What does "trass" mean?

Trass: 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. [Formerly written also tarras, tarrace, terras.] Note: The Dutch trass is made by burning and grinding a soft grayish rock found on the lower Rhine.