Lime types and their meanings explained in our Glossary!

General Glossary of Terms

1. Calcium Carbonate [CaCO3] is the chemical description for pure or high-calcium lime products, normally found in nature (limestone, oyster shells). This material is sometimes sold crushed for use in lawn care and agricultural it is not suitable for mortar.

2. Calcium Oxide [CaO] or Quicklime is produced by firing Calcium Carbonate to 900° and driving off CO2. The result is a dry product that is highly reactive with water that causes very hot steam (see Calcium Hydroxide).

 3. Calcium Hydroxide [Ca(OH)2] is produced when water is reabsorbed by Quicklime. Calcium Hydroxide has a PH of 12 (caustic!) and requires personal protection.  Lime Putty (Calcium Hydroxide), is a highly plastic and workable material with molecular and free water (usually around 50%).

4. Hydrated Lime refers to a form of Calcium Hydroxide that only contains molecular water, leaving a dry powder. Common names are “Hydrated Lime,” “Mason’s Lime,” or “Bag Lime” for building. Type N (normal) or Type S (special) limes are for use in cement-based mortars. They are a “high hydrate” or autoclave (pressure hydrate) form of hydrate. These products can be high-calcium, dolomitic, magnesian, or hydraulic. Type N and S limes require a combined oxide content of 95% without specification as to whether these are calcium or magnesium oxides.

5. Lime Putty is slaked lime, or calcium hydroxide, in paste form. Workable putty derives from slaking from oxide directly to a hydroxide paste.

6. “Fat Lime” denotes a high purity and high plasticity lime putty for structural uses.  The best Mortars, stucco and plaster utilize "fat lime". Traditionally was the result of many years storage of the putty under water to age. “Lean” or "Stiff" limes are harder to work because of chemical impurities in firing and slaking that lessen their plasticity.

7. Slaking refers to the process of adding water to Calcium Oxide to produce Calcium Hydroxide or Calcium Hydrate. Adding water later to a hydrated bagged lime, (Type N or Type S), is called soaking, not slaking, as there is no longer a chemical reaction, only the addition of free water. Type S dolomitic limes (with up to 8% unreacted magnesium oxide),  may benefit from longer soaking times. The oxides over long periods hydrate, limiting the “pitting and popping” that occurs when an oxide later hydrates in plaster or stucco.

8. Dolomitic Lime refers to limes that contain magnesium and calcium. Dolomitic limestone refers to stones with 40-44% magnesium carbonate to 54-58% calcium carbonate. It can pertain to any stone containing in excess of 20% magnesium carbonate. Type N and S limes require a combined oxide content of 95% without specifying whether these are calcium or magnesium oxides.

9. Soft-burned lime refers to a calcine (fired) stone at low and consistent temperatures.  This produces an oxide with high porosity and chemical reactivity.

10. Dead-burned Lime refers to a stone that is calcined (fired) at higher temperatures to process all of the stone into oxide. Magnesian or dolomitic limes use this where the magnesium and calcium components require different temperatures to drive out the CO2 (dissociation). This is less chemically reactive or porous because parts of the stone are over-fired (magnesium carbonate) before the calcium carbonate portion dissociates.

11. High calcium lime is acceptable as a lime containing at least 93% calcium content.  Some suppliers will not sell a lime as “high calcium” if it is not at least 97% calcium. The carbonate cycle describes terms of calcium carbonate, calcium oxide and calcium hydroxide. One can easily overlook the fact that this is largely incorrect when describing most U.S. building limes, since dolomitic lime is common and may include up to 45% magnesium hydroxide, a slow reacting compound.

