Paving joint / gap filler / mortar

I chose a paving system that allow non-level uneven surface. It was uneven beautiful. I assumed to fill the gaps with sand, but soon find out that you will have sand all over the place all the time. Wind and rain and runoff can move the sand. You can't clean the pavement with high pressure jets or strong brushes, or you will have to collect a lot of sand as waste and replace the sand in the gaps.

If you do not fill the gaps, it will be filed with garden waste and soil. It's the same story when you try to hose it or brush it. And if you don't fill it, it's a heaven for insects especially ants.

My problem is, the pavement must drain a lot of water. It's on the bottom of a large slope. Even without the slope, my pavement will collect water here and there because it was not level nor even.

All fillers and mortar are sealants, non permeable. One exception is Ecofil from UK, and Rompox from Germany, but not available anywhere else. There are permeable concrete and asphalt, but even if they are available in gap filler form, they are ugly.

I stumbled upon lime, hydrated lime. You can find a 50lb bag in building materials / landscaping supplies stores for less than $15. It's not in the big chain hardware stores because it's for custom mix of mortar.

Hydrated lime is like white flour. It will turn back to limestone gradually, reacting with carbon dioxide in the air. The proper way is to add enough water to completely hydrate the lime into a putty form. When the water dries the lime will "set" like a mortar, though complete setting will takes years when the lime all turn into limestone. Lime is vapor and moisture permeate. It can drain water fast if sufficient channels are formed.

Round 1: dry brush fill

I applied the lime like sand or polymeric sand. I brushed it in when dry. Then I added water. It doesn't work. Unless added slowly, water rinse away the lime. And when the lime absorbed enough water, it shrinks a lot, leaving large gaps in the joints.

No harm done. Lime is just limestone, caulk, calcium hydroxide. It's totally organic or natural but the bag carries cancer warning. I suppose you shouldn't breath in the dust, and an active, highly alkaline chemical will do some harm. And lime is easy to clean. It's soft and do not adhere to stone strongly. You can just scrub off the surplus lime even when set.

Round 2: pure lime mortar

It's obvious you have to apply lime already wet. So I mix lime powder with water. You have to add a lot of lime to water to get it into a thick paste. Mine wasn't thick enough but a paste nevertheless. I applied with a putty knife.

The good thing is, it sets into a brilliant white joint. Somehow broken stones are glued together.

The bad thing is that it shrinks a lot when dry, creating a lot of cracks. If you push the joint with your finger, part of it will break from the rest and settle more into the gap.

All is not that bad. The cracks look beautiful. They drain water fast.

Round 3: lime and sand mortar

Actually this is standard for repairing historical buildings, which used permeable lime rather than impermeable cement. The mix ratio for me is academic. I can use a lot of sand, as long as the sand are glued together and not blown away by wind or brushed away easily. The lime paste has to be thick enough to stay on the putty knife.

I am supposed to use washed sand so as not to affect the chemical reaction. But imperfection with cracks is always desired. I wanted very coarse sand up to half of 5mm (the gap size) for good drainage. I ended up using the base sand for paving, as I got it in the yard.

The paste seemed to be a lot thicker with sand thrown in. It feels like a mortar and looks like a mortar with a little grey color.

There are still cracks, but a lot smaller, which is a good thing. I think the cracks can be controlled by the thickness of the lime putty. The more lime content the less it will shrink and the smaller the cracks. The full result isn't available yet.

The bad thing is that it's difficult to clean up the paving stone. Walking on it will certainly clean up the excess dry mortar, but it takes a long time. I thought of applying the mix like a caulk gun. Then I just point the mix and there will be no mess. Interestingly there is such a thing as a fillable caulk tube. It's only about $3 each but it's only available over the internet.

Then I think of using my wife's cake making bag. Surprisingly, there is such a thing as a grout bag. It's available on some hardware stores too.