Tried a new lime mix for a chimney rebuild in St. Louis and it set way too fast

I mixed a bag of that NHL 3.5 lime with local sand for a historic repair job, aiming for a slow set to match the old mortar. It went off like a rocket in under an hour, even in the shade, and I barely got the top courses laid. Has anyone else had this happen with natural hydraulic lime in hot weather, and what's your trick for slowing it down?

3 comments

3 Comments

smith.anna4d ago

Hey @young.kim, hot weather absolutely affects NHL sets, so try keeping your water and tools super cold next time!

craig.mila3d ago

Actually heat is a huge deal for NHL. The lime needs time to carbonate and heat speeds up drying way too fast. It can cause major shrinkage cracks. Seen it ruin a whole wall last summer. Keeping things cold helps but you also need to shade the mix and work early.

young.kim4d ago

Sounds like you got the mix wrong honestly. Hot weather barely matters with a proper NHL 3.5 mix. Maybe the sand was too wet or you added too much lime.