Planetary boundary layer. The planetary boundary layer (PBL) is also known as the atmospheric boundary layer (ABL). It is the lowest part of the atmosphere and its behavior is directly influenced by its contact with the ground. It responds to surface forcings in a timescale of an hour or less.
Portland cement is the most common type of cement in general use around the world, used as a basic ingredient of concrete, mortar, stucco, and most non-speciality grout. It was developed from other types of hydraulic lime in England in the mid 19th century and usually originates from limestone.
Jupiterimages/Photos.com/Getty Images. Portland cement is the main component of concrete and it is made using a highly evolved recipe of calcium, silicon, aluminum, iron, gypsum and other ingredients. Modern portland cement is manufactured using giant rock crushers and industrial kilns.