In geology, cementations and concretions occur when a process called lithification takes place, which means that loose particles of rock are bonded together by a mineral such as calcium carbonate (calcite) or iron oxide (limonite). Man has recognized this natural phenomenon and been able to duplicate it from early times for construction purposes. In approximately 200 b.c. the Romans used lime and gypsum to bond sand particles together. The resulting cement was used as mortar and paving material. By adding stones and pottery shards to the cement, they were able to create a material that was adequately hard, but because it required less cement, it was cheaper to make. The Romans also created a forerunner of Portland cement. They mixed volcanic ash with cement to create a variety of cement that could be used under water for aqueducts, drains, and bridges. Concrete and cement...