Can I use gravel as an alternative to sand in my mortar?

Good morning everyone
I wanted to hop on and ask, you may have guessed, "Can I use gravel as an alternative to sand in my mortar?". Here is the situation; my partner and I are building a stone stem wall. The wall sits 16 inches below grade, we are using field stone off of our 'property'. This stem wall is a 30 foot diameter circle, inside the wall we will be tamping in road base (gravel and rock sized 6 inch minus) to raise the floor up. We will be installing an earthen floor so we inevitably will need sand. Whether I buy that sand, or find that sand is a problem for September Amber.

August Amber is trying to avoid buying sand, because we are flat broke. I have already purchased the road base, and have some extra to spare. Can I use some of my road base as a replacement for the sand in my mortar, if I make sure to sift the larger rocks out? I was going to be making a mix of 1 part portland cement, 1/2 part hydrated lime, 2-ish parts sand as needed, in an attempt to make type s mortar for below grade applications. I know when making cement people typically use just portland cement and gravel, but I didn't know if gravel would be applicable for my situation.

Thanks for any advice!

In theory this could maybe work, but in practice, I think it would be a really bad idea. If the road-base rock you have is indeed 6 inch minus (which is what road beds are generally made out of) then you have a mixture of fractured rock, gravel, sand, rock dust, and a bit of dirt that was all able to spill through a 6" grizzly at the rock pit. To call the fines in there "sand" is sort of generous - it is rock powder in various sizes. You will need to sift it into a very narrow range of sizes in order to make a mortar that is workable, and that is going to be a LOT of work.

Mason sand is what you want. If you have a way to go pick it up, you can usually get it by the yard for about 40 bucks at landscape places. Bear in mind, a cubic yard weighs over a ton, so if you dont have a trailer or an F450, just get a half yard at a time and spare your brakes.

One other note - concrete is not made with gravel either. Gravel has sharp edges because it is freshly crushed. Concrete is made with Aggregate - Coarse and Fine. Fine aggregate is just sand, Coarse is generally 3/8" to 3/4" pea-gravel. The rounded edges makes it a lot easier to tamp it into forms, and get a nice finish on flatwork. It is possible to make concrete with gravel, but it must be sifted (at which point you call it Ballast). I have done it in a pinch, and it is a pain to work with.

What do you think about us using granite dust in place of the sand? We have a lot of stone dust from our well being drilled, it is pretty uniform and extremely fine

Sometimes you can find the naturally occuring deposits of sand.
Maybe ask around as see if there is any.
Otherwise, can you learn bartering to get some?
I dont think dust will work, the sand works because of its mechanical properties.

The well is dug now, did you think of catching rainwater as an alternative?

I think granite dust could work, actually, if it is clean enough. Its basically manufactured sand, and if it is fine then it should make a workable mortar when you mix it up. You do not really want to include any silt or clay in your mortar, so you might have to end up washing it. Put a handful of it in a glass jar, fill it up with water and give it a shake. If the "sand" all falls to the bottom and the water is fairly clear - you are golden. If it turns all cloudy and muddy, and stays that way for more than 30 minutes or so. there might be too many fines. This article about sand claims max 4% silt/clay for masonry work.

https://www.diydoctor.org.uk/projects/types-of-sand.htm

If the stuff you have is dirty, I would just save it for the floor you were talking about. Is there really no landscape supply place near you? A lot of them let people come out and get 5 gallon buckets of material for a couple bucks. If you can get ahold of portland cement locally, I can guarantee you that sand is available somewhere too. And at 12-15$ per cubic foot, the portland is going to eat up your budget a whole lot faster than the sand. A 30 foot diameter circle is like 94 feet in circumference - that is going to use a lot of mortar with irregular field stone. Here is a calculator for mortar needed for for placing bricks.

https://www.inchcalculator.com/block-mortar-calculator/

Assuming bricks are about 8" x 4" x 2" then a wall 94' long, 8 inches thick, and 16" high would need about 2250 bricks. That would mean you will need around 1.3 cubic yards of sand. Using rocks that are bigger than bricks will help cut that down, but unless you spend a lot of time finding rocks that fit nicely together, I think you will end up using a lot more mortar than bricks would need.

Anyway, best of luck, and post some pics when your project gets going!

I can agree with Carl if the dust is not only powdery fines that can blow away.
I had assumed that would be what dust is.
But if it has sand sized particles it may work, try it.

Thanks all, I will try to post some pictures later today. We filled a bucket with the granite dust and added water, it is a weird consistency. I'll have to take a photo of the bucket, it looks liquid but when you press into it it is solid. I do have a landscaping place near by, but they don't have "mason" sand. It's like beachy sand and I wasn't sure if that would work. That is what I was hoping to get for the earthen floor, but I wasn't sure about how it'd do in the mortar. I am no stone mason if you hadn't guessed haha
I had figured I'd need about 1.5-2 cubic yards of sand for the stone wall and then 3.5-4 yards for a 2 inch floor. I was able to get ahold of portland cement from Lowe's, hydrated lime was very hard to find. Our local hardware stores are out of mason sand, even home depot and lowe's are sold out of bagged mason sand.

A side note, we will be doing rainwater catchment. However, we are in Maine and have been in a drought for the past few years. This past June we got 0.8 inches of water all month, then in July we got 9 inches. Very unpredictable and irregular

Pea gravel may make placing and finishing concrete easier, but crushed gravel will make it distinctly stronger. The rough edges hold onto the cement better.

For below-grade walls, I think that all-purpose sand will work fine for you. What I have gotten includes some tiny gravel, which would not be a problem for bonding rough stones. The sand may not be sharp like mason sand, so the strength will be a bit less, but as long as the subsoil at the bottom of the trench is fairly uniform and solid, you mostly need gap fillers for the stone foundation. At the top of the wall where it is exposed, I might spring for higher grade materials.

I am wondering if you forgo the mortar entirely and just build a rubble trench foundation if that would also work?

I ended up using the stone dust in the mortar mix and so far it feels very strong! I did consider a rubble trench foundation with earthbags above grade, but ultimately I went with the stone wall both to gain some masonry experience and because a yurt is going on top of the stem wall. Typically yurts are screwed to a wood deck but we happened to have huge piles of field stone on the lot we are building on, so the rock was in a sense 'free' and we had the stone dust lying around.
We plan to build a straw bale home in the future, on a rubble trench foundation. The yurt was to house us for 5-7 years while we build our permanent home and allow us to quickly get out onto the land to start planting and building other structures. Honestly, if I were to do it all over again I would probably get a cheap camper and live in that while we built the straw bale home. Maybe my mind will change though once we actually have the yurt up and move in...right now it's just a big circle in the ground. In the future the yurt will become a work space and center for a future permaculture nursery we are working on getting together.

As for the well being drilled, I think if I could go back I would have also not had this done. I was pretty heavily persuaded by my family that a well was a necessity, we have been in a drought for a few years and after carrying jugs of water to every one of our fruit/nut trees this summer the well sounded like something we needed. We will still utilize rainwater catchment and are setting up swales to help retain water. Hopefully the well will be a secondary source of water that is only needed during certain times of the year.

If you are interested you may want to check out a post I made to keep track of our progress on the structure as we go --> https://permies.com/t/165661/Yurt-stone-foundation-pictures-progress#1300377
We are planning on having the yurt up in a couple of weeks.