pH Testing of Concrete - Clarifying some of the Confusion

pH testing is often misunderstood to the point where some have errantly referred to pH testing as "alkalinity testing" which it DEFINITELY is not alkalinity testing.

pH Changes over the same Concrete Surface

I have seen and read studies where an attempt to quantify or at least qualify pH testing that usually ended up concluding potential issues with and questioning the quality of the pH test strip, pH Paper, pH meter or even pH pencils.

Although possible for inconsistencies to occur, particularly with aging papers, non or improperly calibrated meters, by and large the materials and methods are pretty accurate, with my continuous precaution with ANY test method...tests are stupid, they measure something....it is up to the operator to understand WHAT is being measured and how to properly interpret the data.

pH testing is a simple procedure, but also one of the most challenging since temperatures, interactions of various hygroscopic materials, and concentrations of these various hygroscopic materials can give different results in measurable pH.

Alkalinity - A Focus on Calcium Hydroxide

Calcium hydroxide, also called "portlandite" is a by-product of cement hydration and possesses limited solubility, absent of any other influences. The solubility is so low that it barely qualifies as a soluble chemical compound.

Calcium hydroxide also behaves in a manner that is counter-intuitive if the operator doesn't understand that calcium hydroxide, unlike the more voluminous sodium hydroxide and sodium carbonate becomes more soluble as temperatures decrease. Most chemical reaction increase with increased temperatures, calcium hydroxide is a dramatic exception to this tendency. This is a subtle but important effect.

Calcium hydroxide has a maximum pH of 12.6, which is nearly impossible within a concrete surface since that pH level means the calcium hydroxide is at full saturation, which is an almost non-existent condition within the top surface of concrete, AND absent of other chemicals present that can limit the solubility of calcium hydroxide.

In an unrestrained environment, calcium hydroxide will register a change in pH of .003 units for every 1oC difference in temperature.

In a restrained environment, the presence of sodium hydroxide for example, the solubility of calcium hydroxide is suppressed and the pH and presence of calcium hydroxide may or may not be measurable, again depending upon its concentration and the concentration of the suppressing material.

In these conditions, the pH can be altered with changes in temperature and water volume.

Water can be present, but not yet in saturation when water is added to a surface to measure pH. If this water has an elevated temperature and the pH is approximately 12, this does NOT mean this is measuring the presence or absence of calcium hydroxide, even as it is WAY too often assumed that pH range is identifying its presence

pH, Concentration and Chemistry

With few exceptions, sodium salts tend to be the most voluminous alkaline salt in concrete. Unlike calcium hydroxide which becomes nearly insoluble when carbonated; sodium is no less soluble when it becomes carbonated and the pH can remain quite high in concentration with a pH of 12.

ALL alkaline salts will increase in pH with concentration and decrease pH in dilution.

Where this gets a bit complicated is that all three of these common alkaline salts; calcium hydroxide, sodium hydroxide and sodium carbonate are moderately to highly buffered.

Buffering is a characteristic not equally shared with most chemical compounds where a non-buffered compound has a linear correlation with increases and decreases in concentration.

With a highly buffered alkaline compound, changes in pH can be very gradual over a wide range of concentrations, where even an order of magnitude difference in concentration (depending on the starting point) may not register a difference when using paper, pencils or even with many meter types and further complicated by irregularities in concrete surface texture, porosity and permeability.

As a result, pH testing of a concrete surface merely gives a range of pH that can be quite dynamic at different times of the year and even in different times of the day.

pH testing does not and CANNOT identify what an alkaline compound is unless the pH is 13 or higher.

A pH of 13 or higher rules out calcium hydroxide and sodium carbonate as the alkaline component that is being identified by the pH measurement...HOWEVER, there may be compounds of a much higher concentration (i.e. sodium carbonate) that may be "the culprit" in a partially dissolved adhesive or damaged floor rather than the measured pH of sodium hydroxide, if the sodium hydroxide isn't concentrated.

Sodium carbonate has been historically known as "washing soda" and has been used as a household adhesive remover due to its safer handling as compared with sodium hydroxide which can be extremely hazardous.

Sodium carbonate tends to be a slower and more gradual reaction that softens and dissolves adhesives, whereas sodium hydroxide is much more aggressive and can literally destroy an adhesive.

pH Testing - Managing Expectations

pH testing can be a beneficial way to determine if a condition exists where caution needs to be exercised.

pH testing is NOT an "absolute" for qualification of a safe or unsafe concrete surface. What can be qualified is using the pH test to determine if it is actually concrete being tested.

For several decades I have used a pH of 9 as a benchmark, which has been very effective in assuring there are no objectionable coatings or pore blocking that could interfere with the installation of coatings or adhesives.