Is the type of sand used in GFRC important?

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.