In this video, This Old House landscape contractor Roger Cook shows a no-mortar way to fix a mortared walkway.
1. Lift the loose flagstone from the walkway.
2. Scrub the surface below the flagstone with a wire brush to remove all dirt and mortar.
3. Wipe the surface clean with a dry sponge.
4. Use an air compressor fitted with an air nozzle to blast away any remaining dirt or dust from all cracks and crevices.
5. Apply a continuous bead of masonry adhesive back and forth across the entire surface. Space the beads no more than 1 inch apart.
6. Smooth and flatten the adhesive beads with a flat jointer.
7. Apply a bead of mortar-joint repair adhesive around the perimeter of the flagstone.
8. Set the flagstone back into the walkway, pressing it down into the adhesive.
9. Clean surface of the flagstone with a damp rag or sponge.
10. Fill in any voids or holes around the perimeter of the flagstone with mortar-joint repair adhesive. Smooth the adhesive with the flat jointer.
11. To help conceal the repair, sprinkle a mixture of stone dust and sand over the wet mortar-joint repair adhesive.
12. The next day, sweep off the stone dust and sand.
13. Wait four or five days before walking on the repaired flagstone.
14. Use the remaining mortar joint repair adhesive to fill in any cracks or holes in the walkway.