The lime cycle and sustainability
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Limestone Extraction: The cycle begins with the extraction of limestone (calcium carbonate, CaCO₃), a naturally occurring mineral.
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Calcination: The extracted limestone is then heated in a lime kiln at temperatures of 900-1200°C. This process, known as calcination, drives off carbon dioxide (CO₂) from the limestone, resulting in the formation of quicklime or calcium oxide (CaO).
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Slaking: Quicklime is then mixed with water, a process called slaking, which results in a chemical reaction that produces slaked lime or calcium hydroxide (Ca(OH)₂). This can be in the form of a dry hydrate powder or as a putty, depending on the amount of water used.
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Aging: The slaked lime is then left to age. During this period, it undergoes further reactions and slowly matures, improving its plasticity and workability.
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Carbonation: When the slaked lime is applied in construction (like in plaster or mortar), it eventually absorbs carbon dioxide from the air and gradually converts back to calcium carbonate. This process, known as carbonation, hardens the lime and helps cement the material in place.
This cycle demonstrates the eco-friendly nature of lime, as it essentially reverts to its original mineral form, with carbon dioxide being both released and absorbed during the process.
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Carbon Dioxide Absorption: One of lime's most significant sustainable features is its ability to absorb CO2 during the curing process, partially offsetting the emissions from its production. Over time, lime mortar reabsorbs almost the same amount of CO2 released during calcination, contributing to a lower overall carbon footprint.
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Energy Efficiency in Production: Lime production requires temperatures around 900°C, which is significantly lower than the 1,450°C needed for cement clinker. This results in lower fossil fuel consumption and reduced greenhouse gas emissions.
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Material Longevity and Recyclability: Historic buildings have stood the test of time with lime mortar, proving its durability. Additionally, materials from buildings built with lime and earth-lime mortars can be reused easily as the mortars are soft enough so as not to damage them. Demolished historic buildings such as barn frequently have their timbers and brick/stone turn up at reclamation yards.
