How Lime Mortars Help Reduce Your Home's CO2 Emissions

The cement industry is reportedly responsible for around 8 percent of all carbon dioxide emissions globally. If this industry were a country, it would be the third-largest emitter of greenhouse gasses, behind only China and the United States. Why is this the case? The production of Portland cement relies on making “clinker” wherein ground limestone and clay are super-heated in rotating kilns at temperatures of at least 1,400 degrees Celsius, which is the primary source of carbon emissions from the industry. Every year, enough concrete is produced and poured to cover the entire surface of a country the size of England, thus affecting biodiversity and increasing the harmful environmental effects associated with stormwater runoff.

While sustainable-minded homeowners can opt for more environmentally friendly options for their floors and exterior cladding options, the vast majority of U.S. homes rely on concrete footers for the basis of their home foundation—for a good reason. Concrete foundations offer strength, durability, and protection from moisture. With the average weight of a house at 50 tons, a strong foundation is undoubtedly essential and is one reason why around 81 percent of all U.S. homes rely on poured concrete foundations.

However, Lime is a versatile material in constructing buildings that can be used as mortar in masonry systems and home foundations, offering structural strength and environmental benefits. Below, we look at why opting for lime-based mortars as an alternative to cement and concrete for our home foundations can reduce the carbon footprint of our homes.

The Lime Cycle 

Limestone is one of the most abundant materials on Earth. In the United States, crushed limestone made up 68 percent of all crushed rock produced in the United States in 2007. While limestone is one of the key ingredients in the production of Portland cement, its use predates the cement industry by thousands of years. Architects during the Roman Empire used lime mortars mixed with volcanic rock to build impressive structures (such as the Colosseum) that continue to stand today.

Hydrated lime, or the quick lime used for construction purposes, is formed by burning sources of calcium carbonate (CaCO3), also known as limestone (or occasionally magnesian limestone). The burning process requires high temperatures between 850°C and 1200°C, which forces carbon dioxide out of the limestone (and into the atmosphere) to create calcium oxide (CaO, or quicklime). The calcium oxide is then slaked with water to produce calcium hydroxide eventually (Ca(OH)2) used for construction purposes. While lime also requires burning in ovens at high temperatures, lime requires less energy to produce than cement due to lower temperature requirements.

While building lime does release carbon dioxide into the air during the burning process, lime products can absorb the carbon dioxide in the atmosphere through a process known as the lime cycle. The lime cycle begins when limestone is burned and releases CO2 into the atmosphere. The slaking process eventually creates calcium di-hydroxide, which is sold as a bagged lime powder. This can be mixed with sand or other coarse aggregate and water and used as a more natural mortar (and plaster) alternative. As the mortar hardens, it reabsorbs the carbon dioxide in the atmosphere and eventually re-hardens into limestone. Through this cycle, lime is widely considered a carbon-neutral building material, as it reabsorbs the carbon dioxide released during the manufacturing process.

Lime is a much more environmentally friendly building material because it is biodegradable and can be removed, re-wet, and re-mixed. On the other hand, concrete is rarely recycled and ends up choking landfill sites around the country. Due to its relative flexibility compared to concrete, Lime can also accommodate structural movement as homes settle, thus avoiding structural foundations cracking that can lead to condensation and moisture issues.

Using lime for plasters and mortars can also help to improve the indoor air quality inside our homes. Unlike cement, lime can absorb and release moisture, thus helping to prevent condensation, the leading cause of molds and mildews affecting households around the country. Also, foundations that use lime mortars will not need expansion joints, whose sealants often deteriorate and lead to moisture problems. Lime is highly alkaline and, therefore, naturally antibacterial, antifungal, and antiseptic.

How Do You Build a Rubble Trench and Rock Stem Wall with Lime Mortar?

While most construction teams will have lots of experience with cement footers and concrete foundations, building with lime mortars is much less practiced. Lime mortars are best suited for foundations constructed from solid rock. Instead of digging a trench down to your frost line and filling it with solid concrete, some alternative builders return to the rubble trench, an alternative type of foundation used widely by the famous architect Frank Lloyd Wright. The rubble trench is simply a lined trench dug to below the frost line filled with gravel. This type of footer offers a structurally sound foundation for your home while also helping to improve drainage underneath your house. The trench is usually dug off contour so that excess water can move through the gravel and away from the walls of your home.

On top of a rubble trench, a solid rock stem wall works exceptionally well with lime mortars. It is a much more durable, natural, and aesthetically pleasing option for a home foundation. According to the American Society of Home Inspectors article, “lime mortar sets in several hours, but the final hardening is a prolonged process as the lime re-forms into calcium carbonate. This can take months or years to achieve maximum strength. The result is a mortar that expands slightly as it sets (making shrinkage cracks unlikely) and relatively stable through temperature changes. Lime mortar is also porous enough to allow water vapor to migrate out of the masonry and water-soluble enough to enable it to redeposit itself and ‘heal’ hairline cracks that appear due to settling or other stresses.”

The rock or stone used in this type of natural foundation can be held together with an essential lime mortar made from lime putty (two parts hydrated lime and one part water), four parts coarse sand, ¼ part grog (ground up brick), and water.

An essential element of sustainable building practices is rediscovering the wisdom that was found in vernacular building techniques. While smart home technologies, more energy-efficient home appliances, and modern-day building techniques can improve the energy efficiency and overall sustainability of a house, older procedures and practices that have been used for centuries also have a place in more sustainable homes. For example, replacing our dependence on cement with lime-based mortars is one way to reduce the carbon footprint of our homes.