Portland Cement (İngilizce)
The properties of concrete depend on the quantities and qualities of its components. Because cement is the most active component of concrete and usually has the greatest unit cost, its selection and proper use are important in obtaining most economically the balance of properties desired for any particular concrete mixture.
Type I/II portland cements, which can provide adequate levels of strength and durability, are the most popular cements used by concrete producers. However, some applications require the use of other cements to provide higher levels of properties. The need for high-early strength cements in pavement repairs and the use of blended cements with aggregates susceptible to alkali-aggregate reactions are examples of such applications.
It is essential that highway engineers select the type of cement that will obtain the best performance from the concrete. This choice involves the correct knowledge of the relationship between cement and performance and, in particular, between type of cement and durability of concrete.
Portland Cement (ASTM Types)
ASTM C 150 defines portland cement as “hydraulic cement (cement that not only hardens by reacting with water but also forms a water-resistant product) produced by pulverizing clinkers consisting essentially of hydraulic calcium silicates, usually containing one or more of the forms of calcium sulfate as an inter ground addition.” Clinkers are nodules (diameters, 0.2-1.0 inch [5-25 mm]) of a sintered material that is produced when a raw mixture of predetermined composition is heated to high temperature. The low cost and widespread availability of the limestone, shales, and other naturally occurring materials make portland cement one of the lowest-cost materials widely used over the last century throughout the world. Concrete becomes one of the most versatile construction materials available in the world.
The manufacture and composition of portland cements, hydration processes, and chemical and physical properties have been repeatedly studied and researched, with innumerable reports and papers written on all aspects of these properties.
Types of Portland Cement.
Different types of portland cement are manufactured to meet different physical and chemical requirements for specific purposes, such as durability and high-early strength. Eight types of cement are covered in ASTM C 150 and AASHTO M 85. These types and brief descriptions of their uses are listed in Table 2.1.
More than 92% of portland cement produced in the United States is Type I and II (or Type I/II); Type III accounts for about 3.5% of cement production (U.S. Dept. Int. 1989). Type IV cement is only available on special request, and Type V may also be difficult to obtain (less than 0.5% of production).
Although IA, IIA, and IIIA (air-entraining cements) are available as options, concrete producers prefer to use an air-entraining admixture during concrete manufacture, where they can get better control in obtaining the desired air content. However, this kind of cements can be useful under conditions in which quality control is poor, particularly when no means of measuring the air content of fresh concrete is available (ACI Comm. 225R 1985; Nat. Mat. Ad. Board 1987).
If a given type of cement is not available, comparable results can frequently be obtained by using modifications of available types. High-early strength concrete, for example, can be made by using a higher content of Type I when Type III cement is not available (Nat. Mat. Ad. Board 1987), or by using admixtures such as chemical accelerators or high-range water reducers (HRWR). The availability of portland cements will be affected for years to come by energy and pollution requirements. In fact, the increased attention to pollution abatement and energy conservation has already greatly influenced the cement industry, especially in the production of low-alkali cements. Using high-alkali raw materials in the manufacture of low-alkali cement requires bypass systems to avoid concentrating alkali in the clinkers, which consumes more energy (Energetics, Inc. 1988). It is estimated that 4% of energy used by the cement industry could be saved by relaxing alkali specifications. Limiting use of low-alkali cement to cases in which alkali-reactive aggregates are used could lead to significant improvement in energy efficiency (Energetics, Inc. 1988).
Table 1.1 Portland cement types and their uses.
|I1||General purpose cement, when there are no extenuating conditions|
|II2||Aids in providing moderate resistance to sulfate attack|
|III||When high-early strength is required|
|IV3||When a low heat of hydration is desired (in massive structures)|
|V4||When high sulfate resistance is required|
|IA4||A type I cement containing an integral air-entraining agent|
|IIA4||A type II cement containing an integral air-entraining agent|
|IIIA4||A type III cement containing an integral air-entraining agent|
1 Cements that simultaneously meet requirements of Type I and Type II are also widely available.
2 Type II low alkali (total alkali as Na2O < 0.6%) is often specified in regions where aggregates susceptible to alkali-silica reactivity are employed.
