PLASTICITY, WATER RETENTION, SOUNDNESS AND SAND CARRYING CAPACITY: WHAT A MORTAR NEEDS

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1 PLASTICITY, WATER RETENTION, SOUNDNESS AND SAND CARRYING CAPACITY: WHAT A MORTAR NEEDS Margaret L. Thomson Chemical Lime Company, U.S.A. Abstract Plasticity, water retention, soundness and sand carrying capacity are perhaps the most important properties of mortar. Lime, for the most part, contributes these properties. High calcium lime putty and hydrate as well as dolomitic lime putty and hydrate are investigated for these properties as well as particle size. Dolomitic lime putty and hydrate have the highest values of water retention and plasticity and have a fine particle size. High calcium lime putty has a fine particle size, but not as high a water retention or plasticity. Soundness is related to the presence of impurities in the lime, and is not limited to type. Sand carrying capacity is highest with the hydrates over the putties. 1. Introduction The production of lime for mortars and renders is generally similar throughout the world, but there are some distinct differences. In the British Isles there is much enthusiasm given to aged high calcium lime putties for mortars and renders. In North America dolomitic hydrated lime is by far the most common lime used for mortars and renders. In Germany, Italy and Mexico high calcium lime is common. Dolomitic lime putty is relatively uncommon, but gaining popularity in North America. Users appear satisfied with the performance of the different limes, suggesting there are common qualities which are required. In providing guidelines for the repair of old mortars consideration must be given to those properties of lime that contribute to a good mortar repair. In 1927, Cowper [1] describes the most important property in plaster as its freedom from cracking and pitting or popping." He suggests the cause of cracking is due to either 163

2 movement in a new building, or shrinkage cracks due to insufficient sand in comparison to lime. Lime has an optimum sand carrying capacity, allowing sand grains to touch each other, thus preventing shrinking during the drying process. The most important properties in mortar are slightly different from plaster. The Appendix to ASTM C [2] indicates that the primary purpose of mortar in masonry is to bond masonry units into an assemblage.... Appendix A of the CSA A [3] states that, Plastic properties such as workability and water retentivity, determine construction suitability.... Groot [4] showed that the water retention capacity of the mortar greatly influences the degree of bonding. In repair of old mortars, it is also expected that plasticity, soundness, water retentivity, and sand carrying capacity are required properties. Sand does contribute to these properties, but by far lime is the most influential. The objective of this paper is to present for high calcium lime hydrate, high calcium lime putty, dolomitic lime putty and dolomitic lime hydrate a comparison of plasticity, soundness, water retentivity and sand carrying capacity, and the role of particle size in the development of these suitable properties. 2. Test Results 2.1 Material Dolomitic lime hydrates (DL-H), high calcium lime hydrates (HC-H) and dolomitic lime putties (DL-LP) are all commercially available products in the United States. They are produced by Chemical Lime Company, or by domestic or international manufacturers. The high calcium lime putty (HC-LP) was produced in the laboratory of Chemical Lime Company (Henderson, Nevada) on March 8, 1999 and tested at one week. All the putties were adjusted and tested at 42% solid content. 2.2 Chemistry The summary of the chemistry is presented in Table 1. The major element analysis was conducted according to ASTM C 25 [5] and the sulphur analysis was conducted on a Leco Induction Furnace HF10, and Leco S Titrator The high calcium limes and dolomitic limes show very little variation in Ca and Mg, which is consistent with both limestone or dolomite as the source rock. There is a larger variation in insoluble levels and R 2 O 3 which include clays, iron and quartz (chert). DL-H2, DL-H4, and HC- LP show elevated values. Dolomitic hydrate DL-H3 shows a large value of CO 2 and calculated free MgO. The HC-H3 also shows a high value of CO

