Limestone is primarily calcium carbonate (CaCO3)... its the source of "agricultural lime" or "dolomitic lime" that's used to balance the pH of soil mixes. In a gardening context, it's very alkaline. You something basically inert for the gritty mix, like granite, quartz, and many other silicate/clay minerals.

Could something acidic be added to the mix to neutralize the limestone?

Actually, John - the limestone reacts with the acid to neutralize it. CaCO3 is generally considered insoluble at sizes larger than 200 mesh for agri applications, but that is when adding a few pounds per acre. Using it as a primary fraction of a container media would completely eliminate the buffering capacity of the media and the pH would essentially rise to whatever the pH of the limestone is, which will be very high. There would be so much limestone that you couldn't apply enough acid to neutralize it w/o the limestone becoming soluble in the acid & being washed from the container.

Al