Is the type of sand used in GFRC important?

Article courtesy of Smooth-On, Inc.

Yes. The typical specification is 30-mesh silica: washed, dried and graded. We can look at each requirement for further explanation:

  • Washed: this ensures that impurities that may cause problems are removed. 
  • Dried: wet sand can contain as much as 5% moisture, making it very difficult to ensure a consistent water/cement ratio. This affects color, shrinkage and strengths. 
  • Graded: particles much larger than 30 mesh can block spray nozzles and particles smaller than 100 mesh, referred to as “fines”, require more water for workability due to higher surface area of particles. This creates a number of negative issues. 
  • Silica: natural silica sands have rounded particles, making them easier to pump and spray, creating fewer blockages. They are also less abrasive on equipment. Manufactured, or crushed sands, are angular and therefore, more abrasive. They are also susceptible to packing, making their use in spray applications very problematic. Wet cast or hand layup applications may use crushed sands meeting the other 3 requirements above.

This FAQ article is offered as a guideline and offers possible solutions to problems encountered during mold making and casting. No warranty is implied and it is up to the end user to determine suitability for any specific application. Always refer to the provided Technical Bulletins (TB) & Safety Data Sheets (SDS) before using any material. A small scale test is suggested to determine suitability of any recommendation before trying on a larger scale for any application.