The following letter was received from the Silica Fume Association concerning the article titled “High Performance Concrete in Colorado,” which was published in HPC Bridge Views, Issue 55, May/June 2009.
Editor
The article titled “High Performance Concrete in Colorado” contains a misleading statement regarding the safety of using silica fume in high performance concrete.
A common misconception in the construction industry is that if the labeling of any material contains the word “silica,” it must be a health hazard. Silicas (the common name for silicon dioxides) are all around us, both in natural forms and made-made forms. In fact, silicon ranks second to oxygen in abundance in the earth’s crust in the form of silicate minerals and is present as the oxide (silica) in soils and sediments, in some organisms such as diatoms, and in some plants. Silicas are safe for use in construction and other industries following the specific instructions found in that particular product’s Material Safety Data Sheet (MSDS). The Silica Fume Association has a mandate to educate and, through the following information, is attempting to correct a common misunderstanding.
Silica fume is recognized by standards organizations and industry specification committees such as the Occupational Safety and Health Administration (OSHA), the American College of Governmental Industrial Hygienists (ACGIH), the National Institute for Occupational Safety and Health (NIOSH), and ASTM International as a form of amorphous silica. As an amorphous silica, silica fume does not present the same health risks to concrete workers, as do respirable crystalline silicas.
Amorphous silicas share little in common with crystalline silicas. Unlike crystalline silicas, which have long been recognized as a cause of occupational disease, amorphous silicas are not associated with any permanent or debilitating lung or other disorders.
For more than 30 years, silica exposure levels and their health effects have been intensively studied. As a result, crystalline silicas have become heavily regulated by OSHA, ACGIH, NIOSH, and many other organizations. In 2006, the ACGIH withdrew the threshold limit values (TLV) of respirable silica, (Amorphous – silica fume, Chemical Abstracts Service Registry Number 69012-64-2); thereby supporting the industry practice of following the prescribed cautions in the MSDS for silica fume and treating silica fume as a “nuisance dust” in relation to worker safety.
Silica fume, widely recognized as a valuable ingredient in the production of high performance concrete, makes good sense for producing sustainable concrete structures. Silica fume concrete greatly enhances the service life of many types of structures, especially concrete bridges. We doubt the authors of “High Performance Concrete in Colorado” meant to single out silica fume; rather they may have been highlighting the challenges of using any dusty product in a small bag rather than in bulk silos. We must point out that silica fume is available throughout the United States in bulk form, just like cement and fly ash, and that bulk silica fume is the most common form used in concrete plants. When bulk silica fume is used, the amount of dust created during the batching process is virtually eliminated and workers’ exposure to dust at the plant is significantly reduced.
In closing, great care should be taken when evaluating the health risk of any products used in the construction industry. Close attention must be paid to products labeled “silica” as to their specific form being either crystalline or amorphous and the amount of dust generated when dispensing the product.
Executive Board of the Silica Fume Association
Authors’ Response
The authors would like to thank the Silica Fume Association (SFA) for their clarification of some of the issues surrounding silica fume. As they have pointed out, using bulk silica fume properly can reduce dust exposure as opposed to the small bag process. Our article was intended to highlight the current state of practice in Colorado only, not disparage any material or material supplier. Silica fume has been and will continue to be a vital component of high performance concrete. We’ve met with the Director of SFA and look forward to continue working with SFA as well as other material and concrete associations in reaching our goals for high performance concretes.
Andrew Pott and Jamal Elkaissi