We do not currently know of any synonyms for laitance.
The noun laitance is defined as:
A weak layer of cement and aggregatefines on a concrete surface that is usually caused by an overwetmixture, 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.