12. Magnesium hydroxide is the result of hydrated magnesium oxide, pressurized in an autoclave to force-hydrate the over-burned magnesium resulting from mixtures of magnesium and calcium in dolomitic stone. Magnesium and calcium carbonate dissociate (release CO2) at different temperatures. The magnesium is over-burned (dead-burned) to dissociate. Dead-burned magnesium is less porous, chemically reactive, and difficult to hydrate. Requiring pressure-hydration to hydrate. Autoclaving, is difficult to hydrate magnesium, hence the “unreacted oxides” in Type S mortars. It takes considerable skill and control of high temperatures over long periods to drive off the CO2 from limestone/shell. Historic lime recipes require long slake times and aging. Hydroxides improve with age. ­Particle sizes decrease over time with improvement in plasticity and water retention. Magnesium hydroxide is less reactive, taking years to return to the carbonate state. The magnesium hydroxide component of putty is a binding filler.

13. Carbonation  commonly refers to “curing” or “setting” of lime mortar describes the chemical reaction between carbon dioxide (in the atmosphere or dissolved in rainwater) that reacts with the lime (calcium hydroxide) to create calcium carbonate. This reaction slowly moves the pH of lime from 12 to neutral depending on the rate of carbonation completed. A soft-burned, high-calcium limewash, carbonation may be complete in 36 hours. The magnesium hydroxide component of dolomitic lime in lime stucco or mortar kept from contact with the air may still be uncarbonated hundreds of years later.

14. Harling is a thrown-on mortar application that provides a bond between mortar and substrate while creating a rough surface for keying subsequent coats.  To achieve Harling use a harling trowel and a “soupy” mix of the same mortar used for stucco work and throw with considerable force, in a manner similar to lacrosse techniques.

15. Limewater refers to excess water stored over lime putty or from the settling of lime putty over time, which holds some proportion of lime in solution. This water has a thin crust of calcium carbonate on top from the reaction of the some of the calcium hydroxide component with CO2 where it is in contact with the air. This film and the water protect the lime putty below from CO2 absorption and carbonation indefinitely. The same is true of lime mortars if water covers it.

16. Limewash is a finish treatment consisting of diluting lime putty with water anywhere from 60-90% depending on the desired appearance. Limewash is highly alkaline (pH 12) until CO2 fully adsorbs and the limewash converts to calcium carbonate or calcium and magnesium carbonate and it becomes pH neutral. The high pH at the time of application has made it a useful historical antiseptic method for killing bacteria, algae and mold on buildings, barns and fences.

17. “Green Hard” refers to stucco or plaster that begins to firm up from the loss of free water, but has not yet carbonated. When worked with a float, it is possible to force hairline cracks to close up, but it is not possible to dent or significantly reshape the mortar mass. This is the point when your fingers can no longer make an impression in the stucco or plaster, but you can still easily scratch the surface with your fingernails. At this stage the plaster or stucco feels slightly cool to the touch, indicating that there is still water in the mortar.

18. Void Space Ratio. Lime should fill the entire void space between the sand.  Excess lime pushes the particles apart and weakens the mortar. To little lime will leave voids and weaken the mortar.  A test to determine the void space in sand is fill a clear 100 mL beaker with dry sand.  Measure 100 mL of high proof grain alcohol.  Add enough alcohol to the dry sand to wet the top of the sand and ensure all the sand is saturated. The alcohol will fill the space between the sand particles. The amount of alcohol you add, determines a lime-to-aggregate ratio.  Example: Adding 30 ML of alcohol to 100 mL of sand equals a 30% void space. You will add 30% lime putty per volume to your sand.  This is the method that determines the sand-to-lime ratio for all sands in the making of lime putty mortar.

19. Particle Size Distribution of Aggregates Sand for building should be clean, sharp, and angular. These will pack together tighter, providing a structural matrix. For example, golf balls are somewhere between angular and round because of their multi-faceted surface.  A stack of golf balls has huge gaps between the balls in the same way that sand comprised of only one particle size would not pack together tightly. Mortar strength increases with better packing, building sand has a range of particle sizes from small to large, with the majority of particle sizes in the middle range.  On a graph, this sand has a bell curve shape.

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