Cement

Cement is a common construction material that is used to bond mineral fragments in order to produce a compact whole. The most common types of cement result from the reaction of lime and silica. These are called hydraulic cements for their ability to set and harden underwater.

Types of Cement

By far the most common type of cement is called Portland cement, which was named for its similarity to Portland stone, a type of building stone. Portland cement consists primarily of lime, silica, and alumina. These materials are carefully ground, mixed, and heated to produce a finely powdered gray substance. In the presence of water, these ingredients react to form hydrated calcium silicates that, after setting, form a hardened product. Such a product is classified as a hydraulic cement because of its ability to set and harden under water. The hardening of concrete is not merely a matter of drying out, as the chemical reactions of cement can proceed even underwater.

While Portland cement is by far the most common type of cement, there are others. Most of these, such as high-early-strength cements, slag cements, Portland-pozzolan cements, and expansive cements, are variations on the basic Portland cement. Manufacturers can produce these specialized cements through slight variations of the basic chemical composition of Portland cement and through the use of various additives. Each of these cements is designed for a specific use, and their advantages include lower cost, higher strength, and faster setting times. Another type, high-alumina cement, is not based on Portland cement. Formed by the fusion of limestone and bauxite, high-alumina cement hardens rapidly and withstands the corrosive effects of sulfate waters (unlike basic Portland cement). Its early promise as a structural material has diminished because of a number of failures, but its ability to withstand high temperatures makes it quite useful in constructing furnaces.

Although the term “cement” is often used as a synonym for “concrete,” cement is only the binding agent in concrete. Concrete is a mixture of aggregate (sand, gravel or crushed stone) and cement, which hardens to form a synthetic stone through a series of chemical reactions between the cement and the aggregate.

Hydration

Adding water to dry cement creates a paste that eventually hardens. The reaction of water with cement is known as the hydration process. This reaction involves much more than water molecules attaching themselves to the constituent elements of cement. Rather, the constituents are reorganized to form new, hydrated compounds. One of the first reactions involves the aluminates, particularly tricalcium aluminum oxide (3CaO•Al₂O₃, hereafter abbreviated as C₃A). Note that the C in this formula is not the chemical symbol for carbon but shorthand for calcium oxide, or lime. Although C₃A is undesirable in cement, it forms as a by-product during the manufacturing process as the raw materials are heated. If allowed to react with water unchecked, it would lead to overly rapid setting, not allowing time for working the cement or concrete product. To avoid this difficulty, a carefully controlled amount of gypsum is added to the cement at the time of manufacture. Gypsum slows the hydration of C₃A, giving the more important calcium silicates time to react with water.

Hydration of the calcium silicates occurs more slowly than that of the aluminates but forms the basic, strength-giving structure of hardened Portland cement. Two different calcium silicates 3CaO•SiO₂ (abbreviated as C₃S) and 2CaO•SiO₂ (abbreviated as C₂S) are present in the cement. As with C, the S is not the same as the chemical symbol for sulfur. Both forms of calcium silicate react with water to produce the hydrated calcium silicate (C₃S₂H₃). This product is sometimes called tobermorite because of the resemblance its molecular arrangement bears to that of the natural but rare mineral of the same name (taken from Tobermorey, Scotland). Another product of the reaction is calcium hydroxide, or Ca(OH)₂, which is integral to the microstructure of the hardened cement. It is important to note that as the exact ratios of the different constituents vary, so does the composition of the product. What is actually being produced, at different places and by different manufacturers, is a family of hydrated calcium silicates rather than one precise formula.

Stages of Production

From the time water is added to the time the cement paste sets and fully hardens, cement goes through four general phases. Four main compounds are present during each stage: a gel of the above-mentioned hydrated compounds, crystals of calcium hydroxide, unhydrated cement, and water. The proportions of these compounds change with time; the cement gains rigidity as the percentage of hydrated calcium silicate increases with the attendant drop in the amount of free water.

These compounds eventually arrange themselves in loose, crumpled layers. For the first few minutes after adding water to cement, the two form a paste in which the cement is suspended within the water. At this stage, the cement dissolves in the water. The next stage, sometimes called the dormant period, lasts for one to four hours. During this time, the cement forms a gel and begins to set, losing pliability. The individual cement grains build a coating of hydration products, and loose, crumpled layers of hydrated calcium silicate begin to form. In the microscopic spaces between the cement grains, water is held by the surface forces of the cement particles. Larger spaces, called capillary pores, hold free water, which the cement slowly absorbs for use in the hydration process. During the third stage, which peaks about six hours after water is added, the coating of hydration products around the cement grains ruptures, exposing unhydrated cement to water and further building layers of calcium silicate. As these layers grow, they entrap water, which continues to react with the cement particles. They also contain calcium hydroxide (a by-product of the hydration of calcium silicates), which fills the larger pores and thus apparently contributes to the overall strength of the cement. The fourth stage produces the final setting and hardening of the cement. The hydration process may continue for years, and there will in all probability be a small percentage of the cement that never hydrates. Hydration of C₃S and C₂S occurs at different rates. In the first four weeks, the hydration of the C₃S contributes most to the strength of the hardening cement, and afterward the hydration of the C₂S contributes more to the cement’s strength. After about a year, the hydration rates of the two compounds are roughly equal.

