How Is Natural Cement Formed?

Definition

• Natural cement, in a general sense, refers to cement formed by natural processes, usually involving a combination of limestone and clay. Natural cement was used primarily during the 19th century and early 20th century, but largely disappeared after about 1910, as Portland cement became more popular and began dominating the market. In the early 21st century, Portland cement accounts for about 98 percent of cement products on the market.

Composition

• Natural cement gets its name from the mining of two natural deposits: limestone and clay. To make natural cement, both the limestone and the clay require very specific chemical compositions. For example, the materials must contain no more than 2 percent of insoluble residue, to ensure that the cement is not over-processed. Today, natural cement is used primarily for restoring old and historic buildings.

Basic Forging Process

• As with the more common Portland cement, natural cement comes together from highly concentrated materials and a tremendous amount of heat. The lime and clay are heated in a kiln and ground to a fine powder. When mixed together, the materials harden when submerged in water. British engineer John Smeaton first discovered this chemical-hardening process in 1756, and it ultimately led to the mass production of natural cement.

Portland-Cement Differences

• Like natural cement, Portland cement is a hydraulic product. In other words, it hardens with water and then becomes water-resistant. Rather than containing lime and clay, however, Portland cement consists almost entirely of calcium silicates, including calcium sulfate. Portland cement is stronger and more uniform than traditional cement, which largely accounts for its rise in popularity during the late 19th and early 20th centuries. Portland cement also costs much less to manufacture than natural cement.