Build log - masonry rocket oven

I'm starting this thread here as a journal for my masonry rocket oven build.  Here are the list of materials I'm using:'

About 120 dense firebricks that someone on Craigslist gave me for free

A bucket of pure clay soil I dug out of the ground near a natural lake

About 75 red bricks (again, from Craigslist)

About 3/4 of a bag of Type S masonry mix that was sitting in my house's shed from the previous owner

A bunch of boulders laying around my property

Construction began last weekend. I started in the morning and dug a trench about 2' x 4' and 2' feet deep. For the area in NJ where I live, I should have gone down 3', but oh well. Let's see if it fails. As we dug out the dirt, we screened it and used the gravel to fill and inch or two at the bottom of the trench for drainage. Then we filled with the boulders.

Next step will be the concrete pad. I plan to mix the bit of mortar mix I have with some dirt to make a dirt cement. Will that be strong enough? I need something that I can pour over the rough surface of the rocks.

Andrew,

Mortar Type S has cement to lime to sand ratio: 1 : 0.5 : 4.5, so in the mix 1/6th is the cement. So in case you want to make dirt amended with 10% cement you would need to use 10 part dirt 6 part S Type mix. For 5% stabilization you would use 10 parts dirt and 3 parts S Type.
10% cement in soil makes quite strong blocks and they also last. I have made some 10 years ago and they survived on the ground all the moisture, rains and some flooding of the wet season.
Because of the cement in the block there is possibility that they will be wicking moisture up to your oven's base, and the insulation usually can not cope with moisture, but I would not worry about it, especially if this is your test bench.

Cristobal Cristo wrote:Andrew,

Mortar Type S has cement to lime to sand ratio: 1 : 0.5 : 4.5, so in the mix 1/6th is the cement. So in case you want to make dirt amended with 10% cement you would need to use 10 part dirt 6 part S Type mix. For 5% stabilization you would use 10 parts dirt and 3 parts S Type.

That’s a good point. I was planning to do 10-1, but you’re right, it should be 10-6. This will be my test bed initially, but if it’s strong enough I might just build with mortar here. I’ll see how it goes.

When you say 10 parts dirt to 6 parts mortar mix, is that by volume or weight? If volume, is there a good way to measure the volume? Just buckets I guess.

The specific weight of mineral materials used for mortars are similar so I use volume. Sand is the heaviest, gravel in the middle then cement and lime at the end.
When making a designated strength concrete I have recipes that use weight, so I measured the specific ratios of my materials and still converted ratios to volume.
I recommend to keep the notes and write down all the ratios of materials you use. It helps a lot if you need to extend or repair something in a few years.

A quick update here. I poured my soil-concrete pad last Saturday with some mortar mix I had lying around. The pad is okay. I mixed the soil-concrete in two batches. The first one seemed really wet, so in the second mix I lowered the water by a lot.

After 5 days, both sides are certainly firm enough to walk on. On the side that had the more wet mix, the surface is a bit spongy and when I walk on it, my shoe makes a very small impression (maybe about 1/16" deep or so). One the side with the drier mix, my shoe doesn't make an impression at all. The dry side is quite hard, but I can still scratch away the surface with my fingernail.

Does this sound right? Will this be hard enough to build on?

I should have mentioned before that you should make it only slightly wet. It's the same for cement based concrete. Frequently people add too much water to make it more mixable lowering the final strength. Soil concretes are less predictable than regular concretes, because almost every soil is different in some way. I would give it more time to cure. Regular concrete should reach 90% strength in 3 weeks. Since the soil concrete is weaker, waiting more can result in noticeable improvement. Also covering may help as it will create an environment where warm steam will accelerate curing.
How thick is your slab?

I think you actually did mention it, and I just overlooked your comment. This is how learning occurs. One good thing is that I do have it covered with a tarp. The slab has a variable thickness because it's poured on top of the stones I laid in the hole. On average it is probably about 3 inches I'd guess.

So, I'll wait another two weeks and see if it hardens up sufficiently. How will I know if it is hard enough to build on? Any details on texture, hardness ,etc.? I'm guessing if it's spongy, it's just bad and needs to be dug up.

Also, if the concrete ends up not being hard enough in the end and I need to repour - any tips on getting the mix right? Should I be able form it into a ball? Should it be crumbly? And how do I dispose of the old pour?

A lot of questions here, so thanks!

Certainly it should not be spongy. It may be spongy due to higher clay contents. I just realized now that we did not account for adding more aggregate from the mortar mix, calculating it like only cement was added, so to make 10% cement mix the ratio should be 6 parts mortar+ 4 parts dirt. I would still wait, because different soils react differently to cement and lime. If yours has pozzolanic particles they may additionally react with lime in the mortar.