operation.
3. Address it in the castable itself.
Use low-alkali refractory raw materials and increase the castable's viscosity and hardening speed to slow migration of alkali metal oxides. Strengthen quality control as well, for example by strictly controlling water addition during installation to avoid excess water.
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