business backgrounder | industry
mix and bake
Ash Grove’s Seattle plant makes 2,400 tons of cement a
day. Their finished product becomes the base for today’s
highways, bridges and high-rises, including twin 41-story
story condos currently being built in downtown Seattle.
When a barge heaped with limestone or other minerals
arrives, an Ash Grove stiff-leg crane lifts a front-end loader onto the vessel. The material is unloaded onshore for
storage via a series of conveyor belts. Employees inside a
second-floor control room on the other side of the property
watch it all on video screens and computer flow charts.
They can adjust the mix as needed. A full-time chemist
leads a team that monitors quality throughout the process.
The four ingredients of cement — calcium carbonate, silica, alumina and iron — are pulverized into the consistency
of flour and sent into a 20-story tall pre-heater and then
directly to a rotary kiln. The kiln is fueled by natural gas or
coal, with old tires added to the mix as an eco-friendly way
to recuperate potential energy that would otherwise be lost
to a landfill. The mix is introduced to the rotary kiln where
temperatures reach 2,800 to 3,200 degrees Fahrenheit. The
constituents are chemically combined to form the main
compound that gives cement its strength, which emerges
as small roundish balls known as “clinker.”
“It’s like a cake,” Austell said. “You’ve got to bake it at the
right temperature and for a certain p