The origin of back alkali phenomenon
In a broad sense, refractory castables refer to hydraulic refractory materials, which are composed of refractory particle base materials, powder materials and other additives with different properties. After adding water and stirring, they are unshaped products with a certain hardness and strength. During construction and use, vibrating rods are required for mold casting. It is a refractory material widely used in high-temperature industries. If the refractory castable after the construction is completed and the formwork is removed is not baked in time, during the placement process, long white hairs may appear on the surface, and then sanding and pulverization will gradually appear. Then you can clearly feel the sand drop by touching it with your hands. This phenomenon is called alkali return phenomenon, and it is easier to occur in summer. If measures are not taken in time, the quality of the construction body will be greatly affected in the future.
The cause of reversion phenomenon
There are many reasons for returning to alkali, and personal analysis believes that it is mainly reflected in the following aspects:
01
Influence of Alkaline Substances in Raw Materials
The raw materials prepared by refractory castables contain alkali metal oxides such as Na, K, Fe2O3, etc., and these alkali metal oxides form coagulation and hardening after being combined with water and stirred, forming free water floating on the surface of the water, with loose colloidal white precipitates. This phenomenon is due to the saturated solution formed after the unhydrated cement particles on the surface meet water, followed by the precipitation of aluminum glue and calcium aluminate hydrate, resulting in the reaction of alkali metal oxides with carbon dioxide in the air. Such as refractory aggregate, refractory powder, calcium aluminate cement as a binder in refractory castable raw materials, various microsilica fumes and additives, water reducing agents, coagulants, explosion-proof agents, etc. These materials often contain a small amount of alkali metal oxides, and some of the alkali contained therein can be dissolved in water, especially the alkali in the dispersant, which is almost completely dissolved in water. And these alkali metals dissolved in water can migrate to the surface of the hardened body through the capillary pores, and then react with CO2 in the air to form carbonate to produce alkali and expand after reaching the surface. The main chemical reaction equation is:
Diffusion and reaction of Na+ ions inside the castable system:
Na2O+H2O → 2Na+ + 2OH-2Na++2OH-+CO2+H2O → Na2CO3.xH2O
Carbonates continue to react with the hydration products of aluminate cement:
Na2CO3+CaH10 → CaCO3+Na2O.AL2O3+10H2O
The sodium chlorate formed by the above reaction will continue to react in humid air to regenerate sodium carbonate:
CO2+Na2O.AL2O3 → Na2CO3 + AL2O3
02
Porosity and Pore Size
In the 1970s and 1980s, Lafarge discovered through a large number of studies on castables based on alumina cement that the porosity of castables is one of the main factors affecting alkalization. The relationship between porosity and water-cement ratio has confirmed that the porosity of the binder phase is very important.
03
environment humidity
The following is the effect of different ambient humidity on the pulverization of lightweight castables based on alumina cement bonding. Obviously, the greater the ambient humidity, the more obvious the alkalization phenomenon.
