Masonry Magazine October 1987 Page. 21
NCMA-TEK
An Information series from National Concrete Masonry Association
Mortars for Concrete Masonry
20A
18A
Introduction
It was approximately 4700 years ago, in the year 2690 B.C., that the first recorded use of masonry mortar took place during construction of the great Pyramid of Giza in Egypt. This earliest form of mortar consisted of burned gypsum and sand. The ancient Romans and Greeks later produced mortars from various materials such as burned lime, volcanic tuff, and sand. Even as recent as the settlement of the first colonists in North America, mortar advancement had not progressed beyond the point where a still relatively weak product was being made from lime and sand.
The discovery of portland cement in the early 1800's led to the first major improvement in masonry mortars. This cement, which did not become available in the United States until the latter part of the nineteenth century, was actually a natural cement which had a chemical composition somewhere between certain hydraulic limes (limes containing silica) and portland cements as we know them today. Masons in this country used this natural cement with or without the addition of hydraulic lime. This resulted in the development of a greatly strengthened mortar.
As the manufacture of portland cement evolved to its present state of the art, mortars were produced which possessed far superior strength, durability, resistance to moisture penetration, and more predictable setting characteristics than any of those previously used. It thus became possible to reevaluate previous masonry design considerations, which now provides for the construction of taller and thinner walls.
While mortar represents only a small proportion of the total wall area in concrete masonry construction (approximately 7 percent), its influence on the entire structural performance of a wall is significant. The mortar serves many important functions: it bonds the units together into an integral structure of predictable strength, seals the joints against penetration by air and moisture, accommodates small movements within the wall, secures the joint reinforcement to assist in resisting shrinkage and crack control, and bonds with the steel reinforcement, ties, and anchor bolts so that all elements perform as a unit.
Generally speaking, problems associated with masonry walls have been found to be related to improper design, poor construction practices, and lack of adequate inspection. Proper attention to each of these factors can eliminate a substantial amount of these problems. Some problems, however, such as cracking and rain penetration, have been directly attributed to the use of unsuitable mortar. Since the mortar type and composition directly affect the performance of a wall, mortar selection should be based upon a knowledge of the mortar's characteristics and the intended service conditions.
This TEK emphasizes the role of masonry mortars and provides necessary information leading to the proper selection and use of materials.
Materials
The American Society for Testing and Materials (ASTM) has established standards for materials commonly used in masonry mortars.
Portland cement (ASTM C150) is a hydraulic (able to set under water) cement and one of the main constituents of masonry mortars. Three standard types (I, II, and III) are permitted. Air-entrained portland cements (IA, IIA, and IIIA) may be used as alternatives to each of the standard types. Air-entrained portland cements are manufactured by intergrinding certain agents which produce minute, closely spaced, air bubbles in the masonry mortar.
Masonry cement (ASTM C91) is an inter-ground mixture of portland cement and inert pulverized limestone or hydrated lime, together with other agents introduced to influence such properties as setting time, plasticity, water retention, and durability.
Blended hydraulic cements (ASTM C595) consist of standard portland cement or air-entrained portland cement which has been combined through blending with such materials as blast furnace slag, fly ash, and slag. The resulting products are classified as blast furnace slag cement (Types IS and ISA), portland-pozzolan cement (Types IP and IPA), and slag cement (Types S and SA), respectively. Slag cements (Types S and SA) are approved 00 for use only under the property specification (Table 2) which requires that the masonry mortars be laboratory tested and satisfy the minimum requirements for compressive strength and water retention.
Quicklime (ASTM C5) is calcined (burned-decarbonated) limestone, the major constituents of which are calcium oxide (CaO) and magnesium oxide (MgO). Quicklime requires slaking prior to use and the resultant lime putty must be stored and allowed to hydrate for a period of at least two weeks before use.
Hydrated lime (ASTM C207) was quicklime at one stage in the manufacturing process before it had been hydrated or pressure hydrated and dried. There are two types: Type N (or NA) normal hydrated lime, and Type S (or SA) special hydrated lime for masonry purposes. Types N or NA limes may be permitted if shown by test or performance record to be not detrimental to the soundness of the mortar. Air-entrained limes shall not be Concrete Units