buffering ph by adding minerals

i know you can add various things to your water to buffer the ph
towards a certain value, that value being different depending exactly
which mineral(s) you add, but does anyone know what will buffer to a
ph of 7?
Sodium bicarb buffers to around 8 i think
coral to around 8.2 ish
limestone...not sure

I did try evian! its got 360ppm bicarbonates (kh) and is ph 7.2, so
you can add as much as you need to achieve any kh you want, but its
effects seem to be temporary, and i think i need to add a solid so
that it can slowly dissolve.....but what to add?

chris