How to reattach a broken corner to a concrete planter

Concrete breaks are mostly a fit problem. If the two faces still key together, almost any decent masonry adhesive will hold them, because the interlocking aggregate carries most of the load and the adhesive just stops it moving. If the faces are crushed or a chunk is missing, no adhesive will restore the strength on its own and you are into a fill and reinforce job.

Outdoors, the real enemy is water freezing. A rigid repair holds water in the joint, and the first hard frost splits it again slightly to one side of the original break. MS polymer stays elastic, so the joint can move a fraction and recover. It is also unaffected by UV and by the alkalinity of concrete, which degrades some adhesives.

Dust is the practical failure. A fresh break sheds a fine powder that a cloth will smear rather than remove. Brush it hard, blow it out, and let the faces dry fully before bonding, because concrete holds moisture deep in the pores for a long time after rain.

1. Dry fit the broken corner and check it seats properly. Remove any loose crumbs from both faces.
2. Brush both faces hard with a stiff brush, then blow or vacuum the dust away.
3. Let the planter dry for at least 24 hours in dry weather. Damp concrete will not bond well.
4. Apply MS polymer adhesive to one face in a full covering layer, a little thicker in the middle.
5. Press the corner into place and work it until it seats fully, with a little squeeze out all round.
6. Strap it with ratchet straps, tape or an elastic band, applying pressure across the break rather than along it.
7. Tool the squeeze out into the joint line with a wet finger so it looks like a mortar line.
8. Leave 24 hours strapped, 48 before it takes any weight or gets rained on.