Didomur 1- Dry Mix Mortar - is this ok?

[Michael Thomas]

I have a supply of mortar with these specs (see attached). The mortar is Didomur 1 and is manufactured by RHI. I've attached the specs. Can someone please advise whether this is appropriate for my oven dome?

The mortar I have access to has been kept dry for a few years.

Michael



Hi,

[mongota]

I have a supply of mortar with these specs (see attached). The mortar is Didomur 1 and is manufactured by RHI. I've attached the specs. Can someone please advise whether this is appropriate for my oven dome?

The mortar I have access to has been kept dry for a few years.

Michael
Hi,

I'm not at all familiar with Didomur 1, but I've always given powdered mortar or any bagged portland cement product a general shelf life of around 6 months. Over time, even in a sealed bag, the cement can undergo some hydration simply due to ambient atmospheric moisture.

If you break open the bag and it's still a very fine dry powder that flows easily, it could be fine. But if it is clumpy, personally I'd not use it. It might have undergone partial hydration in the bag, and the resulting brick-to-brick bond may be less than advertised. The good news? WFO construction, everything is on compression. The weight of the dome will keep everything in place. A potential bad news is that if the mortar is weak, it may suffer at the high temperatures seen within the oven, and it may powder out of the joints over time.

Again, I know a fair amount about portland cement, but not a whole lot about refractory or ceramic type mortars, and I am not familiar with Didomur 1 itself. So take my comments not as gospel, but more as simply something to be taken in to consideration. And my comments are directed more towards the age of the product versus if the product is appropriate for a WFO build.

Sorry I can't be of more specific help.

[Michael Thomas]

Thanks so much Mongota, thanks for your advice on this; very helpful tip. I'm going to have a very thorough look through the mortar mix to ensure that there has been no atmospheric absorption.

[Neil.B]

Agree with Mongota regarding shelf life.
The mortar is one of the cheaper parts of the build...is it worth taking the chance?

[Michael Thomas]

Totally, thanks for seconding that. I've just made connection with a mason who will look at the mortar and confirm it's ok to use. He's got no connection to the seller so unbiased. I ended up getting some emails back from the company who made it and everything seems good according to them. The composition matches my bricks and its mortar that can be used for reheating (ie pizza oven or fire place).

thanks again for the conscientious responses. Looking forward to getting my dome started!

[david s]

If it’s years old I wouldn’t use it. Calcium aluminate cements are renowned for going off Keeping them dry is of utmost importance but protecting it from high relative humidity is harder.Most of the calcium aluminate mortars recommend thin mortar joints only. As a brick oven requires joints of varying thickness it may be an unsuitable material, you may like to check this requirement.
In oven builders terms the mortar requirements are to a max temp of around 600 C. You may record a higher temp with an IR temp gun but it’s only the surface temp it’s reading. One inch deep the temp will be much lower.
Calcium aluminate cements are good for around 500C, north of that the temperature itself drives the bonding of the materials to make them permanent. This bonding strength increases from 573C to 1000C The most vulnerable range due to varying thermal expansion rates of the materials used is 500C - 650C where temp rise must be controlled and down to 50C/hr. Unfortunately this is not possible in a WFO so don’t attempt it. You are likely to end up producing micro cracking throughout the material. Beyond 1000C the material becomes harder primarily due to the eutectic (turning into glass) qualities of the materials. The high temperature aggression use in the product the OP provided allow it a max service temp of 1400C. As this is ridiculously high for a WFO, these high temp aggretes are simply a waste of money.
In practice the home brew has proven to be a far more suitable mortar, far cheaper and far more user friendly.
Portland cement begins to break down north of 300c
lime begins to fail north of 500C and the clay(depending on type use) good for anything up to 1110C at least.
So the homebrew relies on at least two, perhaps three cementious systems and explains the high proportion of cement, lime and clay used in the mix. Where one may fail the next takes over. Ther may be a case for using a calcium aluminate based mortar in the upper 1/3 of the dome. But there have been no reports of failure here for homebrew users, so I believe it’s unnecessary.
I hope this information is useful for builders.

[mongota]

Great info from David.

As an aside, how timely is this?

I had a pallet of portland delivered earlier this week. This is basic portland cement, so not the same as yours. But I was out yesterday working on a project and this was I think the 4th bag I opened. This shows hydration having occurred within the bag. Sometimes these chunks powder easily in the hand, the ones in this bag are pretty hard. This is an example of cement that I would NOT use to mix mortar that would be used to set brick or CMU, nor would I use it to make homebrew mortar for a WFO.

Again, this is NOT the same type of product you have. I just thought it odd that the day this topic came up on the forum, it also came up in my back yard. lol

[mongota]

Portland cement begins to break down north of 300c
lime begins to fail north of 500C and the clay(depending on type use) good for anything up to 1110C at least.
So the homebrew relies on at least two, perhaps three cementious systems and explains the high proportion of cement, lime and clay used in the mix. Where one may fail the next takes over.

TERRIFIC info. Thanks for that explanation.

[Michael Thomas]

Thanks Mongo and David s. Good info; will be keeping it in my spreadsheet. I think you've highlighted a good contrast Mongo. I suppose there is a compact clump formed through mechanical adhesion and a reacted clump through chemical adhesion.

[Michael Thomas]

Most of the calcium aluminate mortars recommend thin mortar joints only. As a brick oven requires joints of varying thickness it may be an unsuitable material, you may like to check this requirement.

What is the minimum thickness that you would recommend? Why varying thickness and is there a place where the joints should be thicker or thinner? I was hoping to have thinner mortar joints.

[Michael Thomas]

For anyone following this post. The mortar was tested and it is not suitable despite being dry with no apparent clumps. It deteriorated after a week, while appearing fine on mix, fine on hardening, and looking fine after 4-5 days.

Thanks for the words of caution.

[mongota]

For anyone following this post. The mortar was tested and it is not suitable despite being dry with no apparent clumps. It deteriorated after a week, while appearing fine on mix, fine on hardening, and looking fine after 4-5 days.

Thanks for the words of caution.

Huge credit to you for taking the time to test it prior to implementing it.

[Neil.B]

Glad you found out now, rather than after you built your oven.

[david s]

A higher temperature rated and more expensive mortar does not make it more suitable, especially for our application. Some of the air set mortars designed for high temperatures (>1000C) require careful firing to service temperature to make the materials permanent. Failure to do this results in a failed mortar. As previously posted it is not possible to do this for a WFO where controlled temperature rise is not possible.
Also of interest is that calcium aluminate cement begins to fail north of 500C also. The key word here being “begins”, it’s probably still ok to 600C (can’t give you a figure) but explains why the homebrew is a superior performer. Regarding mortar joint thickness, as the homebrew proves to be a suitable castable joint thickness is no issue for it.