3 Type IV cements are only available on special request.
4 These cements are in limited production and not widely available.
Cement Composition. The composition of portland cements is what distinguishes one type of cement from another. ASTM C 150 and AASHTO M 85 present the standard chemical requirements for each type. The phase compositions in portland cement are denoted by ASTM as tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A), and tetracalcium aluminoferrite (C4AF). However, it should be noted that these compositions would occur at a phase equilibrium of all components in the mix and do not reflect effects of burn temperatures, quenching, oxygen availability, and other real-world kiln conditions. The actual components are often complex chemical crystalline and amorphous structures, denoted by cement chemists as “elite” (C3S), “belite” (C2S), and various forms of aluminates. The behavior of each type of cement depends on the content of these components. Characterization of these compounds, their hydration, and their influence on the behavior of cements are presented in full detail in many texts. Some of the most complete references dealing with the chemistry of cement include those written by Bogue (1955), Taylor (1964), and Lea (1970). Different analytical techniques such as x-ray diffraction and analytical electron microscopy are used by researchers in order to understand fully the reaction of cement with water (hydration process) and to improve its properties.
In simplest terms, results of these studies have shown that early hydration of cement is principally controlled by the amount and activity of C3A, balanced by the amount and type of sulfate interground with the cement. C3A hydrates very rapidly and will influence early bonding characteristics. Abnormal hydration of (C3A) and poor control of this hydration by sulfate can lead to such problems as flash set, false set, slump loss, and cement-admixture incompatibility (Previte 1977; Whiting 1981; Meyer and Perenchio 1979).
Development of the internal structure of hydrated cement (referred to by many researchers as the microstructure) occurs after the concrete has set and continues for months (and even years) after placement. The microstructure of the cement hydrates will determine the mechanical behavior and durability of the concrete. In terms of cement composition, the C3S and C2S will have the primary influence on long term development of structure, although aluminates may contribute to formation of compounds such as ettringite (sulfoaluminate hydrate), which can cause expansive disruption of concrete. Cements high in C3S (especially those that are finely ground) will hydrate more rapidly and lead to higher early strength. However, the hydration products formed will, in effect, make it more difficult for hydration to proceed at later ages, leading to an ultimate strength lower than desired in some cases. Cements high in C2S will hydrate much more slowly, leading to a denser ultimate structure and a higher long-term strength. The relative ratio of C3S to C2S, and the overall fineness of cements, has been steadily increasing over the past few decades. Indeed, Pomeroy (1989) notes that early strengths achievable today in concrete could not have been achieved in the past except at very low water-to-cement ratios (w/c’s), which would have rendered concretes unworkable in the absence of HRWR. This ability to achieve desired strengths at a higher workability (and hence a higher w/c) may account for many durability problems, as it is now established that higher w/c invariably leads to higher permeability in the concrete (Ruettgers, Vidal, and Wing 1935; Whiting, 1988).
One of the major aspects of cement chemistry that concern cement users is the influence of chemical admixtures on portland cement. Since the early 1960s most states have permitted or required the use of water-reducing and other admixtures in highway pavements and structures (Mielenz 1984). A wide variety of chemical admixtures have been introduced to the concrete industry over the last three decades, and engineers are increasingly concerned about the positive and negative effects of these admixtures on cement and concrete performance.
Considerable research dealing with admixtures has been conducted in the United States. Air-entraining agents are widely used in the highway industry in North America, where concrete will be subjected to repeated freeze-thaw cycles. Air-entraining agents have no appreciable effect on the rate of hydration of cement or on the chemical composition of hydration products (Ramachandran and Feldman 1984). However, an increase in cement fineness or a decrease in cement alkali content generally increases the amount of an admixture required for a given air content (ACI Comm. 225R 1985). Water reducers or retarders influence cement compounds and their hydration. Lignosulfonate-based admixtures affect the hydration of C3A, which controls the setting and early hydration of cement. C3S and C4AF hydration is also influenced by water reducers (Ramachandran and Feldman 1984).
Test results presented by Polivka and Klein (1960) showed that alkali and C3A contents influence the required admixtures to achieve the desired mix. It appears that set retarders, for example, are more effective with cement of low alkali and low C3A content, and that water reducers seem to improve the compressive strength of concrete containing cements of low alkali content more than that of the concrete containing cements of high alkali content.
