Originally Posted by 7ways
I've never heard that calcium carbonate is not effective.
I have come across this detail a while back when I found that John Palmer's water spreadsheet didn't get the correct alkalinity from chalk yet the residual alkalinity was still correct.
Here are a few blog posts that shed more light on chalk and its affect on pH in brewing:http://braukaiser.com/blog/?s=chalk
I don't know of any guidelines for using lime or how it would affect hardness, can you make a suggestion based on an anticipated London water profile? And should I add it with the roasted malts, or at the beginning of the mash?
despite its shortcomings with regard to solubility I still think chalk is useful for raising alkalinity in brewing. While I, and others, also have evidence that chalk can raise pH only by 0.2 pH points it is also true that a very high residual alkalinity, for which you wold need lots of chalk, is rarely needed.
Depending on the coarsness of your grist, adding roasted grains towards the end of the mash may also caused them to release less of their pH lowering acids. This also limits the need for chalk or baking soda.
On problem you have is that w/o adding the roasted grains at the beginning and w/o having negative residual alkalinity in the water your mash pH may actually end up a bit high. Most pale base malts give you a mash pH of 5.7 in distilled water (pretty much what you have). That means you want to at least add some calcium salts (Gypsum and/or Calcium Chloride) in the strike water. Maybe aim for 100 ppm Ca.
pH measurement of the boil pH may show if you need to add some chalk or some other alkalinity boosting salt to bring it to ~5.4 at the beginning of the boil.