If I need to adjust the pH of my media - not likely unless I am experimenting - I use hydrated ('slaked') lime which is just CaOH. No Mg in there.
"dolomitic lime is very slow to break down and is intended for outdoor flowerbeds and lawns where it has all season to work."
I don't think the lime is what does buffering. From "The Geography of Soils":
"Buffering action is due mainly to clay and very fine organic matter. Highly weathered tropical clays are less active buffers than most less-weathered silicate clays. Thus with the same amount of acidity, or pH, more lime is required to neutralize ...."
However some support for the idea comes from:
"Buffer compounds, soil: the clay, organic matter, and compounds such as carbonates and phosphates that enable the soil to resist appreciable changes in pH."
It's very difficult to find much information, the above took me over an hour to track down.