Physical Properties of Portland Cements. ASTM C 150 and AASHTO M 85 have specified certain physical requirements for each type of cement. These properties include 1) fineness, 2) soundness, 3) consistency, 4) setting time, 5) compressive strength, 6) heat of hydration, 7) specific gravity, and 8) loss of ignition. Each one of these properties has an influence on the performance of cement in concrete. The fineness of the cement, for example, affects the rate of hydration. Greater fineness increases the surface available for hydration, causing greater early strength and more rapid generation of heat (the fineness of Type III is higher than that of Type I cement) (U.S. Dept. Trans. 1990).
ASTM C 150 and AASHTO M 85 specifications are similar except with regard to fineness of cement. AASHTO M 85 requires coarser cement, which will result in higher ultimate strengths and lower early-strength gain. The Wagner Turbidimeter and the Blaine air permeability test for measuring cement fineness are both required by the American Society for Testing Materials (ASTM) and the American Association for State Highway Transportation Officials (AASHTO). Average Blaine fineness of modern cement ranges from 3,000 to 5,000 cm2/g (300 to 500 m2/kg).
Soundness, which is the ability of hardened cement paste to retain its volume after setting, can be characterized by measuring the expansion of mortar bars in an autoclave (ASTM C 191, AASHTO T 130). The compressive strength of 2-inch (50-mm) mortar cubes after 7 days (as measured by ASTM C 109) should not be less than 2,800 psi (19.3 MPa) for Type I cement. Other physical properties included in both ASTM C 150 and AASHTO M 95 are specific gravity and false set. False set is a significant loss of plasticity shortly after mixing due to the formation of gypsum or the formation of ettringite after mixing. In many cases, workability can be restored by remixing concrete before it is cast.
Influence of Portland Cement on Concrete Properties. Effects of cement on the most important concrete properties are presented in Table 1.2.
Cement composition and fineness play a major role in controlling concrete properties. Fineness of cement affects the placeability, workability, and water content of a concrete mixture much like the amount of cement used in concrete does.
Cement composition affects the permeability of concrete by controlling the rate of hydration. However, the ultimate porosity and permeability are unaffected (ACI Comm. 225R 1985; Powers et al. 1954). The coarse cement tends to produce pastes with higher porosity than that produced by finer cement (Powers et al. 1954). Cement composition has only a minor effect on freeze-thaw resistance. Corrosion of embedded steel has been related to C3A content (Verbeck 1968). The higher the C3A, the more chloride can be tied into chloroaluminate complexes—and thereby be unavailable for catalysis of the corrosion process.
Table 1.2. Effects of cements on concrete properties.
|Cement Property||Cement Effects|
|Placeability||Cement amount, fineness, setting characteristics|
|Strength||Cement composition (C3S, C2S and C3A), loss on ignition, fineness|
|Drying Shrinkage||SO3content, cement composition|
|Permeability||Cement composition, fineness|
|Resistance to sulfate||C3A content|
|Alkali Silica Reactivity||Alkali content|
|Corrosion of embedded steel||Cement Composition (esp. C3A content)|
Storage of Cement. Portland cement is a moisture-sensitive material; if kept dry, it will retain its quality indefinitely. When stored in contact with damp air or moisture, portland cement will set more slowly and has less strength than portland cement that is kept dry. When storing bagged cement, a shaded area or warehouse is preferred. Cracks and openings in storehouses should be closed. When storing bagged cement outdoors, it should be stacked on pallets and covered with a waterproof covering.
Storage of bulk cement should be in a watertight bin or silo. Transportation should be in vehicles with watertight, properly sealed lids. Cement stored for long periods of time should be tested for strength and loss on ignition.
Cement Certification. The current trend in state transportation departments is to accept certification by the cement producer that the cement complies with specifications. Verifications tests are taken by the state DOT to continually monitor specification compliance. The cement producer has a variety of information available from production records and quality control records that may permit certification of conformance without much, if any, additional testing of the product as it is shipped (ACI Comm. 225R 1985).
Blended Portland Cements
Blended cement, as defined in ASTM C 595, is a mixture of portland cement and blast furnace slag (BFS) or a “mixture of portland cement and a pozzolan (most commonly fly ash).”
The use of blended cements in concrete reduces mixing water and bleeding, improves finishability and workability, enhances sulfate resistance, inhibits the alkali-aggregate reaction, and lessens heat evolution during hydration, thus moderating the chances for thermal cracking on cooling.
