Lime mortar is a type of building material that has been used since ancient times. While the actual ingredients within the mortar varied, most would include lime along with some type of aggregate to add strength and substance to the product, with all the ingredients combined with water. There are examples of lime mortar construction dating back to the 4th century BC, with ancient structures in Greece and Rome demonstrating how durable this type of mortar can be.
For many centuries, lime mortar remained one of the most commonly used building materials for a wide range of structures. It was not until the 19th century when various types of cement were developed that the use of the mortar began to decline. Part of this is due to the fact that the newer cements cost less to produce and to purchase while still offering a significant amount of strength and durability. While the use of lime mortar is not as prevalent today, the material is still preferred in some circles when working with terracotta or natural stone to create facades or other forms of construction.
The traditional composition of lime mortar is a lime putty combined with sand, although other types of aggregates may be substituted. Typically, the mixture requires the use of one part lime putty to three parts of the aggregate, then water added to achieve the desired consistency. There is some indication that ancient applications involved preparing the mortar with one part lime to two parts aggregate, an approach that yielded a mortar with a coarse texture that was ideal for specific types of building projects. This alternative combination was sometimes augmented with the addition of horsehair to the mortar, which helped to add strength and reduce the degree of shrinking the material would experience during the drying and curing process.
When choosing to use lime mortar as a building material, it is important to prepare the aggregate carefully. This helps to enhance the overall strength of the mortar and minimize the potential for cracking. Generally, a process known as "washing" is used on sand before it is combined with the lime putty. By running the sand through water, impurities are removed that could create air pockets within the mortar that could cause some decrease in strength.
The application of the finished lime mortar is also important in order to obtain the best results. Typically, applying the mortar in thick coats will increase the potential for cracking or slumping. Ideally, the coating should be applied in a thin and even coat, during a period in which the air temperature is moderate. If possible, the mortar should not be applied in direct sunlight, since this will expedite the drying process and possibly lead to cracking. This creates a situation in which the lime mortar should only be used during certain parts of the day and when the temperatures are neither excessively hot or cold.