Lime in Masonry Mortars (Status Report as of Feb. 1968)

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Technical Note (National Research Council of Canada. Division of Building Research), 1968-11-01

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Lime in Masonry Mortars (Status Report as of Feb. 1968)

Davison, J. I.

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DIVISION OF BUILDING RESEARCH

NATIONAL RESEARCH COUNCIL OF CANADA

']['EClHI N II CAlL

NOTlE

No.

PREPARED BY J.1. Davison CHECKED BY T.R. APPROVED BY N.B.H.

PREPARED FOR

SUBJECT

DATE November 1968

ASTM Committees C-7 and C-12

LIME IN MASONRY MORTARS (Status report as of Feb. 1968)

Prior to the discovery of portland cement near the end of the 19th century, lime and sand were the prime constituents of masonry mortars. There are many fine structures built in past centuries,

and still standing today, that bear witness to the excellence of the lime-sand mortars that remained unchallenged almost from the beginning of time until the advent of cement.

During the first quarter of the 20th century the trend toward increased cement contents in mortars progressed steadily, thus pro-viding more strength to satisfy demands of the building industry for: (l) all-weather year-round construction and (2) larger buildings with thinner walls. Early in the 1930' s however, serious water penetration problems in masonry walls led to a reappraisal of the merits of cement and lime and resulted in a return to the use of cement-lime mortar combinations.

This new thinking was caused by the fact that the harsh cement mortars have poor workability resulting in an inadequate extent of bond between mortar and unit, thus leaving t1unbonded" passages permitting easy access routes for water to enter the structure. On the other hand, the lime mortars, high in workability and capable of an excellent extent· of bond with the masonry units, lacked strength, particularly early

strength, so necessary to meet the new demands of the building industry. A mixture of cement and lime provides a mortar which possesses the good qualities of both ingredients. Varying the amounts of cement and lime provides a flexible range of mortars capable of meeting almost any

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-combination of strength-workability requirem.ent. This principle is recognized in ASTM Specification C 270 on Mortar for Unit Masonry, where five combinations of cement and lime are designated;

More recently, masonry cements have become a big factor in masonry construction and have resulted in reducing the consumption of lime for masonry purposes. In the beginning these products were simply a blended mixture of portland cement and hydrated lime. These packaged materials eased construction-site mixing procedures and pro-vided better quality control over the cementitious material. Eventually, the more expensive hydrated lime was replaced by materials such as clay and limestone, the desirable plasticity being achieved by the ad-dition of airentraining materials and other additives. Currently, masonry cements may also include water repellents, water reducers, accelerators, retarders, colours, etc.

In addition to ensuring for the architect and the builder a better measure of quality control by eliminating some of the haphazard material batching historically associated with mortar mixing, the use of masonry cements has a growing appeal to labour also.

The production of masonry cement, first introduced in the late 1920' s, had reached 23 million barrels in 1966. Despite increasing in-roads made by masonry cement in recent years, there are indications that lime has an opportunity to retain a reasonable share of the masonry mortar market. The brick industry has recently encountered challeng-ing competition in the form of wall panels, made of such materials as precast concrete and metal. In order to meet this competition, the Struc-tural Clay Products Institute (SCPI), representing the clay brick and tile industry, has developed several new programs.

One promotes the use of organic additives in masonry mortars to achieve high bond strength (e. g. 200 psi) in very thin walls. This program _is detrimental to the cause of lime in that the organic additives do not

re-act favourably in the presence of lime, which is therefore excluded from such mortars. There are a number of difficult problems associated with organic modified mortars, however, and after five or six years of ex-pensive development they are still not a significant factor in masonry con-struction. It is interesting to note that one of the early problems was in achieving a satisfactory workability in the absence of lime.

A second program, more favourable to the cause of lime, involves use of load-bearing masonry walls in high-rise buildings. In such systems, 18-storey buildings have been erected with 8-in. brick walls. This approach

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is made possible partly through improved masonry materials and partly by better application and advancement of engineering concepts. Design of these contemporary load-bearing structures requires that walls and floors work together, each giving strength to the other, so that relatively thin walls can carry great loads. Itfollows that bricks and mortar must be of top quality and, for the latter, the SCPI recommend three mortar types, M, S, and N containing portland cement and non-airentraining Type S hydrated lime. They exclude masonry cements from the list of acceptable materials for mortars for this purpose.

SCPI reservations concerning masonry cements are two-fold. First, the lack of a maximum value for air content in specifications for masonry cement causes concern about low bond strength developing between brick and mortar when its air content is high. Secondly, masonry cements are pro-prietary products and so their chemical composition may vary widely and is subject to change without notice. In the 1962 edition of the Brick and Tile Handbook, Harry Plummer states, "Masonry cements are largely proprietary mixtures and their formulae are seldom disclosed by manufacturers. ASTM Specifications place no limitation on their chemical composition and,

con-sequently, there is a wide variation in the constitutuents of the various brands. " Thus, masonry cements, with widely varying air contents and compositions, cannot be recommended for a custom-built product like load-bearing masonry.