Blended cement types and blended ratios are presented in Table 1.3.
Table 1.3 Blended cement types and blended ratios.
|IP||15-40% by weight of pozzolan (fly ash)|
|I(PM)||0-15% by weight of Pozzolan (fly ash)|
|P||15-40% by weitht of pozzolan (fly ash)|
|IS||25-70% by weight of blast furnace slag|
|I(SM)||0-25% by weight of blast furnace slag|
|S||70-100% by weight of blast furnace slag|
The advantages to using mineral admixtures added at the batch plant (Popoff 1991; Massazza 1987).
- Mineral admixture replacement levels can be modified on a day-to-day and job-to-job basis to suit project specifications and needs.
- Cost can be decreased substantially while performance is increased (taking into consideration the fact that the price of blended cement is at least 10% higher than that of Type I/II cement [U.S. Dept. Int. 1989]).
- GGBFS can be ground to its optimum fineness.
- Concrete producers can provide specialty concretes in the concrete product markets.
At the same time, several precautions must be considered when mineral admixtures are added at the batch plant.
- Separate silos are required to store the different hydraulic materials (cements, pozzolans, slags). This might slightly increase the initial capital cost of the plant.
- There is a need to monitor variability in the properties of the cementitious materials, often enough to enable operators to adjust mixtures or obtain alternate materials if problems arise.
- Possibilities of cross-contamination or batching errors are increased as the number of materials that must be stocked and controlled is increased.
Modified Portland Cement (Expansive Cement)
Expansive cement, as well as expansive components, is a cement containing hydraulic calcium silicates (such as those characteristic of portland cement) that, upon being mixed with water, forms a paste, that during the early hydrating period occurring after setting, increases in volume significantly more than does portland cement paste. Expansive cement is used to compensate for volume decrease due to shrinkage and to induce tensile stress in reinforcement.
Expansive cement concrete used to minimize cracking caused by drying shrinkage in concrete slabs, pavements, and structures is termed shrinkage-compensating concrete.
Self-stressing concrete is another expansive cement concrete in which the expansion, if restrained, will induce a compressive stress high enough to result in a significant residual compression in the concrete after drying shrinkage has occurred.
Types of Expansive Cements. Three kinds of expansive cement are defined in ASTM C 845.
- Type K: Contains anhydrous calcium aluminate
- Type M: Contains calcium aluminate and calcium sulfate
- Type S: Contains tricalcium aluminate and calcium sulfate
Only Type K is used in any significant amount in the United States.
Concrete placed in an environment where it begins to dry and lose moisture will begin to shrink. The amount of drying shrinkage that occurs in concrete depends on the characteristics of the materials, mixture proportions, and placing methods. When pavements or other structural members are restrained by subgrade friction, reinforcement, or other portions of the structure, drying shrinkage will induce tensile stresses. These drying shrinkage stresses usually exceed the concrete tensile strengths, causing cracking. The advantage of using expansive cements is to induce stresses large enough to compensate for drying shrinkage stresses and minimize cracking (ACI Comm. 223 1983; Hoff et al. 1977).
Physical and mechanical properties of shrinkage compensating concrete are similar to those of portland cement concrete (PCC). Tensile, flexural, and compressive strengths are comparable to those in PCC. Air-entraining admixtures are as effective with shrinkage-compensating concrete as with portland cement in improving freeze-thaw durability.
Some water-reducing admixtures may be incompatible with expansive cement. Type A water-reducing admixture, for example, may increase the slump loss of shrinkage- compensating concrete (Call 1979). Fly ash and other pozzolans may affect expansion and may also influence strength development and other physical properties.
Structural design considerations and mix proportioning and construction procedures are available in ACI 223-83 (ACI Comm. 223 1983). This report contains several examples of using expansive cements in pavements.
In Japan, admixtures containing expansive compounds are used instead of expansive cements. Tsuji and Miyake (1988) described using expansive admixtures in building chemically prestressed precast concrete box culverts. Bending characteristics of chemically prestressed concrete box culverts were identical to those of reinforced concrete units of greater thickness (Tsuji and Miyake 1988). Expansive compounds are also available in the United States. They can be added to the mix in a way similar to how fly ash is added to concrete mixes.
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