Posts Tagged quicklime

The Lime Revival

J. Speweik during on-site masonry inspection

As a young boy growing up in a family of stone masons mixing mortar was like brushing my teeth…I did it every day, at least during the summer months when school was out. Who would have thought that in the age of technology, speed and convenience that my great great grandfather’s 1846 mortar formulation would return. The trend seems to be one that is sweeping across Scandinavia, Europe and Canada as architects and heritage masons work together to preserve their country’s historic masonry properties built hundreds and often thousands of years ago.  They call it the “Lime Revival” It’s been 30 years for Sweden, 20 years for England, 10 years for Canada….its America’s turn now.

The oldest archaeological sites in the world are, of course, masonry. As early as 2450 B.C., masons began using lime and sand for mortar. Lime is made from limestone (calcium carbonate) which has been heated to temperatures exceeding 1,650F where the heat drives off the carbon dioxide and water turning the limestone into quicklime (calcium oxide). Traditionally this quicklime (sometimes called lump lime or hot lime) was delivered fresh to construction sites or made on-site in a temporary kiln just for the job. The quicklime was mixed with damp sand and stacked up into piles for slaking into a hydrate powder (calcium hydroxide) and run through a screen or the quicklime was combined with water in the ground, formed into a putty (also calcium hydroxide), and mixed with the sand at a later time depending on the project needs. Either way, the mixtures were left to mature or rest for a time before use, due to the expansion of the lime particles during slaking.

The lime was generally mixed with local sand in a ratio of 1 part lime putty to 3 parts sand by volume. Other ingredients like crushed brick, clay, lamp black, and natural cement were sometimes found in smaller quantities before 1870; however, the basic lime putty/quicklime sand mortar formulation has remained unchanged for centuries.

Portland cement was first manufactured in America in 1871, but did not become truly widespread until the 20th century. As late as 1883 there were only three portland manufacturing plants in the U.S. Up until the turn of the last century portland cement was considered an additive, or “minor ingredient” to help accelerate mortar set time. By the 1930s, most masons were using equal parts of portland cement and lime putty or quicklime. Thus, masonry structures built between 1871 and 1930 might be pure lime and sand mixes or a wide range of lime and portland combinations.

What we do know about lime, and the reason for its come-back, is its incredible performance characteristics, and versatility as a time-tested building material – and not just as a masonry mortar either, but also as paint, (limewash/whitewash) exterior stucco/render, and interior plaster as well. Lime, when properly combined with clean, sharp, well graded sand can perform for many centuries in masonry applications. Lime has the ability to handle water without trapping it within a wall structure. It is breathable, flexible, obtains high bond strength to masonry units, it is truly sustainable (less energy is required to heat a ton of lime as compared to a ton of cement) and it has autogenious healing capabilities, often referred to as “self-healing” where hairline cracks do develop over time water combines with the lime again to re-knit the cracks back together. Limes durability comes through a process of what’s called carbonation. Carbonation is a process by which lime turns back to limestone by reabsorbing the CO2 back from the atmosphere though wetting and drying cycles. You can say that the material interacts with nature on a daily basis.

Portland cement mortar "Cover-up"

As portland cement became more widely used many lime sand mortars were being “covered-up” during repair projects. Exterior masonry buildings suffered badly from hard portland cement mortars (1940s until today) which did not accommodate for movement or stresses within the wall systems, and as a result, many historic brick and stone units got damaged by this un-sacrificial material. When cracks did occur, in the portland cement mortars, water would migrate into the wall cavities and not be able to escape or evaporate back out as they once had done with the lime sand combination mortars.

But times are changing. We are seeing signs of the “Lime Revival” hitting the shores of the United States. Mortar manufacturing companies are now offering lime mixes now for restoration and a few specialty companies offer traditional lime putty, quicklime and imported hydraulic lime for sale.

Lime mortar materials, that I am currently aware of, are available from the following U.S. companies listed in alphabetical order for your convenience.  Be sure to ask questions about each of the company’s offerings, as they differ. Some still use portland cement in their lime mortars. It’s best to know what you need first – then go out and find a supplier that can meet that need.

Cathedral Stone Products

Edison Coatings

LimeWorks.us

SpecMix

Transmineral

U.S. Heritage Group

Virginia Lime Works

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Look, I See Lime Inclusions

The next time you come across a historic masonry building take a close look at the surface of the mortar joints. Yes, I know they often get over-looked in competition with the brick or stone, but trust me on this one.  The first thing you should notice is the sand. The sand is the largest part of the mortar by volume and is the material that gives the joint its color, texture and cohesiveness. The next thing you should notice is white specs or small chunks of carbonated lime putty. If this evidence is identified you’ve got yourself a truly historic lime putty mortar. No need to hire a fancy consultant or pay for an expensive mortar test, you can with confidence declare your finding.

Mortars that display lime inclusions were typically mixed using quicklime and sand mixed on the jobsite with a shovel or mixing hoe by hand and with a lot of hard work I might add. Often, the moisture would be added to the sand first then the quicklime added to the damp material. The quicklime would slake first into a hydrate of lime then into putty if more water was added to the mixture.

The batch of mortar would be tossed and turned until the masons yelled out “MUD!” then the material would find its way onto the laborers back then unloaded onto the boards. The mortar would be placed in the wall as construction proceeded. Mortar consistency might certainly vary from batch to batch with this serve as you go system in place. There might be a time when a laborer catches up with the demand for mortar and has more time to mix a particular batch-thus breaking up the lime inclusions into smaller pieces and even dissolving them altogether.

If it is your desire to match these inclusions you have a couple of options. Use a mortar mixture made from damp sand and quicklime (hot lime mortar mix- allow 24 hrs before use), or make lime inclusions from straight lime putty by allowing the material to air dry then running the harden pieces through a series of aggregate sieves to match the inclusion size you specify. Then simply add the inclusions to your lime putty mortar just before application taking care not to over mix. The inclusions in the image above were added to the masons hawk just before installation and were protected from the initial mixing of the lime putty and sand to keep them from breaking apart.

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