Type I Portland cement is most common in GFRC. For architectural GFRC with color, white cement is recommended for color consistency. Gray cements do not normally provide consistent color, even when pigmented.
Disclaimer
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.