diane
New Member
Posts: 2
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Post by diane on Sept 11, 2020 0:13:37 GMT -8
Hi all,
After reading a lot about the topic, we're finally going to actually build a batch rocket (180 mm system). The plan was to start the brickwork today, but after reading about cracks due to high rigidity, I started to question the mortar I want to use.
It will be a double skinned system, I bought pre-mixed refractory mortar for the firechamber and upper part of the bell (from the riser up). For the rest of the bell I have regular bricks and regular cement containing an unknown percentage of lime and some additives (sand and water to be added). I could get the cement for free, so I prefer to use it instead of buying something else...
I'm mostly worried about the lower part of the inner wall, the temperature will still be quite high there and I wonder if the mortar will be flexible enough? It's used for 'normal' outdoor masonry work, so it does have a certain flexibility, I'm just not sure if it's sufficient for this purpose. What do you think?
Thanks in advance, I'm really grateful for the vast amount of information and experience here
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dcp
New Member
Posts: 35
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Post by dcp on Sept 11, 2020 1:11:52 GMT -8
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Post by peterberg on Sept 11, 2020 2:40:55 GMT -8
As far as rigidity is concerned, nothing is more flexible than clay/sand mortar. And it's not agressive to your skin like lime and cement are, plus it is relatively speaking a pleasure to work with.
That said, I used a weak cement/lime/sand mortar a number of instances, proportions approximately 1 cement, 5 lime and 20 sand. When mixed, it feels like a wheelbarrow of sand, a small bucket of lime and a trowel of cement. As far as I know those heaters are still in use without major leaks.
The lime is the actual hydraulic component here, it is hardening quite slow so the cement is there to stiffen up the mortar enough so one can keep building without sagging of the masonry. How it feels while laying bricks is largely depending on the sand, when this is very fine there should be less in the mortar otherwise it feels like laying bricks using wet sand.
So Diane, you could use that ready cement mortar adding more lime and lots of mason's sand. When you are unsure about this, start the double skin some layers (2 or 3) lower than the riser. Only the inner skin is done with refractory mortar, outer skin the same as the lower portion of the bell. The mortar is there to separate the bricks and middle out uneven surfaces, it's NOT there to create a strong bond like glue with the bricks.
NB: I overlooked the fact this is your first post, so welcome to the boards!
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diane
New Member
Posts: 2
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Post by diane on Sept 11, 2020 5:01:16 GMT -8
Thanks for both of your answers. I have enough firebricks to start 2 layers below the riser top, so will do that. By the way, the bell will be completely double skinned, not only the top part. I don't mind if it's very slow, our house is very well insulated so it's an advantage that it provides a slow but steady heat supply.
The clay/sand mixture is indeed probably the best option, but the cement is already here, so I'm gonna go with that for the outer skin, with some added lime (probably 1:1:6 cement, lime, sand). For the lower part of the inner wall, I'll probably make a lime mortar with some cement as you suggested Peter (thanks for your great website by the way!).
I came across more examples where people successfully used cement containing mortars, so I'm confident that this can work, as long as lime is added as well.
When we really start, I'll post some progress pictures!
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