Proper Ratio of Lime to Sand
We get asked the question often “What ratio of lime to sand should I use?” Our answer is “We don’t know until we know what the void space of your sand is,” There are thousands and thousands of sands across the USA, and within those myriad of sands, there are literally countless ways that manufactures sieve, wash and process the sand you purchase at your local masonry or building supply company.
One of our sayings here at Lancaster Lime Works is “Sand is not Sand is not Sand.” Even the mason’s sand that is provided in huge piles at your local masonry supply company will vary from load to load. It is of utmost importance to test the sand that will be used in our lime putty for the following: Void Space and Particle Size/Shape/Distribution.
Our lime putty is the best available. However, if too much or too little lime is added to your sand, product failure will be the result. Therefore we spend just as much time explaining and teaching about sand as we do about installation of lime based mortars
The Basics are Simple:
The void space between the sand is similar to the space between beach balls in a display bin at Walmart. In between these beach balls there is room for smaller soccer balls, then in between the soccer balls there is room for baseballs, then in between the base balls there is room for marbles, then in between the marbles there is still space for even smaller particles such as dust and yet there is still empty space between all these balls. This is called the “void space.” Sand has the same.
The void space within the sand must be filled with the lime putty. Too much lime will push the sand particles apart; not enough lime will leave “holes” in the mortar, and both scenarios will leave you with a weak mortar, stucco or plaster.
Lime Mortars gain the majority of their strength from the sand and rely on the sharp pieces of sand making contact with each other and compacting firmly together. The Lime develops a crystal structure that binds the already sharp, compacted and interlocking sand particles together.
Portland Cement based mortars are comparable to a two part epoxy glue, they gain the majority of their strength from the binder which is the Portland. The Portland basically “glues” the sand together and negates the necessity for grading and evaluating sand that is needed for Lime Mortars. Lime mortar is a cushion between the building stones or bricks, Portland Cement mortar is a glue that holds the stones or bricks together.
The Correct Ratio:
The ratio of lime to sand is always variable. Specifications calling for such ratios of 1:2, 1:3 or 1:1 lime to sand are misleading, confusing and could quite possibly be wrong. The lime:sand ratio will absolutely vary from sand to sand and from load to load. The correct ratio of lime:sand is determined by discovering the void space within the sand chosen for the project. The void space within the sand must be filled with the lime putty – too much lime will push the sand particles apart, not enough lime will leave “holes” in the mortar and both scenarios will leave you with a weak mortar or plaster.
The results of having the correct ratio are fun to watch and are observed when mixing batches of lime mortar. If the ratio of lime to sand is 1:3 (one lime:three sand), that will mean dumping three evenly filled buckets of sand into the mixer and one bucket of putty for a total of four buckets added. If not too much lime has been added, only three buckets of lime mortar will be removed from the mixer because the putty just fills the space between the sand.
We believe that all masons should have the skills to determine the void space within the sand they are using and be able to prove and explain to the architects and engineers that they are using the correct amount of lime for the sand chosen for the project.
The testing method of finding the void space between the particles of sand is relatively simple yet very very important to prevent mortar failure and obtain a strong lime mortar. (more about how to do this in the next blog)
Our goal and Lancaster Lime Works is to provide the training and resources to mason’s and architects to match and install all historic mortars. Please contact us with questions.