Concrete Alkalinity - Possibly the most Overlooked and Under-Researched Influence.

It sounds strange to say that alkalinity is overlooked, particularly when it is brought up in general terms whenever the question of concrete composition comes up. However, alkalinity is addressed by DECADES of research as either "alkalinity" or in a single-minded approach of studying the alkaline by-product of cement hydration; calcium hydroxide.

The alkaline compound: Calcium Hydroxide

Calcium hydroxide is a hydration by-product of clinker - unreacted particles that when hydrated, produces cement with the calcium hydroxide making up as much as 15% of the final hydration product/result. It has been found that calcium hydroxide can provide beneficial as well as negative effects, depending on the desired end use of the concrete. One of the well-established benefits is that the high pH of calcium hydroxide in contact with steel reinforcement, has a corrosion-inhibiting effect (preventing the steel reinforcement from rusting through passivation). Conversely, it has been found that the calcium hydroxide can and will react to carbon dioxide in the air, converting the higher pH calcium hydroxide (12.45-12.5) to the lower pH calcium carbonate (pH of 9-9.8).

Calcium hydroxide, unlike other common hydroxides has very limited solubility, and in absolute terms, is barely soluble and is influenced very uniquely by temperature as compared to the other common hydroxides. For example, as with most chemicals, common hydroxides such as sodium and potassium hydroxide tend to become more soluble and chemically active with increasing temperatures. Calcium hydroxide on the other hand, becomes less soluble with increasing temperatures, with its stated most soluble state at just above the freezing point of water.

This is where we are going to deviate a bit because there is a LOT of confusion regarding BOTH calcium hydroxide and calcium carbonate, where assumptions have been made with dangerously absolute terms.

The "solubility" differences between calcium hydroxide are slight, but impactful since calcium hydroxide, particularly in cooler temperatures is soluble enough to register within the time period allowed for a pH test or phenolphthalein indicator. Calcium carbonate on the other hand is nearly insoluble and even when dissolved in water, it usually undergoes at least a partial conversion to calcium bicarbonate, so the pH being (eventually) registered is slightly misleading. NOTE: field pH "tests" can be profoundly influenced by temperatures, which helps to explain differences in pH when measurements are taken during different times of the day or seasonally. ALWAYS take temperature reading of the ambient conditions and concrete surface when conducting pH tests.

There have been statements where the pH of calcium carbonate can be lower than 9. This is more the result of contamination than an actual measurement of the calcium carbonate itself. ANY soluble or partially soluble compound contaminating calcium carbonate can influence the pH measurements. Environmental pollutants can and WILL interfere with the "purity" of an intended pH measurement (which is one of the original purposes for sanding or grinding the surface of concrete. Unlike countertops, flooring surfaces, etc. Environmental pollutants become imbedded and in many cases, nearly impossible to remove without damage to the concrete itself. All ANYONE has to do is pay attention as a room is cleaned. The weekly accumulation routinely removed from non-porous surfaces can be substantial, yet this obvious accumulation over non porous surface suddenly becomes "different" or a "non-issue" when considering what is imbedded and/or contaminating a concrete surface.

The best question for this "non-practice" is Why? Why are we ignoring the obvious and assuming the concrete surface. ANY concrete surface is pristine enough to give us meaningful measurements or test results? Yeah, it sounds REALLY obvious when said out loud....

Concrete Alkalinity - The Temperature-Based "Tug-of-War"

As noted in my last article, cement tends to be considerably more alkaline than it was in the past. That being said, even with the virtually constant presence of sodium hydroxide and other alkaline compounds in concrete, NONE of these have been separated or evaluated individually or the influence each may have on other alkaline compounds within the concrete...go on, try and find a research study that states or even differentiates "alkalinity". Most of the time, it is simply stated "alkalinity" with no regard for the basic compounds that may exist within the concrete, much less considering the different levels, locations where these alkaline materials are present, with the KNOWN processes of percolation, migration and leaching, yet these are STILL ignored!. That is wrong, has always been wrong, yet this will continue to be ignored due to the potentially deep rabbit holes much of the research can fall into. Example: How many researchers have recognized (with the notable exception on Linked In of Hugh Hou's explanation of this phenomena) that sodium hydroxide will suppress whatever solubility the calcium hydroxide has. This has a compounding effect as temperatures increase since calcium hydroxide is already becoming less soluble, then gets a "chemical boost" towards insolubility in the presence of sodium hydroxide!

