- The ingredients for Portland cement are limestone, seashells or chalk combined with shale, mud or clay.
- To make Portland cement 3 parts of crushed limestone or natural chalk are combined with 1 part crushed and pulverized shale, mud or clay. The ingredients are then heated in a rotary kiln to over 3270° F (1800° C). During heating and mixing CO2 (carbon dioxide) and H2O (water) are removed. Lime from the limestone or chalk fuses with aluminum silicate (from the clay), forming what is called clinker. The clinker is then ground into a fine powder. Gypsum is then added to retard setting of the cement.
- Portland cement contains mostly lime with about 20 percent silica and 5 percent alumina.
- Portland cement with a high silica content produces a slow-setting concrete.
- Portland cement with a high alumina content produces a fast-setting concrete.
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