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The resulting cement, produced from the formerly discarded grappiers, was of much higher quality than that obtained from the unsintered material. This fact was firmly established by the English cement manufacture cementr L. C. Johnson in 1845, and the term "portland cement" has since been applied solely to the cement made from the sintered material. This period marks the real beginning of the portland cement industry.

The principal compounds in port-land cement are: tricalcium silicate (3CaO'SiO2), which is chiefly responsible for initial set and early strength of the cement-water paste; dicalcium silicate (2CaOSiO2), which hardens slowly but contributes notably to strength at ages over a month; tricalcium aluminate (3CaOAl2O3), which liberates a large amount of heat during the first few days of hardening and is rapidly attacked by sulfate solutions; the iron-containing phase (a solid solution that approaches the composition 4CaO'Al2O3'Fe2O3), which is valuable as a flux in manufacture cement; magnesia (MgO), which, if present in excessive amount, may cause expansion of structures exposed to moisture after a number of years; and calcium oxide or free lime (CaO), which results from incomplete reaction in the kiln and, if present in amounts over 2 or 3%, may cause unsoundness and expansion in the cement paste. In addition to the above clinker compounds, gypsum or calcium sulfate hydrate (CaSCX^HsO) is interground with the clinker to control the rate of set of the cement-water paste.

The production of portland cement is a major industry in the United States, increasing from 8 million barrels (1.4 million metric tons) in 1900 —when it trailed natural cement slightly in output—to almost 400 million barrels (68.4 million metric tons) annually. (A 376-pound, or 171-kg, barrel is the standard unit of weight for hydraulic cement in the United States, even though no cement, except for export, is now shipped in barrels. The 94-pound, or 42.7-kg, bag now in general use contains one fourth of a barrel.) The leading cement-producing countries are the United States, the USSR, West Germany, Japan, and France.