Which of the following is a gray powder used in concrete?

Cement is a gray powder that is a key ingredient in concrete. It is made from a mixture of raw materials, primarily limestone and clay, which are heated in a kiln to form clinker. This clinker is then ground into a fine powder, which, when mixed with water, aggregates, and sometimes other additives, hydrates to bind all the components together, creating a solid and durable structure.

In the context of the other options, caulking is a sealant used primarily for filling gaps or joints to prevent water or air leaks, chase refers to a type of channel or groove often used in construction for running electrical wires or plumbing, and cap flashing is a piece of material used to cover the top of a flashing to prevent water intrusion. None of these components serve as a primary binding agent in concrete like cement does.