You get it by the old man writing an haiko, (Basho) Then you finish the bridge by the peices in the right place.
On the bridge
you use it on the bridge
The bag of mortar is near the lake at the far left, on the way to the old man's hut.
You use the mortar to put the blocks together.
First, go all the way to the left of the map. Upon walking up to the small hut that lies there, you will pick up a bag of mortar.
You get the ninja bag from Basho, the ninja master.
The one on Red Dragon Island? Go to the crest of the broken-bridge and use it.
If I remember correctly, you must go to the hut that Basho lives in (found if you keep on going left after the market). There will be a short bridge consisting of three rocks and on the very first rock you see, there will be a bag of mortar. Once you click on it (or rather land on it), it will be put into your backpack. Use the mortar to fix the bridge.
there isn't a special time you need it, you just use it to fix the pridge.
The bag of mortar is found near the lake house to the left of West Edo. Use it to repair the weakened bridge. Talk to the old man on the bridge and then fit the pieces together to reinforce the stone blocks.
You go all the way left until you reach a bag of mortar. Pick it up and it goes in your inventory.
you go to a unfinished wall and click use =)