Thursday, August 27, 2009
Lime and Limestone
Lime is perhaps the most prized and exceedingly versatile building material of the modern world. Early civilizations such as the Egyptians, Greeks and Romans used lime extensively. Many of their works in lime have survived to the present day testifying to its durability and intrinsic beauty (an important factor in sustainability: we tend to preserve what is well designed and are eager to tear down what is not).
Lime is utilized to produce plasters, stucco coatings, paints, mortars and cements. It can be modified to form large, imposing structures such as the Hoover Dam, footings for the Golden Gate Bridge or the space shuttle blast pad at Cape Canaveral. Alternatively it can be prepared as a sophisticated finish material such as Venetian plaster applied with a trowel or as a paint delicately applied al fresco.
Lime prepared for plaster typically is derived from limestone. Limestone is a sedimentary rock that forms from skeletons of marine creatures that have accumulated on the sea floor. With time and pressure these skeletons are pressed together in beds of stone. The White Cliffs of Dover on Britain's southeast coast pictured above provides a stunning example of sedimentary limestone.
Chemically defined, pure limestone is a carbonate of calcium or calcite having the formula CaCO3. Limestone is chemically identical to many materials familiar from everyday life: teeth and bones, chalk, marble and alabaster are common examples.
Pure limestone can be baked and further treated to prepare lime putty, a material very useful as a base for mortars, plasters and paints. Many limestone deposits found in the United States are relatively pure with little contamination from clay or other materials. When exposed to air under normal conditions the processed lime will quickly return to its natural state of calcite, resembling the limestone from which it was derived.
Some limestone contains contaminants of clay or other materials that affect its chemical properties. When these "hydraulic limes" are baked they will readily react and harden when sufficiently hydrated, given enough water. People quickly learned to add certain clays and other materials to pure lime during the baking process to create this effect artificially. This is the basis for cement technology ancient and modern.
In upcoming articles we will consider the lime cycles: the simple non-hydraulic cycle and the somewhat more complicated cementitious or hydraulic cycle.
Contributed by Patrick Webb