Cement is everywhere.  Last year alone, 3.6 billion tons of cement was produced in the world.  This amount could increase by a billion tons by 2050.  Cement is important to us because it is hardening agent in concrete, which is one of our most important and commonly used building materials.  Behind water, concrete is the most commonly used commodity in the world.

Cement production dates to the ancient Roman's, who produced mortars using a mixture of volcanic ash, lime, and crushed clay.  Cement is produced using a two-step process.  The first step is the high-temperature mixing and processing of limestone, sand, and clay to produce a cement powder.  The second step involves the hydration (adding water), mixing, and setting of the cement powder into a final cement product.  The dry portion of Portland cement is composed of about 63% calcium oxide, 20% silica, 6% alumina, 3% iron oxide, and small amounts of other materials.  Cement is then blended with sand, gravel and water to produce concrete.  Depending on the demands of the end use, a variety of different mixtures can be devised to increase the strength and/or utility for the concrete product.

Cement is widely used, as a constituent of concrete, because it is inexpensive, pourable (can take any form), and becomes hard as a rock.  However, cement production emits substantial quantities of carbon dioxide into the atmosphere.  The key ingredient of cement is limestone.  The recipe for making cement calls for heating the limestone, which is a byproduct, result in carbon dioxide emissions.  Limestone is calcium carbonate when heated above 840°C decomposes into calcium oxide (lime) which releases high amounts of carbon dioxide.  Cement production is responsible for 5% of the world's human-produced carbon emissions.  As a result, the cement industry is one of the primary targets of climate change regulations that look to reduce  carbon dioxide emissions.  To address this challenge the industry is developing new ways to produce cement with lower carbon emissions, producing concrete with admixtures and developing green building standards.

Chemical admixtures can be used to improve the performance of concrete blends and also to reduce environmental concerns.  Chemical admixtures are the ingredients in concrete that are added to the mix immediately before or during mixing.  Admixtures are used primarily to reduce the cost of concrete construction; to modify the properties of hardened concrete; and to ensure the quality of concrete during mixing, transporting, placing, and curing.

Cement producers from all over the world are trying to find new ways to make Portland cement more environmentally palatable.  The use of admixtures is an important element in this process.  Nearly 70% of U.S. Portland cement plants use one or more of these byproducts to produce cement. These admixtures are used for a variety of purposes and can be used to reduce the overall quantity of Portland cement in a given blend. 

A number of alternatives or improvements to Portland cement are currently being developed.  Examples include:

·         RHA (rice husk ash) has been found as an eco-friendly addition to conventional concrete.  RHA is an agricultural byproduct left over from harvesting rice.  When burned, rice hulls precipitate silica, which can then be used as a concrete admixture.  It is abundant and cheap.  By adding RHA into a concrete mixture, compressive strength, durability and workability are said to increase.  RHA also reduces the amount of cement required, making it more economical and therefore, lessens the amount of fossil fuels required to produce a given batch of concrete.  Although the concept still needs to be explored, many areas in India are now using RHA as a concrete admixture.

·         A company in California called Calera has found another approach.  A process has been developed to harness carbon dioxide produced from power plants and mixes it with sea water or brine to create carbonates that are used to make cement.  This can then be added to cement to replace some or all of the limestone.

·         An Australian company, Calix, makes cement using superheated steam, modifying the particles in cement.  This reduces impurities and makes the cement more chemically reactive.  This process also separates out carbon dioxide, making it easier to capture the gas and keeps it from entering the atmosphere.

·         In Germany scientists found a group of hydraulic-binding agents, called Celitement.  Celitement is based on hydraulically active calcium hydrosilicates, which needs less lime time (time to cure or harden) during the production process.  Using Celitement, the burning temperature can be reduced to 300°C.  This is a reduction from 1450ºC needed for conventional cement production practices, reducing total energy demand by up to 50%.  However, the structural integrity of this material is not well known.  A failure of the material during construction could have serious consequences for humans and the environment.  Thus, intensive research and testing trials must be conducted before Celitement can be used commercially.

·         Drexel University scientists have created a low-tech, low-energy, low-cost cement that they are hoping to move out of the lab and into the real world.  Tests have shown that the cement is as durable as Portland, but emits 9% less carbon dioxide.  The reduction of energy used to produce the Drexel cement could result in production cost savings of up to 50%.

Governments in Asia and elsewhere are kick-starting green building industries, opening doors for green cements. China's newest cement standards, for instance, require a 15% reduction in energy use.

The science of cement production has extended for more than 3,000 years and continues to improve.  With the emphasis on reduction of greenhouse gas emissions, the need to curtail significant sources of man made carbon dioxide are increasing.  Ongoing research promises to reduce the carbon footprint of the cement industry and improve the characteristics of this product for a variety of applications.