In the article on sealing joints of tilt-up panels (April 1974, page 171) the word "laitance" is used. This word has frequently appeared in our concrete specifications over the years but we have never been able to find it in our dictionary. What is a good definition?
Webster's Third New International Dictionary, Unabridged says that laitance is derived from the French word lait meaning milk plus -ance, which in this instance perhaps means quality. It defines laitance as "an accumulation of fine particles on the surface of freshly placed concrete occurring when there is an upward movement of water through the concrete due to the presence of too much mixing water, to excessive tamping, or to vibration of the concrete." The book "Cement and Concrete Terminology," American Concrete Institute Publication SP-19, defines laitance as "a layer of weak and nondurable material containing cement and fines from aggregates, brought by bleeding to the top of overwet concrete, the amount of which is generally increased by overworking or overmanipulating concrete at the surface, by improper finishing or by job traffic." In general the word laitance is used to identify a thin, flaky layer of hardened but weak hydrated cement and fine sand which began life as a milky scum on the top surface of concrete.