Queston about homemade mortar

[RCon57]

Hello,

I'm new to the forum and starting my oven build. Your response is greatly appreciated.

I plan to mix a mortar consisting of the appropriate ratios of the following materials:

1. Spec mix (a premix blend of Portland cement, lime, sand)
2. Fireclay (per FB instructions)
3. GIRtech Contact DRYMIX Refractory Mortar. This is a dry refractory mortar mixture of fireclay materials and kaolin (different kinds of fireclay, chamotte and aluminosilicate bricks). According to Wikipedia chamotte is has a high percentage of silica and alumina. This dry mix only needs to be mixed with water, implying that there is already a binder within the ingredients.

The reason I am not buying the FB mortar is that I already have much of these materials on hand from a previous project. I want to add the GIRtech dry mix as a means to increase the temperature rating and boost the temperature cycle time longevity. My question is as follows:

Does anyone know of any incompatibility issues between these three materials or more specifically are there any known adverse reactions to combining difference binders (Portland cement and Chamotte)?

Best regards and thank you in advance.

[david s]

The chamotte and fire clay are not reactive and won’t cause any issues, however the mixture of calcium aluminate cement, the reactive ingredient in the refractory mortar with the lime in the premix mortar is likely to act as an accelerant, reducing the already short working time of the refractory mortar. Measure out a small amount, do a test and see how it goes, but I don’t think it will be too successful.

[RCon57]

david s,

I kindly thank you for your prompt reply. Your information in very helpful and I will adjust my mixtures and batch sizes to accommodate for the accelerant. Looking forward to sharing my results with you and the forum.

[david s]

david s,

I kindly thank you for your prompt reply. Your information in very helpful and I will adjust my mixtures and batch sizes to accommodate for the accelerant. Looking forward to sharing my results with you and the forum.

The high temperature aggregates in the refractory mortar instead of sand are overkill because our ovens are not operating in very high temperatures (1200C+). At very high temperatures sand can turn to glass, but this will not occur at the temperature range we use. The lime in the premix if used in conjunction withe calcium aluminate cement will result in the mortar going off so fast that it will quickly become unusable. Unlike portland cement mortars which can be re hydrated once, the calcium aluminate can't. Once it begins to harden it should be discarded.
The beauty of the home-brew mortar 4:1:1:1 sand, portland cement, hydrated lime and powdered clay is that it has a decent working time as well as being adequate for our temperature range, in addition to being cheap. It has proved itself suitable for many years and many ovens. By all means do your own tests, but I think you'll find the problems detailed will lead you back to the homebrew.

[Timothy_W]

The main question is how this mixture will withstand high temperatures. Some materials are absolutely not intended for use in the creation of stoves, baths, saunas, and other objects where the temperature is high.

[david s]

The temperatures we fire to are not considered high. The refractory cement is designed for service temperatures in excess of 1200 C. The problem is the incompatibility of Portland cement, lime and calcium aluminate cement as previously explained.

[RCon57]

Thank you again. Sounds like I don't need to risk making an unstable mixture in an attempt to make a higher temperature rated mortar when a higher temp mortar is overkill anyway. Thank you for your sound advice.