In 1973, the Barrow Lab explored this question and concluded that even high pH's of 10 or 12 were harmless, provided the deacidification solutions used had contained only calcium or magnesium compounds.
Sodium compounds, however, swelled and dissolved the cellulose of the paper at all concentrations used,
This was reported in summary form in The American Archivist, 38 (1): Jan, 1975.
A related report appeared in a recent issue of the same publication, summarizing research done at the National Archives on magnesium bicarbonate solutions The authors found that magnesium hydroxide (the active ingredient in milk of magnesia) made a better solution than magnesium carbonate because it was much easier to dissolve. Both solutions turned into magnesium bicarbonate when CO2 was bubbled through them. Although more concentrated solutions were possible when magnesium hydroxide was used, only slight increases in the pH of paper treated with the solutions resulted; the excess went toward buffering. The upper limit seemed to be just under pH 10,5, and this was about as easy to reach with a weak solution (.02 mole Mg/liter) as with a strong one (.1 mole/liter). The authors say, "One should keep in mind that the more concentrated solutions are not very stable and should be used as soon as possible, Solutions 0.04 molar and weaker appear to be stable, especially if saturated with carbon dioxide and stored in closed containers," It should also be noted that dry MgOH (magnesium hydroxide) is unstable and should be stored in closed containers.
The complete reference to this summary report is. William K. Wilson, Mary C McKiel, James L. Gear, and Robert H. MacClaren. "Preparation of Solutions of Magnesium Bicarbonate for Deacidification " The American Archivist, 41 (1): 67-70, Jan 1978.