Quality Measurement

The quality of Portland cement can be measured by four principal physical characteristics: fineness, soundness, time of set, and compressive strength. The desired specifications for each may vary from one country to another and certainly will vary for the different types of cement, but certain generalizations may be made safely. The fineness of the cement plays a large role in determining the rate of hydration. The finer the cement, the larger the surface area of the cement particles and the faster and more complete the hydration. A method has recently been developed that measures particle fineness in terms of the specific surface (the surface area of cement particles measured in square centimeters per gram of cement). The two most common ways of measuring specific surface are the Wagner turbidimeter test and the Blaine air permeability test. To use the turbidimeter, a sample of cement is dispersed inside a tall glass container of kerosene (which does not react with the cement). A beam of light is then passed through the kerosene at given elevations at a specified time, and the concentration of cement is measured by a photoelectric cell. The specific surface can then be calculated from the photoelectric cell readings.

The air permeability test relies on the fact that the number and size of pores are functions of particle size and distribution. A given volume of air is drawn through a bed of cement, and the time it takes for the air to pass through the cement is used to calculate the specific surface of that sample.

Soundness is another important characteristic of cement. A sound cement is one that will not crack or disintegrate with time. Unsoundness is often caused by the delayed hydration and subsequent expansion of lime. The usual method of testing cement for soundness is in an autoclave. A small sample is placed in the autoclave after curing for twenty-four hours and is subjected to extremely high pressure for three hours. After the sample has cooled, it is measured and compared to its original length. If it has expanded less than 0.8 percent, the cement is usually considered sound. Some aggregate materials, such as chert, cause expansion and cracking as concrete sets and are unsuitable for use in concrete.

Time of setting is tested on fresh cement paste as it hardens. The ability of the paste to sustain a given weight on a needle of given diameter (usually a 300-gram load on a 10-millimeter diameter needle) can be easily correlated to its setting time.

Concrete, like other masonry products, is much stronger in compression (for example, a vertical pillar in which gravity pushes the pillar in on itself) than in tension (a horizontal beam that is bowed such that the bottom half wants to pull apart). Thus, it is most useful to study the compressive strength of concrete. The usual test is to make a two-inch cube of cement and sand (in a 1:2.75 mixture) and compress it until it breaks. Much can be learned by measuring the breaking load, type of fracturing, and other results.

Uses in Construction

Cement has been used for construction purposes since at least 4000 BCE. The Romans used a hydraulic cement based on slaked lime and volcanic ash in many of their construction projects, some of which are still standing.

One important use of cement is as mortar, or cement mixed with sand. Mortar is the substance that binds bricks, stone, and other masonry products. Cement is also the primary ingredient in grout, such as that used between tiles. The most important and common use of cement, however, is in making concrete.

Concrete is the product of mixing cement paste with a mineral aggregate. The aggregate, which acts as a filler, can be a wide variety of materials but is usually a sand or gravel. As a construction material, concrete has many advantages. First, it is inexpensive and readily available. The energy costs alone are a fraction of what they would be for a substance such as steel, and the raw materials for concrete are often available near the construction site, thus saving considerable transportation costs. Another important advantage of concrete is the ability to form it in a wide variety of shapes and sizes, quite often on the job site. Concrete is also known for its long life and low maintenance, due to its strong binding characteristics and resistance to water. Finally, concrete’s high strength in compression and proven long-term performance make it a good choice for many structural components.

Concrete for structural uses comes in four major forms. Ready-mixed concrete is transported to a construction site as a cement paste and is then poured into forms to make roadways, driveways, floor slabs, foundation footings, and many other types of structural foundations. Precast concrete can be used for anything from a birdbath to wall slabs, which are cast at a concrete work and then transported to their intended site. A common example of a precast member might be the beams of a highway overpass. Reinforced concrete is any concrete to which reinforcement (usually steel rods) has been added to increase its strength. Prestressed concrete is a relatively new form of concrete. Developed in the 1920s, prestressed concrete is put under compression through the use of jacks or steel cables, such that a beam is always in compression, whereas an unstressed beam in the same place would experience tension.

These forms of concrete are used, often in combination, in a variety of ways. Slabs, walls, pipes, dams, spillways, and even elegant vaulted roofs (known as thinshell vaulting) are all made of concrete. Cement, especially as it is used in concrete, has played a crucial role in shaping the physical environment. Concrete is the most widely used manufactured construction material in the world. In most modern countries, the ratio of concrete consumption to steel consumption is at least ten to one. Although concrete often is taken for granted, the world is literally built upon it.

Principal Terms

aggregate: a mineral filler such as sand or gravel that, when mixed with cement paste, forms concrete

alumina: sometimes called aluminum sesquioxide; a material found in clay minerals along with silica; tricalcium aluminate acts as a flux in cement manufacturing

clinker: irregular lumps of fused raw materials to which gypsum is added before grinding into finely powdered cement

concrete: a composite construction material that consists of aggregate particles bound by cement

gypsum: a natural mineral, hydrated calcium sulfate; it helps control the setting time of cement

hydraulic cement: any cement that sets and hardens under water; the most common type is known as Portland cement

lime: a common name for calcium oxide; it appears in cement both in an uncombined form and combined with silica and alumina

silica: silicon dioxide; it reacts with lime and alkali oxides, and is a key component in cement

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