Trass
Trass , noun
[Dutch tras or Greek trass, probably from Italian terrazzo terrace. See Terrace.]
(Geology) A white to gray volcanic tufa, formed of decomposed trachytic cinders; -- sometimes used as a cement. Hence, a coarse sort of plaster or mortar, durable in water, and used to line cisterns and other reservoirs of water.
The Dutch trass is made by burning and grinding a soft grayish rock found on the lower Rhine.