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.

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.