Lime has been an important component of mortars for over 2000 years. The characteristics of hydrated lime provide unique benefits in masonry applications that distinguish cement-lime mortars from other masonry mortar materials. Major benefits include:
Flexural bond strength
Cement and Type S Hydrated Lime mortars have been shown to have high levels of flexural bond strength. High tensile bond strength is enhanced by the following properties of cement-lime mortars:
- Tensile Bond Strength - This is the strength of the mortar that holds the masonry units together. High tensile bond strength is developed by the following mortar characteristics:
- Lime provides high water retention that allows for maximum early curing of the cementitious materials.
- High initial flow which permits easy complete coverage of masonry units.
- The low air content of cement-lime mortar increases bond strength.
- Extent of Bond - The extent of bond is the percent of brick to which the mortar adheres. The low air content and the fineness and stickiness of hydrated lime particles increase the extent of bond of mortar to brick. These factors allow cement-lime mortars to penetrate deeply into the brick and seal the brick/mortar interface.
- Durability of Bond - (See Durability section below)
There are a number of studies that demonstrate the superior bond strength of cement-lime mortars. For copies of these studies, pleasecontact Graymont.
Studies have shown that cement-lime mortars can be used to minimize the potential for water penetration into masonry walls.
- Extent of Bond - Low air content, a fine particle size, high plasticity and water retention contribute to excellent extent of bond for cement lime mortars. This eliminates easy migration paths for water penetration.
- Autogenous Healing - When hairline cracks develop in the mortar, hydrated lime reacts with carbon dioxide in the atmosphere. This reaction produces limestone which helps to seal the crack and fill voids in the mortar. This explains the increased moisture resistance noted after six months of curing in two studies.
Masonry construction is a durable, low maintenance system. The use of lime in mortars contributes to the durability of this system. The durability of lime mortar is evidenced as follows:
- Elasticity - Research has shown that high lime content mortars were slow hardening and remained elastic or flexible. Lime, therefore, enhanced the ability of the assemblage to accommodate stresses caused by building movement and cyclical changes without excessive cracking.
- Autogenous Healing - When hairline cracks develop in the mortar, hydrated lime reacts with carbon dioxide in the atmosphere. This reaction produces limestone which helps to seal the crack.
- Proven Performance - Prior to the early 1930's, all masonry buildings were constructed with lime or a mixture of cement and lime. Portland cement was not manufactured in the United States until 1871. Prior to this, lime was used as the primary ingredient of all mortars. The durability of these structures serves as testimony to the lasting durability of lime mortars.
ASTM C270 allows for mortars to be specified by the proportion or property guidelines. Cement-lime (CL) mortars mixed under the proportion specification generally have enough compressive strength to meet the next highest C270 property specification. For example, a Type N cement-lime mortar, as defined by the proportion specification, will have enough strength to meet the Type S mortar property specifications. Specifying CL blends by proportion provides a margin of safety concerning compressive strength. If high compressive strengths are undesirable, lime content can be increased and the property specifications used. In either event, cement-lime mortar compressive strength levels are adjustable and predictable.
Hydrated lime improves the strength of the mortar by several mechanisms:
- Carbonation - Hydrated lime reacts with carbon dioxide in the atmosphere to form limestone.
- Cementitious Reactions - Pozzolonic reactions can occur between hydrated lime and silica compounds in the mortar mix.
- pH - Hydrated lime helps to maintain high pH levels in the mortar mix. This makes siliceous materials more soluble and reactive.
Cement-lime mortars provide uniform performance characteristics in the field. ASTM C270 provides recommended proportions for Type O, N, S and M cement-lime mortars. This specification also requires that hydrated lime products meet ASTM C207 criteria, portland cement meets ASTM C150, and both ASTM C207 and ASTM C150 specify chemical composition as well as physical product qualities. The chemistry of each cement-lime blend is defined and contains a high percentage of cementitious materials (> 95%). Since the chemistry is well defined, performance characteristics such as compressive strength and flexural bond strength are predictable at given proportion levels. Air content of cement-lime mortars are restricted to 12% for Type M and S mortars and 14% for Type N and O mortars. Tighter limits on air content also helps to minimize variation between blends. Preblended cement-lime mortars are also available in most markets in 65-75 lb. bags, bulk bags or silo systems.