For engineered construction and load-bearing applications, mortar performance is as critical as unit strength and workmanship. MORTAR TYPE AND DESIGNATION ASTM C 270 Standard Specification for Mortar for Unit Masonry covers four types of mortars, types M, S, N, and O. These mortar types can be specified by proportion or properties, but not both. PROPERTY VS. PROPORTION SPECS Mortar type classification under the property specifications is dependent on compressive strength, water retention, and air content. These property requirements are for laboratory specimens only and not for job-mixed mortar. The proportions of cement, lime, and sand established in the laboratory to meet ASTM C 270 requirements are required to be used in mixing mortar at the jobsite. CHOOSING THE MORTAR TYPE The architect or engineer should specify the mortar that best meets job requirements. Type M is a high-strength mix that offers greater durability than other mortar types. Type S mortar achieves high tensile bond strengths, about the highest that mortars can obtain. Type N mortar is a general purpose mortar for use in above grade masonry structures. Type O is a high-lime, low strength mortar. For tuckpointing, use only prehydrated mortar. Refractory mortars may be used in residential fireplaces or extremely high-heat industrial boiler incinerators or steel pouring pits. ASTM C 105 covers material standards and properties for ground fire-clay mortar. For high-heat industrial service, ASTM C 64 covers air-setting refractory mortar.