Civil Engineering app for ANDROID

Civil Engineering app for Android

Most Views

  • 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 specific cement 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 (75 m). During the grinding, gypsum is added in small amounts to control the temperature and regulate the cement setting time. The ma terial 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. Portland 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 overall durability. Specific durability cannot be easily measured but can be specified by controlling the cement chemistry and aggregate properties.

    Specifications for Portland Cements

    ASTM C150 defines requirements for eight types of portland cement. The pertinent chemical and physical properties are shown in Table 4.1. 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).
    Most cements will exceed the requirements shown in Table 4.1 by a comfortable margin. Note that the required compressive strengths are minimums. Almost without exception, every portland cement will readily exceed these minimum values.

    However, a caution must be attached to compressive strengths that significantly exceed the minimum values. While there is not a one-to-one correlation between a cement cube strength and the strength of concrete made with that cement (5000- psi cement does not equate to 5000-psi concrete), variations in cube strengths will be reflected in the tested concrete strength. It is imperative that, as the designed concrete strength reaches 5000 psi and greater, the cement cube strength be rigorously monitored. Any lowering of the running average will have a negative effect on the strength of concrete if the concrete mix design is not altered.
    The basic types of portland cement covered by ASTM C150 are as follows:
    Type I, general-purpose cement, is the one commonly used for many structural purposes. Chemical requirements for this type of cement are limited to magnesia and sulfur-trioxide contents and loss on ignition, since the cement is adequately defined by its physical characteristics.
    Type II is a modified cement for use in general concrete where a moderate exposure to sulfate attack may be anticipated or where a moderate heat of hydration is required. These characteristics are attained by placing limitations on the C3S and C3A content of the cement. Type II cement gains strength a little more slowly than Type I but ultimately will achieve equal strength. It is generally available in most sections of the country and is preferred by some engineers over Type I for general construction. Type II cement may also be specified as a low-alkali cement for use where alkali reactive aggregates are present. To do so requires that optional chemical requirements (Table 4.2) be included in the purchase order. Type II low-alkali  cement is commonly specified in California.
    Type III cement attains high early strength. In 7 days, strength of concrete made with it is practically equal to that made with Type I or Type II cement at 28 days.
    This high early strength is attained by finer grinding (although no minimum is placed on the fineness by specification) and by increasing the C3S and C3A content  of the cement. Type III cement, however, has high heat evolution and therefore should not be used in large masses. Because of the higher C3A content, Type III cement also has poor sulfate resistance. Type III cement is not always available from building materials dealers’ stocks but may be obtained by them from the cement manufacturer on short notice. Ready-mix concrete suppliers generally do not stock Type III cement because its shorter set time makes it more volatile to transport and discharge, especially in hot weather.


    Type IV is a low-heat cement that has been developed for mass concrete construction.
    Normal Type I cement, if used in large masses that cannot lose heat by radiation, will liberate enough heat during the hydration of the cement to raise the temperature of the concrete as much as 50 or 60F. This results in a relatively large increase in dimensions while the concrete is still soft and plastic. Later, as the  concrete cools are hardening, shrinkage causes cracks to develop, weakening the  concrete and affording points of attack for aggressive solutions. The potential-phase compounds that make the largest contribution to the heat of hydration are C3S and C3A; so the amounts of these are permitted to be present are limited. Since these compounds also produce the early strength of cement, the limitation results in a cement that gains strength relatively slowly. This is of little importance, however, in the mass concrete for which this type of cement is designed.

    Type V is a portland cement intended for use when high sulfate resistance is required. Its resistance to sulfate attack is attained through the limitation on the C3A content. It is particularly suitable for structures subject to attack by liquors containing sulfates, such as liquids in wastewater treatment plants, seawaters, and some other natural waters.
    Both Type IV and Type V cements are specialty cements. They are not normally available from dealer’s stock but are usually obtainable for use on a large job if arrangements are made with the cement manufacturer in advance.

    Air-Entraining Portland Cements

    For use in the manufacturer of air-entraining concrete, agents may be added to the cement by the manufacturer, thereby producing air-entraining portland cements (‘‘Air-Entraining Additions for Use in the Manufacture of Air-Entraining Portland Cement,’’ ASTM C226). These cements are available as Types IA, IIA, and IIIA.

     

     

     

     

     

     

     


    Tags: , , | 3,337 views