3 2.3 Water Retention The water retention capacity of the lime products was tested according to ASTM C110 [6]. The values for all the physical properties are reported in Table 2. The dolomitic lime hydrates and lime putties have higher water retention values than the high-calcium products. Table 1: Chemistry CaO MgO Insol. R 2 O 3 Leco S Total CO 2 free free MgO CaO DL-H DL-H DL-H DL-H HC-H HC-H2* HC-H3h HC-LP DL-LP DL-LP * Mexican produced, h Italian produced 2.4 Plasticity Plasticity testing is an ASTM C110 [6] test, but not European Norm test. Cowper [1] gives an excellent summary of the basis of the Emley plasticimeter. He points out for plaster there are two issues: water retention and spreadibility.. The Emley plasticimeter incorporates these two parameters in the test. The suction is provided by a porous plate make up of gypsum-based plaster. The spreadibility is provided by a brass disc which sits on the paddy of lime mixed to an initial set consistency. The porous plate base with the lime putty paddy rotates and screws up into the upper brass disc. The resistance to the rotation or torque is measured over time until there is shear failure within the putty. This failure is due to a combination of loss of water and increased pressure. The plasticity values for the dolomitic lime hydrate are above 360. The DL-H4 value of 666 is non-typical. The dolomitic lime putty shows values lower than dolomitic hydrate but above the high calcium hydrate and putty. The high calcium lime hydrate and lime putty values are less than 200 with the exception of HC-H1 with a value of

4 2.5 Soundness The soundness test data presented (Table 2) for the lime hydrates used the pressed disc and autoclave method (ASTM C110 [6]). While the standard requires that there be no pops or pits greater than 1 mm, this scheme does not allow for relative assessment of soundness. A scheme has been developed to record more accurately the number and dimensions of the pops or pits is used. Values of 0 (no pits or pops), 1 (1-3 pops or pits < 1mm) and 2 (4-5 < 1mm ) are within the ASTM standard. Values of 3, 4, 5, and 6 have pits or pops greater than 1 mm and exceed ASTM limits. Table 2. Physical Properties viscosity (10 3 cp) ~42% solid % water retention (mix 1:3) Emley plasticity % held on 75µm mesh soundness % water retention (mix 1:5 ) (% change) surface area (m 2 /g) DL-H (1.51) DL-H n/a DL-H n/a DL-H (2.61) HC-H n/a HC-H (1.41) HC-H n/a HC-LP n/a 74.1 (9.41) DL-LP n/a n/a DL-LP n/a 81.4 (7.50) analysis of lime products conducted in late 1998 and early 1999., DL, dolomitic H- hydrated lime, HC - high calcium, LP- lime putty There is no consistency in soundness in the dolomitic or high calcium hydrates. DL-H3 showed no pits or pops, while HC-H2 showed in excess of 4 pops or pits greater than 1 m. 2.6 Sand carrying capacity The water retention values of lime-sand mixes at a ratio of 1:5 (lime:sand) was used to compare against the standard 1:3 ratio. Both high calcium and dolomitic hydrate types showed the least change in percent water retention at %. The high calcium and dolomitic lime putties showed much larger losses in water retention ( %). 2.7 Particle Size Particle size distribution of hydroxide minerals presents a challenge for measurement. The surface charge due to incomplete crystallization makes the product difficult to 166

5 disperse. In this study several different techniques were used. Sieve analysis, laser techniques and indirect methods of surface area and viscosity were conducted. The amount of material held on 75µm mesh ranges is variable (Table 2). DL-H3 (14.9%) is the high value and HC-H1 (0.6%) is the low. This reflects the nature of the purity of the source rock and the quality of post-production processing. The less than 75 µm particle size distribution was conducted on hydrates using a laser diffraction particle size analyser (Colter, LS100Q). Typical results are shown in Figure 1. The profiles are all bimodal, with a distinct mode at approximately 2µm and the second one between 40 µm. Discriminating between the different products is difficult. Viscosity is measured by the drag exerted on a spindle rotating at a specific rate. A lime slurry with larger particles will be less viscose than one with many smaller particles. Important in the measurement is comparing the same solid content; 42 % is used. There is a wide range in viscosity values for the dolomitic hydrates and high calcium hydrates. DL-H1 is non-typical at a value of 400,000 cp. The visual difference between a viscosity of 400,000 and 20,000 is that between Devon cream and skim milk. The surface area analysis was conducted using BET surface analyser with nitrogen gas (Micromeritics Flowsorb II 2300). The samples of lime putty were oven dried and gently crushed in a mortar and pestle. DL-H1, DL-H2 and HC-P1 show the highest values of surface area, with the HC-H-4 showing the lowest. 3. Discussion 3.1 Water Retention The dolomitic hydrated lime and lime putty have significantly higher retention values than the high calcium hydrated lime and putty. Interestingly, this distinction in water retention defines the difference between Type S and Type N in ASTM C 207 [7]. Water retention and surface area (viscosity) correlate fairly well (Fig. 2) 167

