laitance

noun

1. A weak, milky layer of cement and aggregate fines on a concrete surface that is usually caused by an overwet mixture, overworking the mixture, improper or excessive finishing or combination thereof. Also, could be caused when a slurry mix is used on a deep foundation, where the slurry mix is not completely removed before placing concrete on top.
2. The weak top layer of cement and aggregate fines on a concrete surface that stands between a topical coating and the underlying substrate. This laitance layer prevents a bond forming between the topical coating and substrate, whereby it causes delamination of the topical coating as the laitance layer fails first.