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Portland Cement — The Glue in Concrete

Concrete is the most widely used construction material in the world. It forms at least part of nearly everything we build – foundations, roads, dams, bridges, high-rise structures, sidewalks, and homes. It is comprised of some the most readily available materials on the earth – gravel, sand, water and a special type of cement. What is this special product that makes concrete possible? Its name is Portland cement. Portland cements are a combination of a variety of ingredients, primarily limestone, shale, and clay, and they are classified as hydraulic cements. Hydraulic cements set and harden by reacting chemically with water. This reaction, called hydration, creates a solid, stone-like mass. When combined with aggregates, such as sand and gravel, the Portland cement/water mixture acts like a paste to bind the entire composition together in the product we call concrete. The United States is the third largest producer of Portland Cement in the world, behind China and India. There are 39 companies in the U.S. operating 118 cement plants in 38 states. The total value of U.S. annual production of cement is around $9 billion.

Hydration, the chemical reaction that causes concrete to harden, begins as soon as the cement in the concrete comes in contact with water. The cement particles exhibit a crystalline type growth that envelopes the aggregates and other cement particles. As this growth continues, the mixture hardens and strength develops (evidenced by a lack of workability). Hydration continues as long as there is space available for the crystals to develop and there is sufficient moisture and temperature. The chemical reaction produces heat as the process progresses. This “heat of hydration” helps the concrete cure when placed in colder temperatures. Most of the strength development and hydration takes place in the first 30 days but it continues at a slower rate over a much longer period. Concrete in laboratory conditions has continued to gain strength for over 50 years.

So what is the history of Portland cement? The Assyrians and Babylonians were among the first “constructors” to use cement-like materials. They used clay as the bonding substance or cement to bond together masonry units. The Egyptians came closer to modern-day cement when they used lime and gypsum to form a cement-type product. John Smeaton made the first modern concrete (cement and aggregates mixed together) by adding pebbles as a coarse aggregate and mixing powered brick into cement. Portland cement as we know it today is attributed to a British stone mason named Joseph Aspdin who was searching for a manufactured counterpart to natural or Roman cement – a crude formulation of lime and volcanic ash used as early as 27 BC. This quest led to his discovery and a patent for Portland cement in 1824.

The product was a finely ground powder that hardened when mixed with water. He named it Portland cement because it resembled a stone quarried on the Isle of Portland, just off the British Coast.

The raw materials that comprise Portland cement are natural occurring minerals that are typically mined, where possible, in close proximity to the manufacturing facility. They include combinations of limestone, shells or chalk, and shale, clay, sand or iron ore. The raw materials, typically mined as stone or rock, go through a series of size reductions with rock crushers until they are approximately 3/4” in size. The raw materials are combined at the manufacturing facility using either a wet or dry mixing process. The dry process involves grinding the materials to a fine powder while the wet process involves adding water to the ingredients to create a slurry. The mixed product is then fed into a sloping, rotating, drum kiln where the raw materials are heated to between 2600 and 3000 degrees F. The intense heat actually causes a series of chemical reactions in the ingredients. The product coming out the bottom of the kiln is called a clinker. The cooled clinker is combined with gypsum to slow the curing process and ground to a fine gray powder of around 75 microns in cross section. It is now Portland cement.

Modern Portland cement contains other materials as well. They include calcium oxide, silica, alumina, and iron oxide. During the manufacturing process the composition is frequently monitored to make certain the ingredients are present in the proper proportions.

While there are several types of Portland cement the wall contractor typically only deals with two of them. Each type has a specific formulation and specific properties. Type I is the basic gray product that we use on a day-to-day basis. Type III is referred to as High Early strength cement and is useful in cold weather to hasten the initial set or when it is critical to apply significant loads to the wall early in the curing cycle. The primary difference from Type I is that Type III is ground into finer particles. Other types of cement are designed for low heat production, sulfate resistance, or a combination of properties. Another type of cement that you may encounter is called blended hydraulic cement. Blended cements combine other cementitious materials to reduce the amount of Portland cement. Blending materials include ground, granulated blast furnace slag, a byproduct of the steel manufacturing process; or pozzolans. Fly ash, a byproduct of electric power production in coal-fired plants, is the most common pozzolan. These products reduce the cost of the cement, make it more environmentally friendly (by using waste materials) and modify the properties of the basic Portland Cement. The use of blended cements in concrete reduces mixing water and bleeding, improves finishibality and workability, enhances sulfate resistance, inhibits the alkaliaggregate reaction, and lessens heat evolution during hydration, thus moderating the chances for thermal cracking on cooling.

Most of the concrete used in today’s modern commercial structures are reinforced concrete. Reinforced concrete includes imbedded metal (usually steel) in the structure and is called reinforced concrete or ferroconcrete. It combines the tensile strength of metal and the compression strength of less expensive concrete to withstand heavy loads. It was invented in 1849 by Joseph Monier who received a patent on the product in 1867. Joseph Monier was a gardener who made pots and tubs of concrete reinforced with a metal mesh.

Manufacturing Portland cement requires a tremendous amount of energy which is one of the reasons concrete has come under fire by some environmentalists. When the positive aspects of concrete construction are evaluated, such as the use of locally available materials; the ability to recycle the product when its useful life has expired; low maintenance; thermal storage capacity; and, you factor in the life span of a concrete structure, it fares as well, if not better, than many other common building materials.

For more information on Portland cement, visit the Portland Cement Association web site at www.cement.org/manufacture or www.cement.org/basics/images/flashtour.html.

Ed Sauter, Executive Director, CFA
esauter@cfawalls.org

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Concrete FACTS, a publication of the Concrete Foundations Association, is THE voice for residential concrete industry news, market intelligence, business strategies, technical solutions, product information, and other resources for professionals in the cast-in-place concrete industry. Subscriptions to Concrete FACTS is available to anyone involved or interested in the residential concrete industry as a service to your industry. Please contact CFA Headquarters to find out more about your free subscription or Email Us