Abrasion In Refractory
As a matter of review, refractory concretes/castables are blends of refractory aggregates in a bonding matrix. The matrix includes the cement binder as well as admixtures and fillers that reduce water requirements and other ingredients to control set and flow.
This sketch shows the coarser aggregate in the fine light-colored matrix. What do we need for a good resistant? One feature would be lots of bonds. 40 years ago, before the widespread use of low-cement castables, abrasion resistance castables contained lots of cement, in some cases up to 30%. What else do we need?
Good attachment between the binder and the aggregate, or in chemical terms, good intermolecular bonds. We touched on phosphate-bonded refractories earlier, and good intermolecular attachment is a characteristic of the alumina-phosphate bond, so it is not surprising that phosphate-bonded alumina bricks have great abrasion resistance.
The plate on the right shows a typical result of an ASTM C-704 test on a conventional, high-cement content 40% alumina fireclay castable. A fairly large cavity was eroded by the grit.
Note that the aggregate was not standing very proud, with the aggregate and the cement matrix about evenly eroded, giving a value of 20 cc’s loss.
Let’s evaluate castables with more abrasion-resistant aggregates than 40% alumina fireclay. We’ll select corundum with a Mohs hardness of 9 and then follow up with silicon carbide at about 9.5 Mohs hardness. This is the specimen plate of tabular alumina, also known as corundum, a high-cement content conventional castable. This material is widely recognized as having abrasion resistant properties.
Above to the left, you see that the corundum aggregate is standing proud, but the matrix is eroded. In this type of castable, the cement is not well bonded to the alumina aggregate. The result is 10 cc’s loss.
If we change the castable type to a low-cement castable, and we use an even harder aggregate, silicon carbide, we get better C-704 results, above to the right. Low cement castables are engineered for a closer particle to particle packing, shorter cement bonds, and lower water additions.
The silicon carbide aggregate is again standing proud, the matrix is somewhat eroded, however, less so than in the conventional high-cement content castable. Our result is 6 cc’s loss, which is really good. Low cement castables commonly contain water-reducing additives which increase the strength of the matrix.
The result is less matrix removed by the grit in the test. This is an important point to digress on. Tempering water content. This table shows water contents required for three castables, first, a conventional high-cement content castable. It requires a lot of water, 12% to 14%, for casting. You need a lot of water to hydrate all that cement.
Second is a low cement castable, a material with perhaps 5% to 10% cement, and the water is about half that required for the Conventional Castable. Simply put, there is less cement for the water to hydrate. Less water at 6.7% is needed for pumping because the low cement contains a water-reducing additive. The last product is an extreme service castable. For vibration casting the extreme service takes only 4.3% water by weight, about one- third of the Conventional’s amount.
We saw this abrasion plate of a conventional high cement castable before, with a fairly large cavity eroded for the conventional castable.
This photo compares the abrasion plates for the Extreme Service on the left at about 4 cc’s, and the pumpable low cement castable at about 9 cc’s.
Reduced water content gives better abrasion resistance. The water addition for mixing has the single greatest impact on the abrasion resistance of the things that you can control. If you have a particularly abrasive application, the best installation method is vibration casting, using as little water as you can.
Minimum water content means that the castable material will be pretty stiff and only flow under vibration.