When working with fresh concrete, care should be taken to avoid skin irritation or chemical burns. Prolonged contact between fresh concrete and skin surfaces, eyes, and clothing may result in burns that are quite severe, including third-degree burns. If irritation persists consult a physician. For deep burns or large affected skin areas, seek medical attention immediately.
The A-B-Cs of fresh concrete's effect on skin are:
Abrasive Sand contained in fresh concrete is abrasive to bare skin.
Basic & portland cement is alkaline in nature, so wet
Caustic concrete and other cement mixtures are strongly basic (pH of 12 to 13). Strong bases-like strong acids-are harmful, or caustic to skin.
Drying portland cement is hygroscopic-it absorbs water. In fact, portland cement needs water to harden. It will draw water away from any material it contacts-including skin.
Clothing worn as protection from fresh concrete should not be allowed to become saturated with moisture from fresh concrete because saturated clothing can transmit alkaline or hygroscopic effects to the skin.
Waterproof gloves, a long-sleeved shirt, and long pants should be worn. If you must stand in fresh concrete while it is being placed, screeded, or floated, wear rubber boots high enough to prevent concrete from getting into them.
The best way to avoid skin irritation is to wash frequently with pH neutral soap and clean water.
Source: http://www.cement.org/cement-concrete-basics/working-with-concrete/working-safely-with-concrete