you use it on the bridge
On the bridge
You use the mortar to put the blocks together.
The bag of mortar is near the lake at the far left, on the way to the old man's hut.
there isn't a special time you need it, you just use it to fix the pridge.
You get it by the old man writing an haiko, (Basho) Then you finish the bridge by the peices in the right place.
you use the mortar at the great bridge to fix it
you go to a unfinished wall and click use =)
You go all the way left until you reach a bag of mortar. Pick it up and it goes in your inventory.
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.
The mortar to fix the bridge is at the lake, far left of the city, past West Edo.
You put it on the bridge with all the workers where there is a crack.
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.