Portland cement
Portland cement is produced by burning limestone containing clay in its natural state or limestone mixed artificially in the required proportion with an appropriate amount of clay. Other substances such as silicon dioxide, alumina, or iron-containing materials may also be added. As a result of the burning process, semi-finished products known as clinker are obtained. These clinkers undergo further grinding to produce Portland cement, into which additives and accelerators may be introduced to modify its hydraulic properties. The main types of Portland cement are normal Portland cement, moderate Portland cement, and white Portland cement.
Source: "Explanations to the Harmonized System of the Russian Federation's Foreign Economic Activity (HS Russia)" (prepared by the Customs Authority of the Russian Federation) (Volume 1, Sections I - VI, Groups 1 - 29)