Would this mortar work?

[dbhansen]

I'm just curious if anyone sees a problem with using "Mt. Savage Waterproof Hi-Temp Mortar" for a WFO. The specs seem similar to RefMix, except the Alumina is 11% higher and Silica is 9% lower. Would I notice the difference?

Max. Service Temp: 2400 deg. F
Modulus of Rupture: 500-700 psi
Silica: 41.2%
Alumina: 40.9%
Iron Oxide: 4.7%
Lime: 10%
P.C.E. (whatever that is): 12

The sales rep is looking into how fast it sets, but the specs say to only mix enough as can be used in 45 minutes or less.

[dbhansen]

Bumping this up. Anyone?

[Ed_]

I'm not in a position to tell you if it would work, but... PCE might be a coefficient of expansion, which (I think) is often given in units of 10^-6/K. That is to say, the expansion would be 12 parts per million per degree K of temp increase. This is similar to concrete. Not sure how it compares to firebrick, but you'd want the mortar and brick to have reasonably well matched expansion coefficients to avoid cracking.

Where's dmun?

[dmun]

Where's dmun?

I'm here, but I don't know the answer. I haven't heard about this brand. I'm good at remembering what's worked before, but not so good on predicting the future, especially since I'm allergic to reading material data sheets.

I looked for the MDS for heat-stop so you could compare them, but I can't find it on line.

[dbhansen]

I appreciate the input, thanks. Good idea to compare it to HeatStop, if I can find the specs.

[gjbingham]

Ed,
I agree with your thoughts on trying to match the coefficients of expansion of the two different materials. I didn't even consider it during my build. In theory (perhaps a half-assed theory), if the two materials had identical CEs, there would be no cracking.

[Mojoe]

That type of mortar is used in more of a continuous duty industrial type setting...blast furnaces, coke ovens, cement kilns. I agree with Ed in that you want a close match to your brick which will be around 30% silica and 15% alumina? (don't remember alumina content but its much lower than silica). I would think your joints would get hotter than the brick and, in turn, cool down slower than the brick which over time would lead to premature failure...brick falling out. These types of mortars also typically have strict curing schedules at a set temperature which may be hard to hold with your oven.
Unless you have a contact, this type of material is also typically very expensive.

[Ed_]

Well, matching coefficients of thermal expansion would (at least in theory) prevent cracking due to brick and mortar expanding. I suspect that most firebrick and most refractory mortars will be reasonably close (but I could be very wrong).

I get the impression, though, that most of the cracking takes place during the dessication of the mortar. I guess I assumed it was the result of too much water expanding to steam too quickly inside the mortar. My (possibly hair-brained) theory was that if you had the patience to let the dome cure for a month, then gently heated it to 150-200C and held it there for maybe 8-12 hours, you'd be unlikely to see any serious cracking.

Not that most people have that kind of patience.

[dbhansen]

Here's what I learned about Heat Stop 50, from the manufacturer:

40% alumina
50% silica
2% iron oxide

According to the FB plans, low-duty firebrick is roughly 30% alumina and 50% silica, the same as the RefMix mortar.

fyi, Mt. Savage recommended their mortar (see post #1) with a (albeit high-duty) fire brick that is 35% alumina and 58% silica, with a PCE of 31.5. The PCE test is based on "ASTM C24-01(2006) Standard Test Method for Pyrometric Cone Equivalent (PCE) of Fireclay and High Alumina Refractory Materials."

That's enough mumbo-jumbo for me! I guess I'll just have to keep looking for a source for HeatStop 50.

[Mojoe]

Don't forget to check the Mortar primer sticky under this board. I opted for the do-it-yourself fireclay mortar and haven't experienced any problems since construction 8 months ago.

As for the patience, I agree Ed as I could only resist the urge for 3 weeks until I struck the first match.