Excessive moisture vapor movement will bring additional hydroxides (alkalis) to the surface and will cause the pH level to increase. Alkaline salts in solution with moisture, which exude from concrete or which work their way up from the earth in concrete on grade or below grade, have a tendency to destroy satisfactory bonding of adhesives by sheer physical displacement. They can leave unsightly salt deposits at the seams of sheet materials and joints of tiles. The surface of the concrete needs to be exposed to carbon dioxide, which neutralizes the existing pH on the concrete’s surface and will penetrate down about one-millimeter. If you were to dig down beneath the carbonation layer you will find the internal pH of the slab to have a pH of about 12.0. Remember the pH scale is a logarithmic scale with each number being 10-time greater than the previous one. Example: a ph of 7.0 is neutral and a pH of 10.0 is 1,000 times greater than a pH of 7.0. Most adhesives on the market have a pH tolerance of 9.0 with some going as high as a pH of 10.0.