Cement and concrete
Cement and concrete are fundamental materials in construction, significantly influencing the built environment. Concrete, the most widely used manufactured construction material globally, is created by combining cement with aggregates like sand, gravel, or crushed stone, resulting in a durable and load-bearing product. Cement itself is a versatile binder that hardens over time, primarily produced from raw materials such as limestone, clay, and gypsum. The history of cement dates back over six thousand years, evolving from simple mixtures used by ancient civilizations, including the Egyptians, Greeks, and Romans, to modern hydraulic cements like portland cement, which is named after a specific type of limestone.
Manufacturing cement involves extracting raw materials, which are then ground, mixed, and heated in large kilns to create clinker, a key component of cement. Once cooled, the clinker is ground into a fine powder and combined with gypsum to produce the final product. Concrete is utilized in various forms, including ready-mixed, precast, reinforced, and prestressed concrete, each serving distinct applications from roadways to structural components. This multi-faceted use underscores the importance of cement and concrete in contemporary construction, reflecting both their historical significance and modern engineering advancements.
Subject Terms
Cement and concrete
Cement and concrete have played crucial roles in shaping humankind’s physical environment. Of all manufactured construction materials worldwide, concrete is the most widely used.
Background
Cement is an important construction material because of the ready availability of its raw materials, its capacity to be shaped prior to setting, and its durability after hardening. When combined with an (such as sand, gravel, or crushed rock), cement becomes concrete—a durable, load-bearing construction material.
Cements with the ability to set and harden underwater are called hydraulic cements. The most common of these is portland cement, consisting of compounds of lime mixed with silica, alumina, and iron oxide. Gypsum is also added to retard the setting time. When water is added, these ingredients react to form hydrated calcium silicates that will set into a hardened product.
History
Cement has been used for construction purposes for the past six thousand years. The Egyptians are known to have used a simple cement, and the Greeks and Romans advanced the technology by creating hydraulic cements from various volcanic materials and lime. Many examples of their concrete structures remain today—some underwater, where they were used in harbors.
The quality of cementing materials declined greatly during the Middle Ages but began to improve again in the late eighteenth century. In 1756, the famed British engineer John Smeaton was commissioned to rebuild the Eddystone Lighthouse near Cornwall, England. He undertook a search for lime mortars that would resist the action of the water and discovered that the best limestone contains a high proportion of clayey material. For his project he used lime mixed with pozzolana from Italy (the same volcanic material the Romans had used). Smeaton was followed by a number of researchers, including Joseph Aspdin, a Leeds builder, who patented “portland” cement, named for the high-quality building quarried at Portland, England.
Manufacturing Cement
Cement is a manufactured product, made from raw materials that are found relatively easily in nature. Cement manufacturers have a number of sources for lime, but the most common are and chalk. Coral and marine shell deposits are also used as sources of lime, when available. Silica, alumina, and iron are found in clays, shales, slates, and certain muds. Some raw materials contain almost all the ingredients of cement, especially marl (a compact clay), cement rock, and blast-furnace slag. Industrial wastes such as fly ash and calcium carbonate are also used as raw materials for cement, but not on a large scale.
Raw materials in the form of hard rock—such as limestone, slate, and some shales—are usually quarried, but they may also be mined. If the limestone is of low quality, it may need to go through a concentrating process. Softer materials such as chalk, clay, and mud can be dug by various types of machinery, depending on the physical setting and type of material. Once extracted, the raw materials are transported to the cement manufacturing plant by truck, rail, conveyor belt, or pipeline (when in a slurry).
At the plant, the raw materials are ground into a fine powder and then mixed in predetermined ratios. The mixing can be done wet, semidry, or dry. In the wet process the materials are ground wet and mixed into a slurry. In the semidry process they are ground dry, then moistened for adhesion; and in the dry process the raw materials remain dry throughout.
After mixing, the raw materials are burned in a large rotating kiln. Kilns are usually from four to eight meters in diameter and from 90 to 200 meters long, and they consist of a steel cylindrical shell inclined slightly from the horizontal. The mixture is introduced at the upper end of the kiln, and as it flows down the incline (with the help of gravity and the kiln’s steady rotation), it reaches a maximum temperature between 1,300° Celsius and 1,500° Celsius, at which point the raw materials interact to form calcium silicates. The heated material exits the kiln in the form of rough lumps or pellets—called clinker—no larger than 5 centimeters in diameter. After the clinker cools, the manufacturer adds gypsum and grinds the mixture into the fine powder known as portland cement.
Uses of Concrete
Concrete is generally used in four common forms: ready-mixed, precast, reinforced, and prestressed. Ready-mixed concrete is transported to a construction site as a cement paste and is then poured into forms to make roadways, foundations, driveways, floor slabs, and much more. Precast concrete—cast at a plant and then transported to the site—is used for everything from lawn ornaments to major structural elements. Reinforced concrete is created by adding steel mesh, reinforcing bars, or any other stiffening member to the concrete before it sets. Prestressed concrete, the most recently developed form, increases the strength of a beam by using reinforcing steel to keep the entire beam under compression. Concrete is much stronger under compression (pushed in on itself) than under tension (pulled apart).
Bibliography
Gani, M. S. J. Cement and Concrete. New York: Chapman & Hall, 1997.
"How Is Cement Made? Concrete vs Cement: What's the Difference?" Chart Industries, 9 Nov. 2023, www.chartindustries.com/Articles/how-is-cement-made. Accessed 26 Dec. 2024.
Lea, F. M. Lea’s Chemistry of Cement and Concrete. 4th ed. Edited by Peter C. Hewlett. New York: J. Wiley, 1998.
Mehta, P. K., and Paulo J. M. Monteiro. Concrete: Microstructure, Properties, and Materials. 3d ed. New York: McGraw-Hill, 2005.
Mindess, Sidney, J. Francis Young, and David Darwin. Concrete. 2d ed. Upper Saddle River, N.J.: Prentice Hall, 2003.
Neville, A. M. Properties of Concrete. 4th ed. Harlow, Essex, England: Longman Group, 1995.
Pollman, Herbert, Ruben Snellings, and Luca Valenti. "Cement and Concrete doi.org/10.2138/gselements.18.5.295. Accessed 26 Dec. 2024.