pH is a measure of the concentration of acid ions in a solution. The pH scale runs from 0 to 14; 7 is neutral. Below 7 is considered acidic and above 7 is basic. It is often incorrectly called “alkalinity” which really refers to the concentration of ions of calcium, magnesium, sodium and potassium.
The pH of new concrete will be approximately 12 to 13 mostly due to calcium hydroxide, which is a normally by-product of cement hydration. As a concrete surface reacts with carbon dioxide in air, the pH of the surface gradually is reduced to about 8.5 through a process called carbonation. A dry, normally carbonated concrete surface is ideal for flooring installation and adhesive performance. A high pH surface with excessive moisture can damage floor coverings and break down adhesives leading to flooring failures.
In the presence of a continuing water source, dissolved alkalies and hydroxides can be carried to the surface of a concrete slab. Therefore, if testing shows high pH, it is important to determine the cause.
Alkaline salts in solution with moisture, which exude from concrete or which work their way up from the earth into on-grade or below-grade concrete slabs, 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. They can also have a deteriorating effect on the overall installation.
Concrete floors can be tested for pH following procedures in ASTM F 710 before the installation of resilient flooring. Distilled water is placed on the slab for 60 seconds and a pH indicator strip is placed into the water. A chart normally supplied with the strips allows you to determine the pH based on the color change of the test strip.
Rinsing and vacuuming with potable water is the best way to lower pH, but it cannot prevent the future deposit of salts on the surface of the slab. Do not use acid rinses to “neutralize” a high-pH concrete surface. The acid will deposit unwanted salts and can attack interior building finishes and be detrimental to the final installation.
The testing of concrete for pH can show the pH only at the time the test is run and cannot be used to predict long-term conditions.