The lime industry, however, should not adopt a complacent attitude because of their favoured position with SCPI. First of all, the masonry cement industry, concerned about the SCPI attitude, and well aware that the load-bearing masonry concept is gaining favour, are endeavouring to re-solve their problems. Their opposition to imposition of a maximum air content level has relaxed to the point where a maximum value of 24 per cent is to be inserted in their masonry cement specification. Although this value is considered unrealistically high, it is a step in the right direction and it will undoubtedly be modified when a meaningful figure based on reliable tef!t data can be obtained. One of the real problems in this regard is the

lack of a reproducible bond strength test. There should be no doubt, however, that the masonry cement industry is fully aware of its problem areas with SCPI and is taking steps to have them resolved.

SCPI also has some reservations about lime, and if the lime in-dustry is sincerely interested in retaining its favoured position with this group, these reservations must be recognized and resolved with the least

possible delay. In February 1967, SCPI representative asked ASTM Committee C7 on Lime to alter ASTM Specification C207 on Hydrated Lime for Masonry

non-airentraining hydrates are required for certain mortars for load-bearing masonry and in some areas, it is very difficult to obtain these.

It is well known that, although leaders of the lime industry publicly decry the deleterious effect of organic materials in masonry mortars, most of the major producers are using these materials to produce air entraining hydrated limes. The request to alter C207 was rejected because there was no method of test to measure air content. It was, however, recommended that the industry label their hydrates trairentraining" or "non-airentraining.II

Industry response to this request is not .known.

Since then, a method of test for measuring air content in hydrated lime is being investigated by Committee C7and a method may soon be adopted. When this has been accomplished the lime industry could enhance its position with SCPI by writing a specification for air entraining hydrated lime containing a realistic maximum value. There can be no doubt that such action would be favourably received in the light of the reluctance of the

masonry cement industry to take similar action.

The other disagreement between SCPI and the lime interests con-cerns the continued use of Type N hydrated lime in masonry mortars. SCPI recommends only Type S hydrates and current proposals for revision

of C210 on Mortar for Unit Masonry include dropping Type N hydrated lime from the list of acceptable materials. Opposition to Type N hydrates is apparently based on construction problems that occurred some 30 years ago and were attributed, rightly or wrongly, to expansion caused by dolomitic hydrates. Similar problems with dolomitic hydrates in finish plaster - - a totally different situation -- have undoubtedly contributed to the prejudice agains.t Type N hydrates.

The problem could be solved in two steps. Those who oppose in-clusion of Type N hydrates in C270 would accept a Type N with a maximum limit on unhydrated oxides. This limit would be designed to permit most high calcium Type N hydrates to qualify but would exclude certain Type N dolomitic hydrates.

At present, 95 per cent of U. S. production of mason's hydrated lime is Type S. The remaining 5 per cent is Type N, of which 3 per cent is high calcium ｾｮ､ 2 per cent is dolomitic. The industry ITlUSt decide if

it really wants to retain the Type N dolomitic hydrate in the mortar speci-fication. If so, there should be no difficulty in presenting factual evidence to support its case. There is a long record of the satisfactory performance with masonry mortars containing dolomitic hydrates in Canada, where in fact, there is no Type S hydrated lime, the total production in the country being Type N. A paper by T. Ritchie at the Division of Building Research

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-wall are sufficient to overcome any expansive tendencies in a dolomitic hydrate extensively used in Canada

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A similar research program, conceivably a project sponsored by the National Lime Association at a university or some other competent research laboratory, inc::luding dolomitic hydrates representative of cur-rent U. S. production, could be designed to produce information that should convince those concerned of the suitability of the Type N dolomitic hydrate for use in mortar.

At present, supporters of Type N lime are basing their arguments for its retention in the mortar specification on a lack of evidence to support its rejection as an acceptable material after many years of apparently satis-factory performance. Although this argument has some merit, it is obvious, after three or four years, that there are still too many persons who have reservations about dolomitic hydrates for these hydrates to be accepted. Until some specific information to substantiate the satisfactory performance of dolomitic hydrates currently produced in the U. S. A. can be compiled, it would appear wise to cease arguing.

In summary, it would be advisable to ensure a sound liaison with

SCPI, who are currently recommending the use of lime, by resolving

their reservations about air entrainment and Type N hydrated lirtle. Such action would appear to be a positive step in consolidating the present position of lime in the masonry mortar market.

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mortars. Materials Research and Standards, Vol. 4, Jan. 1964, p. 15-19 (Reprinted as NRC No. 7751).