Masonry Magazine August 1970 Page. 15
NGMATEK 20
An Information series from National Concrete Masonry Association ©1970, National Concrete Masonry Association
MORTARS
FOR CONCRETE MASONRY
Introduction
History records that masonry mortars were used as far back as the year 2690 B.C. when the Great Pyramid of Giza was constructed in Egypt. The massive stone blocks of this 40-story wonder of the world were cemented together with a mortar made of burned gypsum and sand. Later in ancient Greece and Rome, mortars were produced from various materials such as burned lime, volcanic tuff, and sand. Although many years passed before the first settlements were begun 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 manufacture of portland cement beginning in the early part of the 20th century led to greatly strengthened mortar, either when portland cement was used alone or in combination with lime. In general, modern day mortar is still made from portland cement and lime, or a recently developed product called masonry cement is employed.
The relatively small proportion of mortar in concrete masonry construction significantly influences the total structural performance. The mortar not only binds the individual masonry units together into a single structure of predictable strength characteristics, but also seals the joints against penetration by air and moisture. It bonds with steel reinforcement, ties, and anchor bolts, so that they perform integrally with the masonry. In many cases, problems which have arisen in masonry walls, such as cracking and rain penetration, have been directly attributed to the use of unsuitable mortar. Mortar for concrete masonry structures should therefore be composed of materials which will produce the best combination of mortar properties for the intended service conditions.
Materials
The American Society for Testing and Materials has established standards for materials commonly used in mortars, as summarized in table 1.
Portland cement, a hydraulic cement, is a principal cementitious ingredient in masonry mortars. Three types are permitted by ASTM specifications:
Type I-for general purpose construction.
Type II-for resistance to sulphate action.
Type III-for high early strength development.
Air entraining portland cement is interchangeable with portlant cement in the ASTM specification for masonry mortar (C-270). It is manufactured by the addition of certain agents to the cement mix, so as to produce minute, closely spaced, air bubbles in the cementitious binder.
| MATERIAL | ASTM DESIGNATION |
| ----------- | ----------- |
| Portland Cement (Types I, II, III) | C150 |
| Air Entraining Portland Cement (Types IA, IIA, IIIА) | C175 |
| Blended Cement (Types IS, ISA, IP, IPA, S, SA) | C 595 |
| Masonry Cement | C91 |
| Quicklime | CS |
| Hydrated Lime | C 207 |
| Aggregate | C144 |
Table 1. Mortar Materials
| SIEVE SIZE NUMBER | PERCENT PASSING |
| ----------- | ----------- |
| 4 | 100 |
| 8 | 95-100 |
| 100 | 25 maximum |
| 200 | 10 maximum |
Table 2. Gradation of Aggregate (ASTM C-144)
Of the Blended cements, portland blast furnace slag cement (Type IS or ISA) is also interchangeable with portland cement in the ASTM specification. Other blended cements include portland-pozzolan cement (types IP and IPA) and slag cement (Types S and SA). Blended cements are not used as extensively as other types of cement.
Masonry cement is a mixture of portland cement and finely ground inert limestone in approximately equal amounts. Hydrated lime may be used instead of limestone. Gypsum is sometimes added to regulate the setting time, and other agents may be added to influence properties such as plasticity, water retentivity, and durability.
Quicklime is essentially calcium oxide (CaO) which must be slaked before use. Slaking the quicklime gives off heat, requiring precautions to be observed during the process. After slaking, the resultant lime putty must be stored for a period of up to two weeks before use.
Hydrated lime is a dry powder obtained from quicklime which has been slaked to satisfy its chemical affinity for water. Calcium oxide is converted to calcium hydroxide,