Rediscovering Lime Plaster In Costa Rica

This research and investigation was done as part of a natural building course I co-instructed with Liz Johndrow of Earthen Endeavors at Rancho Mastatal in Mastatal, Costa Rica. Rancho Mastatal is a permaculture farm and has been a pioneer for natural building and sustainable living technology in Central America. What I found out about lime in that part of the world was eye-opening and enlightening.

By Ryan Chivers

Historically, as in almost every other society in the world, lime was used as the primary binder for masonry, plaster and render in Costa Rica. It was not until the 1950s and 60s that cement production became widespread eventually replacing lime altogether. Today, the building industry in all of Latin America is dominated by Portland cement with most of the markets being controlled by a handful of billion-dollar multinational companies.

It is unheard of for a modern Latin American mason or plasterer to be using pure lime mixes with no cement. In Costa Rica, they don’t use lime in construction at all. At builders and masonry supply yards it is virtually impossible to find a bag of hydrated lime.

Fortunately, lime is one of the most widely used man made materials and is critical for almost everything that is produced in the modern world.   Limestone is abundant. It makes up 20% or so of the earth’s crust, and its abundance makes it a material that is relatively cheap to produce. For industry and agriculture it is bought and sold in huge quantities, and so is almost always sourced relatively locally.

This report will focus on lime technology as it relates to building construction. The main uses will be formulations for plasters, stuccos and mortars for masonry. Two related raw materials will be covered. The first one is hydrated lime, which has been fired in a kiln and slaked with water. The second material is crushed limestone, which is essentially a sand of different gradations that could be used as the aggregate portion of all lime formulations.

It has been my experience that the use of calcium carbonate based aggregates in pure lime formulations yields plasters and mortars that are stronger and more workable than more commonly used silica based pit or quarry sands.   The theoretical reasoning is that calcium carbonate sands allow for faster and more thorough carbonation, creating a mortar or plaster that becomes chemically pure CaCo3, or limestone, when it has fully cured.

Costa Rica Lime

In Costa Rica, a large majority of processed lime and crushed limestone is used for agriculture. Almost all industrial agriculture, bananas, coffee, palm oil, pineapple, etc. use lime or crushed limestone as soil amendments, pH buffers, and anti-fungal treatment.   They are both produced in large quantities.   We discovered two large lime producing areas in the country. One is in the south at Palmar Norte, and the other is 10 km or so south of San Jose, at Patarra.

This research and investigation was done as part of a natural building course at Rancho Mastatal in Mastatal, Costa Rica. Rancho Mastatal is a permaculture farm and has been a pioneer for natural building technology in Central America.

Crushed Limestone

At Palmar Norte, we found lime and stone production happening at small quarries adjacent to each other. Crushed stone and lime production were separate operations. The crushed limestone was screened into 3 different grades as follows:

• Primera was the finest sand. It has a top size of approximately 70 mesh, with gradations down to 325 mesh fine powder. This material is their main product and is used as a soil amendment in agriculture.     In building, this fine sand could be used in very fine finish plaster work.

• Segunda is the next size up. It has a top size of approximately 16 mesh, down to a small amount of sub 200 mesh particles.   This would be the main aggregate for use in finish coats, tadelakt and thin base coats and small mortar joints.

• Trecera is the coarsest sand they produce. It contains particles from approximately 10 to 30 mesh. This material works well for thick base coats and mortar joints. Because of its lack of fines content, workability would be greatly improved by adding a small percentage of Segunda and Primera to this aggregate for base coats.

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*Note the angular particle shape of the sands, this property is a basic principle for creating strong mortars and plasters. Crushed stone particles are generally quite angular.

All three of these materials are for sale at less than $3.00 US per 100 kilos.

Lime

The lime kiln and process at Palmar was as close as you can get to lime production in ancient times. Limestone is quarried from a hillside and loaded into a very basic pot type kiln approximately 18 feet deep and 12 feet across. The kiln is built into the earth and lined with stone. Limestone is loaded from above.

There is a fire box below where scrap wood is burned as fuel. When the firing is complete, the quicklime is removed from the bottom of the kiln and slaked to a dry hydrate by sprinkling it with water. It is then shoveled into 23 kilo bags for sale. It is approximately $3.00 USD per bag.

Having been produced in such a traditional way this lime had relatively low levels of plasticity and water retention. It also contained several pieces of unburned stone fragments. In order to use this lime, we had to make some adjustments and take some steps to make it workable.

At Rancho Mastatal they have been doing work with natural building since their beginning in the late 1990’s. In 2008, they bought a load of quicklime from the kilns at Pattara. They slaked it in water and left it to soak from that time on. Since that time, they had used it to create some lime plasters and lime washes that were of variable quality.

The lime putty was soaking for several years in 200-liter barrels, and it had about 8-12 inches of water on top of it. The water was covered with floating carbonated lime indicating that it was chemically reactive and therefore able to carbonate. It had a thin layer of creamy material at the top, but quickly became thick and chunky, seeming to be chunkier deeper in the barrel.

This stiffening over time may indicate that this lime may have very slightly hydraulic properties.

The water was poured off and the putty was broken into pieces. We noticed that physically beating the putty seemed to change the viscosity quickly.

The next step was to powerfully knock the putty up with a mixing drill. Surprisingly, the putty quickly changed into a milkshake like consistency.

Next, we filtered the putty through a window screen to remove the unburned stone particles.

Using Lime in the Workshop

At Rancho Mastatal there were several projects that needed lime. A tadelakt bathroom and shower, some fresh lime wash over previously lime washed walls, and some exterior lime finish plaster over wattle and daub walls. We had a workshop with 16 mostly inexperienced students to complete the work. Overall, the plasters turned out beautifully. The workability of the mixes was good and the plasters seemed to cure strong and hard. For this project, we only had access to the “Primera” sand, having not yet discovered the other two gradations.

The ability to source local lime and crushed limestone resources to create functional and beautiful plasters is something that I have done in several locations throughout the United States and Canada. This latest work with more crudely produced materials in a less developed country like Costa Rica was very exciting.

With the ubiquity of lime and limestone production throughout the world, I believe that similar results to these could be reproduced almost anywhere.   The ability to create locally sourced lime mortars and plasters could be a powerful way to reduce dependence on expensive and poorly performing cement-based materials. Adding functional beauty to a world dominated by stucco and latex paint.

Lime and crushed limestone are already being produced for other industries. They are typically readily available, cheap, and potentially produced by small scale local operations in the developing world. The development of effective and functional lime plasters and mortars could also play a key factor in the further adoption of viable and appropriate earth building technologies.

The Finished Work

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