6 DL-H1 HC-H3 DL-H3 HC-H2 Figure 1. Particle size distribution of dolomitic and high calcium hydrates. 168

7 surface area (m2/g) DL-H HC-H DL-P HC-P % water retention Figure 2. Relationship of surface area to water retention. 3.2 Plasticity Plasticity values are highest for the dolomitic hydrates and putties. Surface area (viscosity) does not, however, correlate well with plasticity (Fig. 3). DL-H4 has a plasticity value of 666, and a surface area of only 18 g/m2. HC-LP has a plasticity value of 481 and a surface area value of 26.6 g/m2. Plasticity testing measures a fairly complex dynamic, replicating the trowelling action of smooth finishes. Although, highly speculative, lime with lower water retention has coarser grained lime particles which may provide a ball-bearing effect increasing ductility in the plaster. 3.3 Soundness Soundness appears to be related to the presence of impurities in the source rock. HC-H2 and DL-H2 both showed poor soundness, high insoluble residue and R2O3 values. Although unhydrated MgO is generally associated with soundness failure, only DL-H2 showed associated high free MgO with poor soundness. DL-H3 and DL-H4 showed no soundness problems yet their calculated free MgO values are high. Accordingly, soundness must be tested, regardless of the source of the lime and regardless of its chemistry. This is especially important when evaluating use in interior plasters. 169

8 700 Emely plasticity DL-H HC-H DL-P HC-P surface area (m 2 /g) Figure 3. Relationship of surface area to Emley plasticity. 3.4 Sand Carrying Capacity High calcium and dolomitic hydrate limes showed the least loss in water retention with increased sand load. The lime putties showed the most. This result is inconsistent with the water retention values of the dolomitic lime putty. It would be expected the higher the water retention the better the sand carrying capacity. It is possible that the solid content of the lime putty is reduced compared to the hydrate, but this is not reflected in the other properties. In this current study, the sand carrying capacity probably has not been adequately tested. 3.5 Particle Size The particle size of some dolomitic hydrates (Dl-H1, Dl-H2) and putty (DL-LP2) and high calcium putty is finer than high calcium hydrate. The reason for this is certainly beyond the scope of this discussion as it involves the kinetics and thermodynamics of the slaking and hydrating process. It is commonly suggested that aging of high calcium lime putty results in particle size decreases. Equilibrium thermodynamics suggests that particle size increases with time under saturated conditions. The surface area value for the high calcium lime putty produced in the laboratory indicates a very small particle size. It would be remarkable if, over time, the particle size would decrease. Aging, therefore, probably represents more a completion of the slaking process, eliminating coarser grains through hydration, rather than a decrease in size of the hydrated lime particle. 170

9 4. Conclusions determination of lime properties is critical to evaluating the best product for repair dolomitic hydrated lime (type S), dolomitic lime putty, and high calcium lime putty (in decreasing order) provide better water retention and plasticity over high calcium hydrated lime soundness must be tested for, especially for interior finish 5. Acknowledgments Mr. Kurt Scow conducted the testing with his usual attention to detail. Dr. Tim Salter (CLC) provided surface area analysis and Dr. Deitum Walter (Lhoist) provided size distribution analysis. Dr. Gary Suter planted the seed for this paper several years ago. Chemical Lime Company provided the opportunity and encouragement. Mrs. Angela Katherine Thomson ( ), my mother, is profoundly missed. 6. References 1. Cowper, A. D., Lime and Lime Mortars. First Published for the Building Research Station, by HM Stationery Office, London, Send publication, Building Research Establishment Ltd., Donhead Publishing Ltd., Dorset, England. 2. ASTM C , Standard Specification for Mortar for Unit Masonry, American Society for Testing and Material, Philadelphia, U.S.A. 3. CSA A179-94, Mortar and Grout for Unit Masonry, Canadian Standards Association, Toronto, Canada, pp.46, Groot, C.J.W.P, 1993, Effects of water in mortar-brick bond, Ph.D. thesis, Delft University of Technology, the Netherlands. 5. ASTM C 25-95b., Standard Test Methods for Chemical Analysis of Limestone, Quicklime, Hydrated Lime, American Society for Testing and Material. Philadelphia, U.S.A. 6. ASTM C b, Standard Test Methods for Physical Testing of Quicklime, Hydrated and Limestone, American Society for Testing and Material. Philadelphia, U.S.A. 7. ASTM C , Standard Specification for Hydrated Lime for Masonry Purposes, American Society for Testing and Material, Philadelphia, U.S.A. 171