Cement Manufacturing

What is Portland cement?

Many people confuse the terms “cement” and “concrete.” Portland cement is a manufactured product that is an ingredient in various concrete products. Portland cement is sold as a fine powder that is blended with water and aggregates to make Portland cement concrete (PCC).  Portland cement is  composed of calcium silicates, calcium aluminates, calcium aluminoferrites and usually some gypsum. When water is added to the cement, the calcium minerals hydrate and form a gel. This gel is what holds the aggregate together in concrete. 

There are eight types of Portland cements, each with special use and chemical requirements.  The manufacturing process, however, is essentially the same and is a product of a kiln operation.  Cement manufacturing is a two-stage process.  Materials such as limestone that contain calcium oxide are mixed with silica and alumina materials such as sand, shale or clay.  The raw materials are usually dried and ground, and then the mixture is heated in a rotary kiln to form clinker. The clinker is then mixed with gypsum and other materials and ground into the fine powder (200 mesh sieve) known as Portland cement.

How is Foundry Sand used in manufacturing Portland cement?

Foundry sand is considered an “alternative material” which can be substituted for virgin raw materials.  Raw materials used in Portland cement manufacturing must contain appropriate proportions of calcium oxide, silica, alumina, and iron oxide.  Portland cement mixtures typically contain 10-12% silica by weight and alumina and iron oxides (2-5% by weight).  These mineral components are significant components of most foundry sands, which can therefore replace virgin minerals.  According to a 2008 survey by the Portland Cement Association, 13 of the nation’s cement kilns were sourcing foundry sand.  Ground cupola slag can also be used as raw material for cement manufacture.

How does Portland cement perform using Foundry Sand?

Portland cement manufactured with foundry sand meets all the quality and performance requirements of Portland cement made solely with virgin materials.  A study by the American Foundry Society indicated that Portland cement manufactured with foundry sand may exhibit higher compressive strengths than Portland cement made with conventional raw materials.

Foundry sand is used as a manufacturing feedstock in the production of Portland cement.  Different types of Portland cement have different chemical and physical requirements because of the varying uses of the cement.  ASTM Standard C-150, Standard Specification for Portland Cement, designates the different types of Portland cement. 

What are the technical issues associated with Foundry Sand in Portland cement manufacturing?

Chemical consistency of foundry sands are more important than physical characteristics in determining suitability for Portland cement manufacturing.  The silica content of foundry sands exceeds the 80% minimum silica content that Portland cement kilns require and the presence of other elements such as iron and aluminum is an asset.  Clay content of foundry sand can be an issue if it poses flowability problems at cement kilns with wet raw material mixing processes.

Although foundry sand can be an excellent feedstock for Portland cement manufacturing, transportation distances may be an impediment to sourcing more foundry sands for Portland cement kilns.  There are only 118 cement kilns in the United States, some of which own adjacent quarries where limestone and other virgin aggregates are processed for cement production.

Are there any specific QA/QC issues that suppliers and/or end users need to be aware of?

Screened foundry sand can be a desirable material for cement manufacture feedstock if transportation distances do not preclude its use. The kiln feeder systems are designed to handle grain-sized materials and screened foundry sand is already a fine aggregate.  That means that the use of foundry sand avoids the costs and environmental impacts of grinding other raw materials. 

Foundry sand destined for a cement kiln should be screened to be free of all debris, core butts, tramp metal and extraneous materials. Tramp metal, in particular, can damage the kiln feeder systems.  Core butts can be ground and mixed with other sand streams so long as chemical consistency is maintained.  Sodium silicate foundry sands should be avoided as that particular binder type changes the chemistry of the cement. The largest detriment to increasing the use of foundry sand in cement manufacturing is obtaining necessary quantities of screened sand on a consistent basis. 

Are there any specific environmental issues associated with the use of Foundry Sand in Portland cement?

The Portland Cement Association has established a program to make the manufacture and use of Portland cement more sustainable.  Using foundry sand in the manufacturing process of Portland cement helps to create a more sustainable product.  Tremendous amounts of raw materials containing lime, silica, alumina and iron oxide are required to meet the demand for Portland cement. Cement manufacturing is a very energy-intensive industrial activity.  In addition to reducing virgin material usage, foundry sand use saves energy through avoidance of mining, crushing and grinding virgin raw materials down to grain size.  

Click on the links below for additional resources and information on cement.

Portland Cement Association