Areas with respirator crystalline silica must be marked to prevent unauthorized entry. The requirement for signs are different for construction and general industry.
Silica Signs for General Industry
1910.1053(j)(2) requires that silica regulated areas be marked with a sign that reads:
RESPIRABLE CRYSTALLINE SILICA
MAY CAUSE CANCER
CAUSES DAMAGE TO LUNGS
WEAR RESPIRATORY PROTECTION IN
AUTHORIZED PERSONNEL ONLY
You can find the respirable crystalline silica danger sign here.
Silica Signs for Construction
Silica construction regulations have no specific requirements for signs. 1926.1153(g)(1)(iv) does require the employer to include the method for restricting access to silica hazard areas as part of the exposure control plan. This would include signs or markings of some sort, but the method of communication is up to the employer.