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Portland cement is obtained by firing limestone containing in its natural state, or mixed artificially with, a suitable proportion of clay. Other materials such as silica, alumina or iron bearing substances may also be added. As a result of the firing process, semi‑finished products known as clinkers are obtained. These clinkers are subsequently ground to produce Portland cement, which may incorporate additives and accelerators to modify its hydraulic properties. The principal types of Portland cement are normal Portland cement, moderate Portland cement and white Portland cement.
The heading also covers aluminous cement, slag cement, supersulphate cement (ground blast furnace slag mixed with an accelerator and calcined gypsum), pozzolana cement, Roman cement, etc., and mixtures of the above‑mentioned cements.
The cements of this heading may be coloured.
This heading does not include certain products sometimes known under the name of cement, such as, Keene's cement or English cement (alumed plaster gypsum) (heading 25.20), and pozzolana earth, santorin earth and similar substances, sometimes called natural cements (heading 25.30).The heading also excludes : (a) Finely ground blast furnace slag which requires the addition of a small quantity of accelerator at the time of making up (heading 26.19); the ground slag mixed with an accelerator, ready for use, does, however, fall in this heading. (b) Dental cements and bone reconstruction cements (heading 30.06). (c) Cements of heading 32.14. (d) Refractory cements and mortars, based on chamotte or dinas earths, etc. (heading 38.16). (e) Non‑refractory mortars and concretes (heading 38.24). Subheading Explanatory Note. Subheadings 2523.21 and 2523.29
For the purposes of subheadings 2523.21 and 2523.29, "Portland cement" means cement obtained by grinding Portland clinker with the possible addition of a small quantity of calcium sulphate. It is to be noted :
‑ that Portland clinker is a product of subheading 2523.10 consisting mostly of calcium silicates which is obtained by heating to partial fusion a predetermined and homogeneous mixture of materials principally containing lime (CaO) and silica (SiO2) with a smaller proportion of alumina (Al2O3) and iron oxide (Fe2O3) and;
‑ that the term "calcium sulphate" covers gypsum and its derivatives and anhydrite and other calcium sulphate products appropriate to the manufacture of cements.
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