Think about the ramifications of just THESE influences. For example, with pH testing, if the pH is higher than 12.5 it ISN'T calcium hydroxide. However, if the pH testing shows a pH of 12, it doesn't necessarily mean it is calcium hydroxide you are measuring! Likewise, with a phenolphthalein indicator (used for over 80 years), if the concrete turned bright red or purple, it was ASSUMED the concrete wasn't carbonated, but if there was no color change, the concrete is carbonated...well, ONE of these statements is true, which is the no color change. The color change on the other hand is yet ONE reason we NEED to start separating the different alkaline compounds when evaluating concrete. If concrete is fully carbonated, it simply means the hydroxides have converted to carbonate. Sodium hydroxide is extremely soluble and has a high pH, well, once it converts to a carbonate (sodium carbonate), it is STILL extremely soluble with a high (albeit slightly lower) pH, which just so happens to be right in the sweet spot of a phenolphthalein indicator.

This is yet another area where the ramifications are alarming...how many times have researchers, engineers, consultants, etc. gone out, measured the steel reinforced concrete structures (condos, bridges, etc.) and declared them NOT carbonated, when in fact they may be partially or fully carbonated. The common belief is the steel reinforcement is no longer passivated when the pH falls below 10.5. This however does NOT take into account that a "transition" exists where steel is only partially protected within the lower pH range where color change of phenolphthalein will STILL occur, or that the "non-carbonated" concrete gives a false sense of security that the concrete density has not allowed carbonation, when in fact, it has! Depth of carbonation has been inextricably linked to concrete permeability..so if the phenolphthalein indicator leads the field tech to believe the concrete isn't carbonated, there may be a progressing damage that will only be noted after a substantive failure has occurred.

Simplifying the Complex While Complicating the Simple

It is a constant source of irritation to me that this happens on a routine basis, particularly when testing concrete moisture.

Moisture testing is made out to sound complicated, when in reality it isn't. I will go a step further and state that nearly ALL moisture test methods are pretty simple as most are quite accurate; what ISN'T accurate is the interpretation, or the implied reasoning of the data. I will make a statement where two ASTM Standards NEED to be re-categorized; ASTM F 1869 and ASTM F 2170 - BOTH are stated to be quantitative....no, within the context as presented NEITHER is quantitative. A quantitative result should be an absolute (such as gravimetric), BOTH methods suffer from chemical and preparation influences, and limitations of measurements, which is almost NEVER addressed. The presence of alkalinity WILL influence the measurements of either method (particularly the F 2170 method)...since this context is never stated, not qualified, nor quantified, these methods should be redefined as "qualitative".

What IS complicated is the concrete itself; over the past two decades, it was discovered that ALL concrete, to greater and lesser degrees will eventually experience what is known as hysteresis. Hysteresis of any given concrete for a given location is applicable ONLY for that specific concrete. Hysteresis is a designation given to a complex process that alters what should be a predictable inflow/outflow of moisture. Hysteresis is noted when the outflow is slows and in many instances, continues to become progressively slower than the inflow.

Absent of hysteresis, a diffusion model can be used and is commonly used as a predictor of many characteristics of concrete. However, once concrete hysteresis is introduced to any given site condition, any and ALL diffusion models have been rendered inaccurate.

The reason I introduce hysteresis within this article, is that I believe alkalinity plays a significant role in the emergence and development of hysteresis, yet, as with other studies over the past few decades, the role of alkalinity is once again not being considered.

In my view, alkalinity and hysteresis are not generally addressed is due in large part to laboratory conditions. The kinetics of field conditions are absent, thereby eliminating most of the most significant impacts that lead to alkaline migration and leaching, as well as the development of hysteresis.