Properties of Lime Mortar UUnderstandingnderstanding thethe NatureNature 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. It requires that there be meet the requirements of C 1489 when slaked < 15% hydrated lime coarser than 200 mesh to a lime putty. or 75µm and that there is no component that can cause pits or pops. Properties of Lime Mortars Plasticity values over 200 are required Hydrated lime or lime putty will not set within 30 minutes after mixing for Type S Figure 2: Scanning Electron Photomicrograph of hydrated lime and lime putty used for mortar under water. It is not hydraulic as is portland cement. For a lime-sand mortar to harden, it the calcium carbonate patina on lime-sand mortar. and fi nishing plaster. Type N hydrated lime This surface is developed initially but is added to must achieve plasticity values of 200 after 16 must fi rst lose its excess water by suction to over repetitions of wetting and drying hours, but less than 24 hours of soaking for the backing material and evaporation to the STRUCTURE magazine • May 2005 27 The following is taken Benefi t Drawback from Hook & Johnson Highly plastic and workable mortar (1929, p. 978): Slow to harden (must carbonate) (mason effi ciency) The Building Code Committee of the U.S. High water retentivity Low near term compressive strength Department of Commerce in (brick compatibility) compared to lime-cement mortars March 1926, prepared a compilation of data on High ductility to joint and Lime bloom compressive tests on brick massive masonry masonry. The strengths of straight lime mortar, show High open porosity results 95% of which Dissolution/reprecipitation vary from a maximum of (autogenous healing) 2000 psi to a minimum of approximately 500 No salts to effl oresce psi. Taking the average minimum strength of Table 2: Lime-Sand Mortar 500 psi and applying a factor of safety of 4, a safe working ©load of 125 saturated solution, and carbonate, autogenousCopyright also be specifi ed. The air content produced psi is obtained for straight lime mortar. This is self-healing the crack. by air entraining agents of the hydrated lime entirely logical code strength and is developed The most obvious use of lime-sand mortars or lime putty will be greater than 7% and along legitimate engineering lines. is in masonry restoration, particularly for re- less than 14%. However, C270 indicates that It is well-known fact that, in general, designers pointing, of historic structures which used cement-lime mortars when used with steel do not fi gure the stresses on brickwork, and for lime-sand. Older clay bricks which are relatively reinforcement may not have air contents in this reason little consideration has been given to soft and porous benefi t from lime-sand mortars excess of 12%. Air content of the cement-lime its economical features. In order to form a more that bear the moisture/vapor transport load, mortars produced by air entraining agents careful consideration of masonry, the Department which includes salts, so that the masonry with the lime will vary with the type of sand of Commerce Building Code Committee, as the units remain undamaged. Where the masonry and cement used, and typically will be reduce result of various tests, saw fi t to include a clause unit is very dense stone, lime-sand mortar from measured for the e building lime only. in its code which permits a load of 135 psi on provides the optimum route for moisture/ If specifying hydrated lime or lime putty straight lime mortar when stresses are calculated. vapor transport, providing for drainage. For outside of ASTM specifi cations, the user Current guidance for specifi cation of lime- the same reason, lime-sand mortars should should understand the potential consequences sand mortars comes from the National Park always be considered fi rst when exterior stucco of not knowing the chemical and physical Service Preservation Briefs (1998). The is considered for historic masonry. properties of the hydrated lime or putty being document is an excellent guide on the use used. It is recommended that some basic of lime-sand mortars but does not provide Specifying Lime for Masonry information on the product should include engineering guidance or testing methods.▪ chemistry (Ca+Mg, CO content) and plus WhenSTRUCTURE specifying lime for masonry mortar, 2 under ASTM C270 Specifi cation for Mortar 30 mesh values. for Unit Masonry, use Type S Hydrated Lime ASTM C 270 for mortar was fi rst pub- or lime putty (ASTM C5, C207, C1489). lished as a temporary standard in 1954. The magazinebinder materials were a combination of Type Margaret Thomson is the Technical Type N hydrated lime is permitted if shown S-special hydrated with Portland cement or Manager for Building Lime, Chemical Lime by test or performance record to not be Company and is a member of the National detrimental to the soundness of the mortar. masonry cement. The designations M(1:0:3), S(1:½:4½), N(1:1:6), O (1:2:9) K(1:3:12) Lime Association Building Lime Group. Air entrained hydrated lime, Type SA or She participates in numerous masonry Type NA hydrated lime or lime putty, may were adopted. The ratios represent the ce- ment:lime:sand volume ratios. Decreasing ce- organizations which include modern and ment content results in a) an increase in lime historic masonry. content to maintain a 3 to 1 sand to binder ratio, and b) a reduced compressive strength. K-type mortar was dropped in the 1980’s. Lime-sand mortars were never included in ASTM C270. In response to this absence, there is a task group charged with developing a specifi cation for mortars for historic masonry (ASTM C12.03.03). The most critical aspect is to develop appropriate test methods to assess the properties of mortars such as lime- Figure 3: Transmitting Light Photomicrograph of crack in line-sand mortar healed with later calcium sand in the context of when they have reached carbonate or calcite. (Width of photomicrograph 3.94 at least 75% of their fi nal properties. The mm. Photography courtesy of Dr. B. Middendorf, specifi cation is probably at least two years in Lime stone quarry. Note the truck on the lower Kassel University, Germany) the offi ng. bench for scale (Apex, Nevada, Chemical Lime Company)

