Where Does The Lime Go?
The Effect of Acids On Cured Lime Mortar
In a typical mortar analysis, dilute muriatic acid is used to digest the binder out of a mortar sample to get a good look at the sand. After you dry, sieve, and examine the sand, you can identify different characteristics that guide your search for a sand match. Acid digestion is often the first step in a mortar analysis.
When you put ground mortar in a beaker and add water with 5 or 8% muriatic acid, it begins foaming and bubbling as the acid reacts with the carbonate, releasing CO2. This reaction leaves a murky liquid with silts suspended and sand sinking to the bottom.
After acid digesting numerous mortar samples, I finally asked, "Where does the lime go?" The answer: after the CO2 has been released during acid digestion, the calcium, now in solution, has become calcium chloride, a salt.
So I decided to try a little experiment to find out exactly how much salt was produced.
The Experiment:
1. After I digested the binder and the bubbling has stopped, I pulled 10 CCs into a syringe. |
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2. You can see that the sand has sunk to the bottom. |
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3. I weighed a large evaporation dish. |
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4. I put the 10 CCs into the dish. |
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5. I heated it to evaporate the water, which left a gummy salt. |
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6. With additional heat, a dry, hard lens of salt was left. |
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7. I weighed the dish with the salt. What I saw was interesting and a bit alarming. The weight of the salt from just 10 CCs of liquid was more than one gram. |
I emailed my friend Richard Wolbers, a chemist at the University of Delaware, and asked him for help in understanding this process a little better. I asked him for the equations for the chemical reactions for both hydrochloric acid as well as acetic acid. He replied,
"Here are the balanced equations:
CaCO3 + 2 HCl reacts to give CaCl2 + H2O + CO2
CaCO3 + 2 CH3COOH reacts to give Ca (CH3COO-)2 + H20 + CO2
The CaCl2 and Ca (CH3COO-)2 ( calcium chloride and calcium acetate) both are deliquescent, both take up a specific molar "volume" (which as you've suggested, may be greater than the pore size of the stone material) as crystalline materials, and both weigh more mole for mole than CO2 (100.07MW): Ca Acetate (138.07MW), Ca Chloride (110.97MW).
It is alarming. Hope this helps!"
This simple experiment demonstrates that when acid comes in contact with masonry surfaces--bricks, mortar, stone--it can create other compounds, in this case calcium chloride. Calcium chloride is one of the more deleterious salts for masonry, as it tends to expand in supersaturated solutions inside smaller pores. As pressure breaks the masonry pores apart, the salts go back into solution, travel with the water, and crystallize and recrystallize again. Even as we clean buildings we are converting the dissolved lime into a salt that is left behind on the brick surfaces and over time may damage the building.
This becomes a problem when people use acid-bearing compounds to clean masonry. Even if the building has been thoroughly pre-wet and flushed, there is still the potential for creating other compounds when strong acids or based are used on masonry. Salts of one sort or another are created, and because these reactions occur over time, the byproducts may not be immediately apparent. With time, however, staining and efflorescence can develop on masonry along with disintegration of the masonry.