Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar, stucco, and non-speciality grout. It was developed from other types of hydraulic lime in England in the mid 19th century, and usually originates from limestone. It is a fine powder, produced by heating limestone and clay minerals in a kiln to form clinker, grinding the clinker, and adding small amounts of other materials.
“In the mid-first century BC, there was an explosive advance of technology in concrete,” Jackson said. “The Romans needed to find a mix that would work for building harbors for shipping and military purposes.”
X-ray spectroscopy indicated that the aluminum substitution may be responsible for the durability of the seawater concrete: the ancient material contained a rare hydrothermal mineral, aluminum tobermorite, that is absent in modern concrete formulations. The researchers determined tobermorite has higher stiffness than modern concrete binders and may serve as a model for future concrete strength and durability.
We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness.
Article Five of the United States Constitution describes the process whereby the Constitution, the nation's frame of government, may be altered. Altering the Constitution consists of proposing an amendment or amendments and subsequent ratification. Amendments may be proposed either by the Congress with a two-thirds vote in both the House of Representatives and the Senate or by a convention of states called for by two-thirds of the state legislatures. To become part of the Constitution, an amendment must be ratified by either—as determined by Congress—the legislatures of three-quarters of the states or state ratifying conventions in three-quarters of the states. The vote of each state (to either ratify or reject a proposed amendment) carries equal weight, regardless of a state's population or length of time in the Union.