Usually that warning applies only in California—most states are not at risk. :-)
The carcinogen involved is cement dust, specifically its components of tri- and dicalcium silicate, alumina, tricalcium aluminate, and iron oxide. The hazard extends to skin, eyes, mouth, and respiration.
To be completely protected, a moon suit (made of Tyvek in the photo) is in line with the most pessimistic expectations. However, the practical alternative (and recommended by CDC and OSHA) is any quarter-mask respirator with an N-95, R, or P filter to arrest fine particles, along with gloves, and enclosed eye protection—and avoiding unnecessary cement contact.
Many workers are skeptical of the carcinogenic property of cement dust: it is common to see experienced workers taking only minor precautions. Usually wearing only a mask while handling the powder. Once it is moistened and and dust is no longer a factor, most don't do anything special. But it is a strong caustic alkalai, so do take care not to get it inside or on you for an extended time.