28 STRUCTURE magazine • May 2005 Archaic Materials The following is taken from the California Historical Building Code (2001), describing allowable conditions for specifi c archaic materials. It does not give a clear path for the use of lime-sand mortars, nor does it preclude it’s use. Rather it indicates that the masonry be assessed for its conditions and loads required.

SECTION 8-804 – Allowable Conditions for Specific Materials Archaic materials which exist and are to remain in historic structures shall be evaluated for their conditions and for loads required by this code. The structural survey required in Section 8-703 of this code shall document existing conditions, reinforcement, anchorage, deterioration and other factors pertinent to establishing allowable stresses and adequacy of the archaic materials. The remaining portion of this chapter provides additional specifi c requirements for commonly encountered archaic materials. 8-805.1 Existing Solid Masonry. Existing solid masonry walls of any type, except adobe, may be allowed, without testing, a maximum value of 3 pounds per square inch (20.7 kPa) in shear where there is a qualifying statement by the architect or engineer that an inspection has been made, that mortar joints are fi lled and that both brick and mortar are reasonably good. The allowable shear stress above applies to unreinforced masonry, except adobe, where the maximum ration of unsupported height or length© to thickness does not exceed 12, andCopyright where the minimum quality mortar is used or exists. Wall height or length is measured to supporting or resisting elements that are at least twice as stiff as the tributary wall. Stiffness is based on the gross section. Allowable shear stress may be increased by the addition of 10 percent of the axial direct stress due to the weight of the wall directly above. Higher-quality mortar may provide a greater shear value and shall be tested in accordance with UBC Standard 21-6.▪

The Building Lime Group The 2005 International Building Lime Symposium was held March 9-11 in Orlando. Sponsored by the National Lime References Association (NLA) Building Lime Group, the symposium addressed contemporary American Society for Testing and Materials, West Conshohocken, PA, U.S.A. and STRUCTUREhistoric uses of lime in mortars, ASTM C 1489. Standard Specifi cation for Lime Putty for Structural Purposes plasters, and stucco. The NLA is a trade ASTM, C 206. Standard Specifi cation for Finishing Hydrated Lime. association of American and Canadian manufacturer’s of high calcium and ASTM, C 207. Standard Specifi cation for Hydrated Lime for Masonry Purposes dolomitic lime products.magazine The Building ASTM, C 5. Standard Specifi cation for Quicklime for Structural Purposes. Lime Group promotes high standards for ASTM C270. Standard Specifi cation for Mortars for Unit Masonry. materials and construction techniques California Historical Building Code, 2001. California Code of Regualtions, through symposiums, research, technical publications, and codes and standards. Title 24, Part 8. Alternative Regulations for qualifi ed historical buildings. The symposium included 39 papers International Conference of Building Offi ciails, Whittier, Ca. and four workshops. Some topics of http://www.bsc.ca.gov/title_24/documents/Part8/2001_part8.pdf. interest to structural engineers included: Hool, G. A. and Johnson, N.C, 1929. Handbook of Building Construction. Restoration with Lime Mortars Lime Renders McGraw-Hill Book Company, Inc., New York, New York. Pp1611 Stuccos and Plasters Mack, R.C., and Speweik, J. P., 1998. Repointing Mortar Joints in Historic Sustainable Design Masonry Buildings, National Park Service, Preservation Briefs 2. Lime Mortars: Case Studies http://www.cr.nps.gov/hps/tps/briefs/brief02.htm Lime in Building Construction The peer-reviewed proceedings are available from the National Lime Association by calling 703-243-5463, x222. Abstracts are available on-line at www.lime.org/BLG/Abstracts.pdf▪

STRUCTURE magazine • May 2005 29