General Glossary of Terms

This picture shows a chimney top rebuild after the 2011 earthquake

1. Calcium Carbonate [CaCO3] is the chemical description for pure or high-calcium lime products as normally found in nature (limestone, oystershells). This material is sometimes sold crushed for use in lawncare and agricultural activities and is not suitable for mortar making or structural uses.

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, causing very hot steam (see Calcium Hydroxide).

3. Calcium Hydroxide [Ca(OH)2] is produced when water is reabsorbed by Quicklime. It is a caustic substance requiring personal protection with a pH of 12 (until carbonation occurs). Calcium hydroxide is also known as Lime Putty, 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. Commonly used names for this product are “Hydrated Lime,” “Mason’s Lime,” or just “Bag Lime” for building. The Type N (normal) or Type S (special) limes designed for use with cement-based mortars are a “highly hydrated” or autoclaved (pressure hydrated) form of hydrate. These products can be high-calcium, dolomitic, magnesian, or hydraulic. Both Type N and Type S commercial lime products simply require a combined oxide content of 95% without specifying whether these are calcium or magnesium oxides.

5. Lime Putty is slaked lime, or calcium hydroxide, in paste form. More workable putty derives from slaking from oxide directly to a hydroxide paste as opposed to pressure hydration into a powdered hydrate that is later soaked to form the paste.

6. “Fat Lime” denotes a high purity and high plasticity lime putty for structural uses that has always been highly regarded for mortars, particularly in stucco and plaster and traditionally was the result of many years storage of the putty under water to age. Harder to work limes, both because of chemical impurities or indifferent firing and slaking procedures that lessen their plasticity are commonly referred to as “Lean” or stiff.

7. Slaking refers to the process of adding water to Calcium Oxide to produce Calcium Hydroxide or Calcium Hydrate. If water is later added to a hydrated bagged lime (such as Type N or Type S), this process is soaking, not slaking, as there is no longer a chemical reaction, only the addition of free water. It should be noted, however, that with Type S limes that are dolomitic (with up to 8% unreacted magnesium oxide allowed), there may be an additional benefit to longer soaking times, in that the oxides over long periods may hydrate, limiting the “pitting and popping” that occurs when an oxide later hydrates in a finished plaster or stucco finish.

8. Dolomitic Lime refers to limes that contain magnesium and calcium. Dolomitic limestone technically refer to stones with 40-44% magnesium carbonate to 54-58% calcium carbonate, although in general use it may be used for any stone containing in excess of 20% magnesium carbonate. Both Type N and Type S commercial lime products simply require a combined oxide content of 95% without specifying whether these are calcium or magnesium oxides.

9. Soft-burned lime refers to a stone that is calcined (fired) at low and consistent temperatures to produce an oxide with high porosity and chemical reactivity.

10. Dead-burned Lime refers to a stone that is calcined (fired) at much higher temperatures in order to process all of the stone into oxide. This is normally undertaken for magnesian or dolomitic limes where the magnesium and calcium components of the stone require very different temperatures to drive out all of the CO2 (dissociation). The resulting product is generally less chemically reactive or porous because some parts of the stone will have been over-fired (magnesium carbonate) before the calcium carbonate portion dissociates.

11. High calcium lime is generally accepted as a lime containing at least 93% calcium content, although some commercial suppliers will not sell a lime as “high calcium” if it is not at least 97% calcium. Since the carbonate cycle is described in 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, generally accomplished under high pressure in an autoclave in order to force-hydrate the over-burned magnesium resulting from mixtures of magnesium and calcium in a dolomitic stone. Since the magnesium carbonate and calcium carbonate dissociate (release CO2) at very different temperatures, the magnesium is over-burned (dead-burned) to release the CO2. This dead-burned magnesium portion then is much less porous and less chemically reactive, and therefore practically impossible to hydrate quickly in normal atmospheres, instead requiring pressure-hydration (autoclaving) to hydrate. Even with autoclaves, it is difficult to fully hydrate the magnesium, hence the “unreacted oxides” content allowed in Type S mortars. Limes that are not commercially calcined and slaked, will have a more unpredictable level of complete dissociation (it takes considerable skill and control of high temperatures over long periods to drive off all of the CO2 from limestone or shell) and historic lime recipes consistently call for long slaking times, not infrequently calling for the lime to be many years old before use. While all hydroxides improve with age ­ the particle sizes decreasing over time with a resulting improvement in plasticity and water retention ­ this long slaking period also assists hydration of magnesium hydroxide components when the benefit of autoclaving equipment is not available. It should be noted, however, that magnesium hydroxide is also less reactive, commonly not returning to the carbonate state for many years. The magnesium hydroxide component of architectural putty then, should be considered more as inert filler than a part of the cementitious binding matrix.

13. Carbonation, also sometimes commonly referred to as “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 the lime from 12 down to neutral depending on the rate or degree of carbonation completed. With a soft-burned, high-calcium limewash, carbonation may be complete in 36 hours whereas the magnesium hydroxide component of a dolomitic lime in a lime stucco or mortar kept several inches from contact with the air may still be uncarbonated hundreds of years later.

14. Harling is a thrown-on mortar application technique that provides a very intimate bond between mortar and substrate while creating a rough surface for keying subsequent coats. Harling is normally achieved with a harling trowel and a “soupy” mix of the same mortar used for stuccowork that is thrown with considerable force, in a manner similar to lacrosse techniques.

15. Limewater refers to any excess water stored over lime putty or resulting from the settling of lime putty over time, which holds some proportion of lime in solution. This water is normally seen to have 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 they are kept covered with water.

16. Limewash is an inexpensive finish treatment made by diluting lime putty with water to anywhere from 60-90% depending on the desired appearance. Again limewash is highly alkaline (pH 12) until CO2 has been fully adsorbed and the limewash has been converted to calcium carbonate or calcium and magnesium carbonate when 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 (recall Huck “white washing” in the introduction to Mark Twain’s Huckleberry Finn).

17. “Green Hard” refers to stucco or plaster that has begun to firm up from the loss of free water, but that has not begun to carbonate. When worked with a float, it is possible to force any hairline cracks to close up at this point, but it is no longer possible to dent or significantly reshape the mortar mass. It is sometimes described as 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 should still feel slightly cool to the touch, indicating that there is still water held in the mortar and that it is not fully dried out.

18. Void Space Ratio. Lime should just coat the sand particles to bind everything together without having excess lime to push the particles apart and weaken the mortar. A simple on-site test to determine the void space in a given sand is to tamp down a completely dry sand sample to 100mL before measuring in the amount of 200-proof alcohol necessary to just wet to the top of the sand. By determining the ratio of alcohol added, a lime-to-aggregate ratio can be derived from the void (alcohol) to sand ratio. This is the method that should determine the sand-to-lime ratio for all sands to be used to make mortar.

19. Particle Size Distribution of Aggregates Sand used for building should ideally be clean and sharp or angular, not rounded, as these will pack together more tightly, providing a more structural matrix. For demonstration purposes, golf balls might be seen as somewhere between angular and rounded because of their multi-faceted surface, but spherical shape. A stack of golf balls would leave huge gaps between the balls in the same way that a sand comprised of only one particle size would not pack together tightly. Mortar strength increases with better packing, so appropriate building sand has a range of particle sizes from small to large, with the majority of particle sizes in the middle range. If graphed, this sand is said to have a bell curve shape.