Properties of Lime Mortar UUnderstandingnderstanding thethe NNatureature ooff LLime-Sandime-Sand MMortarsortars By Margaret Thomson Stand in the street of just about any major urban center of a North American city, and look around at the buildings. It is very likely that one of those massive stone or brick buildings was built using a mortar consisting of lime and sand – no cement. This © realization might bring a structural engineerCopyright to contemplate if they could do the same thing today. As with every good engineering question, the answer is a fi rm maybe. Modern masonry mortars are composed primarily of portland cement, hydrated lime and sand as aggregate. However, prior to the development of portland cement in 1864 and Figure 1: Simplifi ed diagram of the lime cycle in the context of manufacturing building lime its introduction and commercial production in the United States in the 1880s, lime- applications. The following is a summary of • High calcium hydrated lime (normal) sand mortars were the standard for building the terms commonly used by United States – atmospheric hydrating conditions construction. In a modern context, lime-sand lime industry for quicklime. The terms are produces a powder of calcium hydroxide derived from the different rock types. mortars are still appropriate. Let’s explore (Ca(OH)2 ) containing generally 23 to the nature of a lime-sand mortar in terms of • High calcium quicklime – derived from 24 percent chemically combined water. materials, properties and current standard limestone containing the mineral calcite • Dolomitic hydrated lime (normal) specifi cations and codes. and 0 to 5 percent magnesium in the – atmospheric hydrating conditions ⇒ calcite structure (CaCO3 CaO). produces a powder of calcium hydroxide • Magnesian quicklime – derived from What is Lime? (Ca(OH)2) with some magnesium limestone containing 5 to 20 percent hydroxide (Mg(OH) ), but also some STRUCTURE2 Lime is a generic term referring to the magnesium in the calcite structure magnesium oxide (MgO) component, calcium oxide component of a material. (Ca(Mg) CO ⇒ CaO + minor MgO) generally containing 15 to 17 percent When the term lime is used, it should always 3 • Dolomitic quicklime – derived from the rock chemically combined water. be followed by anothermagazine term. For instance, dolomite, made up of the mineral dolomite • Dolomitic hydrated lime (special) lime in terms of a rock type is limestone and containing the ratio of 40 to 44% calcium – pressure (100-150 psi) hydrating lime in the context of mortar is quicklime, and 54 to 58% magnesium (CaMg(CO ) conditions produces calcium hydroxide lime putty and hydrated lime. When a farmer 3 2 ⇒ CaO + MgO) (Ca(OH) ) and magnesium hydroxide wants to lime his fi eld he will use crushed 2 Today in the United States, hydrated lime (Mg(OH) ) with no magnesium oxide limestone, and when he wants to white wash 2 for masonry, stucco and plasters is dominantly (MgO), generally containing 25 to 27 or lime wash his dairy, he may use crushed from dolomitic quicklime. Lime putty when percent chemically combined water. limestone in the form of chalk or may use formed directly from quicklime is dominantly Lime putty is manufactured by adding an hydrated lime in a wet slurry. To only say from high calcium quicklime. excess amount of water to the quicklime. It lime is not often enough information. Always is a slurry that ranges in consistency from think about which lime is being discussed. yogurt to a farmer’s cheese and may vary in To manufacture hydrated lime, one must What are the Differences solid content (hydroxide minerals) from 40 to fi rst calcine (heat) limestone to quicklime, between Hydrated Lime and 80%. The source of the quicklime does not then add water to hydrate the quicklime to Lime Putty? differ from that of hydrated lime. get hydrated lime (see Figure 1). The phrase Hydrated lime is a dry powder manufactured Limestone and dolomite are rocks. While quicklime refers to very rapid exothermic by water to quicklime only in a suffi cient lime producers pride themselves in having reaction that occurs on the addition of water to amount to satisfy the chemical reaction very pure carbonate deposits, there are always the calcium oxide (CaO). Once the reaction of converting the oxides to hydroxides. impurities. These might include iron, clays, starts, it is quick and hot. Depending on the type of quicklime used which contribute to color variations and other The added complexity is that not only and the hydrating conditions employed, the minerals which might produce grittiness. limestone, which is pure or high in calcium, amount of water in chemical combination Calcination produces variability within the is calcined to be used for a varied number of varies, as follows: resulting quicklime, although the objective is 26 STRUCTURE magazine • May 2005 to calcine each rock to the same degree. The Type S fi nal processing may involve modifi cation of Requirement ASTM C5, C206, C207, C1489 the upper size fraction by screening, grinding or air separation. All of this leads to variations, Chemical Properties which the manufacturer should aim to keep at Calcium and magnesium oxides 95 a minimum. (non-volatile basis), min. % Converting quicklime to hydroxide is Carbon dioxide, min. % 5 often termed slaking. A slaked lime implies that the oxide phases have been converted Unhydrated oxides, max. % 8 Understanding the Nature of Lime-Sand Mortars to hydroxide phases. It does not imply how Physical Properties this was done. There is often considerable .05 confusion about whether or not hydrated lime No. 30 mesh (600 µm), max. % or if greater is slaked. It is! One can also make a lime no pits or pops putty out of hydrated lime powder, and call it µ a slaked lime putty. There is implication that No. 200 mesh (75 m), max. % no value assigned the very process of slaking quicklime produces Not applicable unless No. 30 mesh > 0.5%, Pits or pops a product which is superior to a hydrated then none lime. Each process can produce high quality Plasticity, min. Emley value 200 within 30 min. building lime. Copyright © What are the Characteristics Water retention, min. % 85 within 30 min. of Building Lime as Defined by ASTM? Table 1: Specifi cation Requirements for Type N and Type S Hydrated Lime The current ASTM standard specifi cations for building limes are listed later. Defi ning, the fi nishing lime and has no requirement for atmosphere, which makes it stiff, and then and quantifying properties was completed by mortar applications. carbonate, which makes it harden. the National Bureau of Standards between + Water retention values are only required for Ca(OH)2 (calcium hydroxide) CO2 (gas 1906 and about 1956, and adopted by ASTM ⇒ the mortar application. For Type N hydrated from atmosphere) CaCO3 (calcite) in the specifi cations. lime, a value of 75% must be met after 16, but This process has advantages and disadvan- There is a requirement for chemical purity, less than 24 hours of soaking; whereas, the tages are summarized in Table 2. completeness of hydration, limited coarse Type S hydrated lime a value of 85% must Lime-sand mortar produces a highly plastic fraction, but also key physical properties of be met within 30 minutes after mixing. and workable mortar that is prized by masons. water retentivity and plasticity. Table 1 is a There is no water retention requirement for With a water retentivity requirement of generalized summary of the specifi cations fi nishing lime. If the plasticity value is over 85% wet lime will hold on to water, which for Type N (normal) and Type S (special). 200 Emley units, then the water retention promotes an even moisture loss due to suction For specific details please refer to the value will be greater than 85%. or evaporation. standard specifications. A Special hydrated lime for fi nishing Over time, lime-sand mortar develops a TheSTRUCTURE chemical specifi cations for all are the purposes can be designated a Type N in either surface coating of calcium carbonate. This is same. They require that the product contain specifi cation, but not vice versa. What makes formed initially, but also tends to build up, as no less then 95% combined values of calcium a Type S hydrated lime different from a Type calcium hydroxide saturated water is moved to or magnesium and notmagazine more than 5% carbon N is limiting the amount of unhydrated the surface and carbonates. This surface, often dioxide. These dictate that the source rock oxides, obtaining a plasticity value of 200 referred as the patina and is almost marble must be pure and that the calcination must be within 30 minutes of mixing and requiring like, provides excellent resistance to water virtually complete. a water retention value of 85%. There is no penetration, but allows for vapor penetration For mortar: The coarse fraction for hydrates distinction made based on whether the source (see Figure 2). Similarly fi ne cracks that might is limited to not more then 0.5 weight percent is limestone (high calcium), magnesium- developed are fi lled with a calcium hydroxide µ held on a 30 mesh screen (600 m). By testing, limestone or dolomite. a small amount of coarse fraction can be ASTM C1489 Specifi cation for Lime Putty tolerated only if it is essentially inert, that is, for Structural Purposes became a specifi cation no pits or pops. in 2000, and is meant to refl ect the spirit of For fi nishing applications: No more than ASTM C5 Specifi cation for Quicklime for 0.5% of a 30 mesh screen and no more than Structural Purposes. If quicklime is purchased µ 15% on a 200 mesh screen (75 m) and no pits under ASTM C5 qualifi cations it needs to or pops are allowed.