Differentiating Lime Mortars, Cement Mortars & Hydraulic Limes

John Robert D. General, RN, MSN

Masonry is a highly durable form of construction. Getting the masonry job will never be complete without the pieces of building blocks needed for construction. The common materials of masonry construction are brick, stone, marble, granite, travertine, limestone, cast stone, concrete block, glass block, stucco, and tile. The materials used, the quality of the mortar and workmanship, and the pattern in which the units are assembled can significantly affect the durability of the overall masonry construction.

One material commonly used in masonry is the lime mortar. It is a type of mortar composed of lime and an aggregate such as sand mixed with water. It is one of the oldest known types of mortar, dating back to the 4th century BC and widely used in Ancient Rome and Greece. It was discovered that limestone, when burnt and combined with water, produced a material that would harden with age, thus making the constructed material durable and proven to last a lifetime. The earliest documented use of lime as a construction material was approximately 4000 B.C. when it was used in Egypt for plastering the pyramids.

Traditionally, lime mortar is a combination of lime putty and aggregate which is usually sand. A typical modern lime mortar mix would be 1 part lime putty to 3 parts washed, well graded, sharp sand. Other materials have been used as aggregate instead of sand. Its importance can never be discounted since lime mortar is primarily used in the restoration of buildings originally built using lime mortar. Lime provides high water retention that allows for maximum early curing of the cementitious materials. Its high initial flow permits easy complete coverage of masonry units, and the low air content of cement-lime mortar increases bond strength.

Moreover, the low air content, 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. 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. Research also 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. Lime has been an important component of mortars for over 2000 years and continues to be useful today.

Recent addition to the previous masonry work is the introduction of cement mortars, usually combines with lime or used singly. Cement mortar is a building compound created by mixing sand and a selection of aggregates with a specified amount of water. The mortar can be used for a number of applications, such as plastering over bricks or other forms of masonry. Cement mortar continues to be used in many different types of construction. Professional building projects often employ mortar as the binder between bricks in walls, fences, and walkways. Around the house, cement mortar is often employed to make quick repairs in patio slabs and reset loosened stones or bricks in a walkway or retaining wall.

Cement mortar also makes an excellent medium for creating a smooth surface to walls made from bricks and other forms of masonry. The ingredients in cement mortar vary somewhat, depending on the manufacturer specifications. A typical mortar will include both sand and cement, with lime added to the mix. Other types of aggregates may be added, depending on the texture that is desired for the mortar.

In recent years, the inclusion of synthetic materials such as polymers have helped to create cement mortar products that provide additional flexibility without negatively impacting the binding powers of the cement mortar. Another type of mortar material that interests masonry is the hydraulic lime. It is a type of lime, or calcium carbonate, which is used to make mortar and plaster products. The lime is heated, and then mixed with an aggregate material like sand or stone. Once this mixture is blended with water to form mortar, it can be used in many types of masonry construction projects. The mortar can be placed between bricks or blocks to bind them together, or may even be applied to the surface of the masonry to create a plaster or stucco-like application. Hydraulic lime is used for providing a faster initial set than ordinary lime in more extreme conditions (including under water).

To understand how hydraulic lime works, it is important to first understand how traditional hydrated lime works. When hydrated lime is mixed with water to form mortar, it has a consistency similar to peanut butter. As the mortar reacts with the air, it absorbs carbon dioxide, causing it to harden or cure. Hydraulic lime mortar, or the other hand, begins to harden when it is exposed to water. Over time, it will also absorb carbon dioxide from the air to experience a second phase of hardening or curing. Due to these differing reactions, hydrated lime and hydraulic lime are very different products and are not interchangeable.

Hydraulic lime offers a number of benefits over traditional lime mortar blends. The most important is its ability to cure and harden when wet, which means it can be used in many applications where other mortar products would fail. It also has low elasticity, resulting in fewer cracks due to expansion and contraction. Hydraulic lime is also very porous, allowing excess moisture to escape rather than collecting inside the wall structure. This makes it very popular for historic preservation applications, where traditional hydrated lime would crack or crumble over time due to trapped moisture.

With the knowledge that hydraulic lime had already been in use since the Roman Times, its significance is still remarkably recognized even this time. This means that lime could be used in a wide variety of applications – singly, mixed with cement or be created with another curing method. Mortars as indispensible products have evolved over time, making masonry outcomes more reliable, genuine and timeless.