Portland cement, the most common of the modern cements, is made by carefully blending selected raw materials to produce a finished material meeting the requirements of ASTM C150 for one of eight specific cement types. Four major compounds [lime (CaO), iron (Fe2O3), silica (SiO2), and alumina (Al2O3)] and two minor compounds [gypsum (CaSO4 2H2O) and magnesia (MgO)] constitute the raw materials. The calcareous (CaO) materials typically come from limestone, calcite, marl, or shale. The argillaceous (SiO2 and Al2O3) materials are derived from clay, shale, and sand. The materials used for the manufacture of any specificcement are dependent on the manufacturing plant’s location and availability of raw materials. Portland cement can be made of a wide variety of industrial by-products.
In the manufacture of cement, the raw materials are first mined and then ground to a powder before blending in predetermined proportions. The blend is fed into the upper end of a rotary kiln heated to 2600 to 3000F by burning oil, gas, or powdered coal. Because cement production is an energy-intensive process, reheaters and the use of alternative fuel sources, such as old tires, are used to reduce the fuel cost. (Burning tires provide heat to produce the clinker and the steel belts provide the iron constituent.) Exposure to the elevated temperature chemically fuses the raw materials together into hard nodules called cement clinker. After cooling,the clinker is passed through a ball mill and ground to a fineness where essentially all of it will pass a No. 200 sieve . During the grinding, gypsum is added in small amounts to control the temperature and regulate the cement setting time. The material that exits the ball mill is portland cement. It is normally sold in bags containing 94 lb of cement
Concrete, the most common use for portland cement, is a complex material consisting of portland cement, aggregates, water, and possibly chemical and mineral admixtures. Only rarely is portland cement used alone, such as for a cement slurry for filling well holes or for a fine grout. Therefore, it is important to examine the relationship between the various portland cement properties and their potential effect upon the finished concrete. cement concrete is generally selected for structural use because of its strength and durability. Strength is easily measured and can be used as a general directly proportional indicator of over all durability. Specific durability cannot be easily measured but can be specified by controlling the cement chemistry and aggregate properties.
specification for portland cement
ASTM C150 defines requirements for eight types of cement. The chemical composition of portland cement is expressed in a cement-chemistry shorthand based on four phase compounds: tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A), and tetracalcium aluminum ferrite (C4AF). C2S and C3S are termed the calcium silicate hydrates (CSH).
References:
- BUILDING DESIGN AND CONSTRUCTION HANDBOOK for Frederick S. Merritt & Jonathan T. Ricketts