Laitance
Laitance turns up in 2 fields: concrete, waterproofing. Here is what it means to the people in each one.
Laitance in concrete
The soft, chalky skin of fines that bleed water leaves on top of concrete, too weak to bond to.
Bleed water carries fine cement particles, aggregate dust and impurities to the top, where they settle as a soft film with almost no strength. Laitance has to be removed by grinding, shotblasting, wire brushing or green cutting before a topping, overlay or the next lift goes on, or the new layer bonds to a weak skin and delaminates. Over-wet mixes and heavy bleeding produce the most.
"Shotblast the laitance off the whole deck before the topping, I don't want to be back here chipping delams."
Laitance in waterproofing
A weak, powdery skin of cement and fines on the surface of concrete that sealants and membranes cannot hold onto.
Bleed water carries cement and fine particles to the top of fresh concrete, where they harden into a thin, weak, often dusty layer. Sealants and membranes bonded to laitance fail with it. Grinding, wire brushing, abrasive blasting or shot blasting removes it to expose sound concrete. Joint faces in precast and tilt-up are ground for the same reason.
"Joint faces are all laitance, grind them back to hard concrete or the sealant will come off with a skin on it."