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Alkali resistant castables

HASLE have a long tradition for producing highly alkali resistant refractory castables.


Article By: Kjartan Butzbach  - kbu@hasle-refractories.com
Product manager - Head of R&D | February 2019


Alkalies like potassium (K) and sodium (Na) are something we often hear about as a threat to the life of refractory linings. Managing alkalies and how to defend against alkali attack is an ongoing battle in many industries.

Alkalies are present in a wide array of fuels and raw materials used in high temperature industries. Alkalies are nutrients for all plant life, and therefore materials that “used to be plants” like e.g. oil and coal will have a certain alkali content. RDF (Refuse Derived Fuel) will typically also contain alkalies as will some raw materials like clay and lime.

An interesting feature when discussing alkali attack and how to counter it is that it goes against one of the “old school” perceptions of refractories saying that the more alumina (Al2O3) in a castable the better. While a high alumina content brings good refractoriness (ability to withstand heat), the alumina also reacts willingly with alkalis, resulting in a decrease in alkali resistance.

Alkali resistance shown as function of allumina content.

What happens when alkalies react with alumnina?

When alkalies react with aluminasilicates (especially mullite), it results in formation of feldspars that have a higher volume than the original minerals. This in turn leads to expansion resulting in fracturing known as alkali-spalling.

Fracturing known as alkali-spalling.

The way HASLE counter this is a through a twin approach.


The “composition approach”; by keeping a relatively low alumina content in the castables, the alkalies have less material to react with, and thereby the impact of alkali attack will be less.
Further to the “composition approach” to alkali resistant castables described above, HASLE also takes a “physical approach” to mitigate alkali- and other chemical attacks. This is done through optimizing PSD (Particle Size Distribution) to obtain low open porosities in the 10-12% range. The low porosity makes it harder for harmful compounds to penetrate and attack the castable from within.
“Cup tests” performed on HASLE alkali resistant castables D59A. The D59A cup was fully intact after the test and the edges on the cup sharp. On the photo it has been cut in half to examine penetrations pattern (no penetration or discoloration seen).
 “Cup tests” performed on HASLE alkali resistant castables D59A.

The approach to alkali resistance makes HASLE Castables a premium choice


The effective combined approach to alkali resistance makes HASLE castable a good choice for high alkali environments in applications ranging from small household boilers over waste-to-energy district heating to heavy industry like cement factories.

Cup test on a “normal” castable that fractured in 4 pieces due to alkali spalling.
HASLE offer monolithics in vibro-casting, easy-flow and gunning versions as well as pre-cast solutions with excellent alkali resistance.

Installations all over the world

HASLE Castables are used with very good references  in a variety of Industries throughout the world, among these in Cement factories, Boilers and Incineration Plants. 

› See also our Case Stories and References 

HASLE look forward to becoming part of the solution to your alkali problems. We are happy to offer advice and technical sparring through our international sales network. Feel free to contact HASLE for a noncommittal visit or recommendation.

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