The careful selection of mortar is paramount in masonry restoration, ensuring not only the structural integrity of a building but also its long-term health and historical authenticity. Different eras of construction utilized distinct mortar technologies, and using an inappropriate modern mortar on an older structure can lead to irreversible damage. This article explores several key mortar types, from modern cement-lime blends to traditional pozzolanic hydraulic limes, and their appropriate applications in restoration work.
Modern and Transitional Mortars: Type N and Type O
Type N Mortar: A familiar name in modern masonry, Type N mortar is a medium-strength formulation typically composed of one part Portland cement, one part lime, and six parts sand. This 50/50 blend of Portland cement and lime by volume of cementitious material results in a mortar with a compressive strength generally around 750 pounds per square inch (psi), which can reach up to 1200 psi when fully cured.
Its balanced characteristics make Type N suitable for a variety of contemporary applications, including repointing many modern masonry projects, general brick and stone construction above grade, and interior load-bearing walls. It can also be appropriate for some soft stone masonry due to its adequate flexibility compared to stronger Portland cement-based mortars. Historically, structures built after events like the Great Chicago Fire, which saw advancements in building materials, may be candidates for Type N mortar in restoration if the original mortar composition aligns.
Type O Mortar: Representing a shift from purely lime-based mortars, Type O is a lower-strength, lime-rich mortar that incorporates a small amount of Portland cement. A common mix proportion is one part Portland cement, two parts lime, and nine parts sand, yielding a compressive strength of approximately 350 psi.
Type O mortar is often considered a "transition-period" mortar, reflecting construction practices from around the early 1900s up to the 1940s. During this era, builders began experimenting with Portland cement, adding it to traditional lime mortars to achieve a faster set and increased strength, though not to the levels seen in later, harder cement mortars. In restoration, Type O is valuable for repointing interior, non-load-bearing historic masonry that originally used similar softer, gauged lime mortars. It's also considered for historic masonry where hydraulic binders were not originally present but a slight increase in performance over pure lime might be needed in specific, sheltered conditions. Care must be taken to ensure its lower strength and higher vapor permeability are compatible with the surrounding masonry units.
Pure Lime Mortars for Historic Masonry: Pozzolanic Hydraulic Limes (PHL)
For structures predating the widespread use of Portland cement, particularly mass-masonry constructions common before the twentieth century, Pozzolanic Hydraulic Lime (PHL) mortars offer a historically appropriate and technically superior solution. These mortars are pure lime-based, containing no Portland cement. They achieve their hydraulic set (the ability to set in the presence of water) through the reaction of lime with pozzolans – siliceous or alumino-siliceous materials like volcanic ash, certain clays, or rice husk ash. This hydraulic property, combined with the inherent qualities of lime, provides the crucial flexibility and self-healing capabilities necessary for the preservation of older buildings.
The Importance of Flexibility and Self-Healing: Mass-masonry walls were designed to move. They experience thermal expansion and contraction, slight settlement, and respond to changes in humidity. Mortars used in these constructions needed to be flexible enough to accommodate these movements without cracking the masonry units themselves. Softer lime mortars act sacrificially; they are intended to be slightly weaker than the bricks or stones, so any cracking occurs in the mortar joints, which can be more easily repaired.
Furthermore, lime mortars possess a remarkable "self-healing" property. This natural phenomenon involves the dissolution of free lime (calcium hydroxide) by water entering the mortar. This lime-laden water then travels into any micro-cracks that have formed. As the water evaporates and through carbonation (reaction with atmospheric carbon dioxide), the calcium hydroxide re-crystallizes as calcium carbonate, effectively sealing the cracks and restoring the mortar's integrity. This process contributes significantly to the long-term durability of historic structures.
PHL 2.0: This is the softest of the PHL mortars, designed for the most delicate applications. With a lower hydraulic strength, PHL 2.0 is used for interior plasterwork and the repointing of extremely fragile or historically significant masonry where the original mortar was very weak. Its use is typically restricted to sheltered locations as it offers the least weather resistance among the PHL grades. Its high breathability is essential for managing moisture within sensitive historic fabric.
PHL 3.5: Considered a general-purpose pozzolanic hydraulic lime mortar, PHL 3.5 strikes a balance between strength, flexibility, and weather resistance. It typically has a compressive strength around 500-600 psi after 28 days. This makes it suitable for a wide range of interior and exterior applications in historic building restoration. It offers good workability for masons and provides the necessary vapor permeability to allow historic walls to "breathe," preventing trapped moisture and associated decay. Common mix ratios are around 1 part PHL 3.5 binder to 3 parts sand. It is also finding use in sustainable new construction, such as in strawbale or hempcrete buildings, due to its lower embodied energy and breathability.
PHL 5.0: This is the strongest of the commonly used PHL mortars, with a compressive strength typically around 750 psi after 28 days. It is formulated for situations where a pure lime mortar is historically appropriate but greater durability and weather resistance are required. Such applications include exterior chimneys, parapet walls, and other masonry elements exposed to harsh weather conditions and high moisture levels. Even with its increased strength, PHL 5.0 retains the essential flexibility and breathability characteristic of lime mortars, ensuring compatibility with older masonry units. Typical mix ratios are approximately 1 part PHL 5.0 binder to 2.5-3 parts sand.
In Summary
The successful restoration of masonry structures hinges on an understanding of their original materials and construction techniques. Choosing the correct mortar type is not merely a matter of filling joints; it is about ensuring the continued performance, health, and historical integrity of the building. Modern mortars like Type N have their place in contemporary construction and specific later historic applications. Transition-period mortars like Type O bridge the gap to an era of early cement use. However, for the vast majority of pre-twentieth-century mass-masonry buildings, Pozzolanic Hydraulic Lime mortars (PHL 2.0, 3.5, and 5.0) offer the essential properties of flexibility, breathability, and self-healing that are critical for their long-term preservation, ensuring these invaluable structures endure for future generations.