Mortar Combat?

[zekebv]

Greetings all - I'm looking for some clarification on the home brew recipe.

I've been studying several build threads, thought I was clear, but just finished a lengthy discussion on mortar that talked about 6-1-1-1 and 3-1-1 so now I'm thoroughly perplexed.

I'd be grateful for clarification on this!

[Les]

Re: Mortar Combat?

Greetings all - I'm looking for some clarification on the home brew recipe.

I've been studying several build threads, thought I was clear, but just finished a lengthy discussion on mortar that talked about 6-1-1-1 and 3-1-1 so now I'm thoroughly perplexed.

I'd be grateful for clarification on this!

No clue where you were reading. Have you downloaded the FREE plans - everything is pretty well documented (within reason)

I think the "roll your own" mix is one part fireclay, one part portland cement, one part lime and three parts sand, sorry it's been a while...

[stonecutter]

Re: Mortar Combat?

I'd be grateful for clarification on this!

I know the thread you mean.....the one where we discussed how rich the 3:1:1:1 is. To simplify...

3:1:1:1 is a rich mix, meaning there is a high binder to aggregate ratio. 6:1:1:1 is closer to a traditional ratio, with a higher aggregate ratio...(leaner). Mixes like this tend to have less shrink cracking.

Stay somewhere between these ranges for a Pompeii build and you'll be fine. Remember that no matter what ratio you use for this mix, a slow cure means a stronger mortar. Keep it covered after finishing..draping a damp drop cloth over the work then covering with plastic works great.

[Tscarborough]

Re: Mortar Combat?

As shown in that thread, either will work fine. Richer is easier to work with, leaner is (in my opinion and depending upon the aggregate used) better from a structural standpoint.

[stonecutter]

Re: Mortar Combat?

......leaner is (in my opinion and depending upon the aggregate used) better from a structural standpoint.

And that would be a professional opinion, which I also agree with 100%

[Tscarborough]

Re: Mortar Combat?

And just to clarify my reasoning on the matter, let me explain what a "mortar" is and what what it is designed to do.

Mortar is used to fill the gaps between imperfect masonry units and allow them to be laid in a uniform pattern. As such, it can be said to hold the units apart, not together. That is not to say that it should not have an excellent bond to the masonry units, but it does indicate that it should never have a compressive strength greater than the units being laid.

The mix design of a mortar should allow for a matrix of aggregates graded such that there is a minimal amount of space between particles while allowing each particle to be coated with the cementious material used. Too rich, and the particles are separated by too much cementious material, too little and they are not fully coated.

Mortar strength does not rely strictly on how much cementious material is used, the aggregate will dictate the greater part of the strength. Gradation is the key, and it is also related to the anticipated joint size. Bigger joints=larger aggregate (but the gradation ratios will remain constant to allow for the point made in para 3).

When using a portland cement based mix, it can be assumed that the portland cement portion of the mix will fail shortly after curing. It is there as a gauging agent, that is, to provide a rapid initial set. Therefore, simple logic would assume that the more of it there is, the worse the degradation.

The less there is, the better. So why not decrease the portland component to a formula: 1/2 portland, 1 lime/1 fireclay/5 sand? You could and it would work just fine if not better, but it would be harder to work with. Typical lime mortars and stucco use less than 5% gauging material as an example, but they are not refractory.

[Tscarborough]

Re: Mortar Combat?

One last note. The structural requirements for a hemispherical dome are so low you could use almost anything that would hold together long enough to construct the dome and not powder off under firing to be sufficient. Cow S@#% and sand would work just fine.

[ATK406]

Re: Mortar Combat?

Hello Zekebv,
You need no more information than has been provided above. I know it is not in keeping with the concept of an open forum, but you have gotten as clear a definition of the mortar requirements as you are ever going to get from a couple of the most respected members of this site. If there were a way to freeze a thread at the point when no further comment was necessary it is now...err before the start of this post. Good Luck on the rest of your build and disregard everything else that is added to this thread (IMHO)...sorry guys.

Best Regards,
AT

[Les]

Re: Mortar Combat?

Good Luck on the rest of your build and disregard everything else that is added to this thread (IMHO)...sorry guys.

I have a vision of a guy looking for a cow that shits sand Zek - it's pretty forgiving, don't fret.

[zekebv]

Re: Mortar Combat?

Aahh, the fog is lifting and I see light peeking through. Gentlemen, thanks very much for the additional explanation.

One last question - in his build, Aceves mentioned mixing a stickier mortar for the later courses by increasing the amount of fireclay. Any thoughts on this?

[david s]

Re: Mortar Combat?

If you increase the clay content you increase the amount of shrinkage and hence the propensity to shrinkage cracks.

[zekebv]

Re: Mortar Combat?

Thanks - I'm not going to monkey with the proven recipe!

[zekebv]

Re: Mortar Combat?

One last mortar question, I hope, regarding sand - what is preferable, fine, medium or coarse? Thanks.

[Tscarborough]

Re: Mortar Combat?

It depends upon the anticipated joint size. For normal masonry (1/4 to 3/8" joints) it is regulated by ASTM C-144. For fireplaces the aggregate is very fine, but for oven that have both large and small joints, masonry sand or play sand or #4 blast sand will all work just fine. Concrete sand/bedding sand/plaster sand will be too coarse.

[zekebv]

Re: Mortar Combat?

Somewhere on one of the posts I read #12 silica sand, but then I've also seen reference to fine sand, which I'm guessing is the play sand. I looked at #12 sand at the rockery and it looks pretty course to me. So I'll check check out the masonry, play or #4 blast sand.

Thanks for the quick feedback.