Perfect imperfection: Lime Sand permeable paving mortar

There's no permeable paving for residential use. Those permeable concrete and asphalt are ugly and hard to install. Usual permeable paving for the home are stone (or similar material) slabs, relying on the gaps between stones to drain water.

Why you want permeable paving? The big issue is to prevent urban runoff, with the rain and storm carrying dirt into the ocean. Soil around your home absorbs the excess water, and then release it up on sunny days.

But for the home owner, leveling a pavement is the big issue. That's impossible if you pave an old path or area with things there at different levels over the years. I tried to lay the stone directly on grass and let the grass die and let the stones sink a bit into the soil at different degrees. If your pavement or patio is not perfectly level (with a few degrees off to drain water), water puddles can form every time after rain or washing.

For home paving, you make a level bed of rocks and sand, then add the top layer of stones, brick or other slabs. Finally you fill the gaps with something.

I'm using like thin tiles of stone modules hold in place in a plastic grid. The untended side effect is that it can go up and down to fit my irregular base. With some modifications it would be a pleasure to install. Though you still need a good base. Sadly, the current design is unpopular because of a few flaws. I managed to get it cents for dollars.

Permeable gap fillers are aggregates of stone fragments or sand. If you don't fill it it will be filled with soil or dirt. The problem is that you can't brush it, you can't hose it. And if you don't fill it, insects such as ants like it very much.

Other ways to fill the gaps are more like mortars. The popular way is to use polymeric sand, easy to apply and some retains it's soft sealing property. But they are not permeable. I found about two permeable types on the web, but only available in Europe or commercially.

Modern mortars are impermeable, they will make sure of it to prevent water seeping into the whatever base.

My perfect permeable mortar is lime and sand mix, which is actually used for old buildings. Here it is:

1 cup water
2 cup Type S hydrated lime (~$14 / 50 lb)
2 cup washed plaster sand (~$3 / 50 lb)

Type S hydrated lime is the most common lime you can get from building material suppliers, except for the home improvement hardware stores because they don't think people will mix their own mortar. Any hydrated lime will do but you ,may have to vary the ratios. Standard lime allows the builders to mix consistence mortars.

Any sand will do but beware that if they are not properly graded (filtered), any large particle will block your grout bad tip, and you have no easy to unblock it but to empty your whole bag. Sea salt will affect the long term chemical reaction of lime reverting to lime stone, but you probably don't really need that. Play sand is probably OK - washed and safe to play on. Paving sand will not be OK because my grout bag was blocked.

The exact ratio is not critical. The above amount will give you mortar for some 10 sq ft of 5 mm shallow gaps. Add 1 cup of water to 2 cup of lime will give you a soft paste that will not drip. Without sand the lime will be very soft. More sand the finish will be rougher, may be stronger and may be more permeable.

Features

The mix is brilliant white. The color of sand is immaterial unless you have a very high proportion of it.

The lime sand mix is soft. You can crack it with your finger nails. But you can brush it or hose it without anything noticeable falling off. That's a big advantage over any aggregates. Even if you brush a little lime (and a little sand) off the surface, it just lime stone and sand, perfectly environment friendly.

The lime sand mortar is strong enough to glue lose stone or fragments into position.

I cannot guarantee that the mortar is permeable. But for me it's highly permeable. The mortar do not have strong adhesive properties attached to the stone, and pure lime shrinks a lot when dry leaving a lot of tiny cracks. As a result I tend to create a lot of tiny cracks during the filling process. They look beautiful with my natural stones anyway. If you use pure lime with little sand, the minute cracks due to shrinking when dry will certain be highly permeable. The other extreme is to use very little lime so basically it's all sand so it must be permeable.

Procedure

You have to use a grout bag, like apply cream to a cake. Lime will stain everything brilliant translucent white. Lime is soft and the stain will eventually fall off but fine dust takes time.

For 5mm gaps or larger, the tip of the bag should go inside the gap and you should fill the gap without the mortar overflowing out of the gap.

Then use a pointing knife to "point" the mortar. It's basically a sharp object to smooth out the mortar. If no mortar overflow outside the gap, you are done. Or you have to remove the excess mortar with a putty knife, which is like a pizza server, or anything with a thin strong edge. Excess mortar will stain the edge with a white haze. It doesn't look bad and it will wear off with time.

You should let the pavement from sun and rain for a couple of hours or days. The proper way is to keep it moist under a wet blanket. I don't do any of these, which defeats the purpose of easiness. The mixture dries out in a few hours at most, and good enough to brush and rinse the next day for certain. The carbonate process is a long term process.

Unused mortar dries very slowly with a lid on. You can do a little bit everyday or every weekend. The tools are easily washed clean with water. There's no environment impact because it's lime stone and sand. I pour the used water into other parts of the permeable pavement.

Hydrated lime is very alkaline before turning back into lime stone with carbon dioxide in the air. The lime is fine as dust. Maybe for this reason the package says there are cancer causing agents. I doubt if they have to add any chemical to lime.