By Julie Etra
First, a primer. Concrete consists of three basic components: 1. water; 2. aggregate (rock, sand, or gravel); and 3. cement.
Portland cement. Portland cement consists of limestone and clay. The limestone is essential, of course, but minor elements in cement can vary as a function of the parent material (the native rock formation). Limestone is by definition calcium carbonate (CaCO3) and is a precipitate, or a sedimentary rock, formed from calcareous parts of marine animals. Chalk is a fine form of limestone. Cement is usually in powder form and acts as a binding agent when mixed with water and aggregates.
Portland cement was named for Portland stone, quarried from the Isle of Portland in Dorset, England. It is manufactured through controlled chemical combination of the calcium in limestone, silicon, aluminum, iron and other ingredients. Common materials used to manufacture cement include limestone (shells, chalk) and marl (or marlstone, which is a calcium carbonate or lime-rich mud or mudstone that contains variable amounts of clays and silt), combined with shale, clay, slate, blast furnace slag (clinkers), silica sand, and iron ore.
Gypsum concrete. Gypsum is calcium sulfate (CaSO4), instead of calcium carbonate. It is a mixture of gypsum plaster, Portland cement, and sand. The brand name “Gyp-Crete” has come into general use by U.S. construction professionals and architects to describe gypsum concrete.
Dolomite is calcium magnesium carbonate (C2CaMgO6), also used in some cement blends and is a viable solution for producing Portland dolomite limestone cement, especially where quarries have dolomitic inclusions or overburden.
When was concrete first used in construction? The Romans made concrete by mixing lime and volcanic rock to form a mortar. To build underwater structures, this mortar and volcanic “tuff” were packed into wooden forms. In addition to being more durable than Portland cement, experts argue, Roman concrete may be less damaging to the environment to produce. The Romans did not use any reinforcement, which was first invented in 1849 (reinforcement bar, or rebar). In 1889 the first concrete reinforced bridge was built, and the first large concrete dams in 1936 (Hoover and Grand Coulee Dams).
In Mexico, the largest producer of cement is CEMEX S.A. de C.V. (Sociedad Anonima de Capital Variable, basically “Inc.”), headquartered in Monterrey, capital of the state of Nuevo Leon. They produce three types of cement: mortero, Óptimo gray, and white, with different characteristics and different purposes. CEMEX also owns Construrama.
Founded in Hidalgo, Mexico, in 1906, CEMEX is a multinational building materials company that manufactures and distributes cement, ready-mix concrete and aggregate in over 50 countries. As of the writing of this article, it is the second largest building materials company in the world. There are 64 quarries in the mountains surrounding the city of Monterrey, mostly owned by CEMEX, and anyone who has traveled in the area can testify to the massive earthworks.
There is a downside to production in this arid area. According to a 2013 article in La Jornada, the extraction and processing has resulted in an air quality problem with an annual average concentration of 85.9 micrograms per cubic meter of suspended particles less than 10 microns, four times more than the limit recommended by the World Health Organization. (The data came from the latest report of the Clean Air Institute, entitled Air Quality in Latin America: An Overview .) One of the many potentially irreversible damages is against ecological reserves such as the Sierra Picachos, the largest protected area in Mexico from the point of view of biodiversity, with 75,850 hectares. Cerro de las Mitras has several quarries at its base and numerous other quarries are located north and west of the city. In summary, this area is one of the largest sources of Portland Cement in the world.
CEMEX of course has numerous quarries besides those in Monterrey, given the ubiquity of the product and market. In the U.S. they operate nearly 60 aggregate quarries, for example in Lyons, Colorado; and Victorville, Santa Barbara, Lytle Creek, and Monterey, California, just to name a few. If you start an internet search for retail outlets the list is huge. One map of the Bay Area of California and environs north and south yielded 14 suppliers of cement and aggregates. CEMEX is present everywhere across the USA.
Not entirely surprising, CEMEX has had its share of legal problems in the USA. It has been accused of violating the Clean Air Act in Lyons, Victorville, and Santa Barbara, CA. Dusty business. There can be minor carcinogenic minerals in cement, such as chromium VI (also known as hexavalent chromium). This is the contaminant that inspired the movie Erin Brockovich. Ten years ago, during tests conducted from June to August, the Monterey Bay Unified Air Pollution Control District reported high levels of chromium VI in Davenport, California. The toxic substance apparently originated from dust emitted by the CEMEX plant, with chromium VI measuring 8 to 10 times the air district’s acceptable level. Environmentalists and scientists remain concerned for the Monterey Bay coastline.
The other major supplier in Mexico is Cemento Cruz Azul, or Cement Blue Cross, but that is a topic for another article.