It’s often said that lime mortar and render (sand and either hydraulic lime or lime putty) is lower carbon than the same made with cement, but is it really? Lime manufacture produces less carbon emissions per kilogram of material than cement, and lime has a lower density than cement so the same volume will weigh less. On the other hand a greater volume of lime is used in a mortar mix.
To find out what’s really happening I calculated the embodied carbon (EC) per cubic metre of a full usable mix of each (the amount of carbon dioxide released in production of the materials) and the answer is a resounding “yes!” to lime mortars and renders being responsible for less carbon emissions. As the table below shows, there are 24.9 kgs less CO2 released per cubic metre of lime mortar/render than per cubic metre of cement mortar/render.
Lime render also has added benefits to the health of a building: being vapour-open it allows moisture to escape from the structure of a building, whilst still keeping the rain out (the analogy often used is of a breathable water-proof jacket).
The amount of carbon released in the manufacture of building materials is an important factor in deciding their suitability for sustainable building. If you want to compare the CO2 emissions of different types of construction I can calculate this for you, and advise on related issues such as resource-depletion and the potential toxicity or otherwise of production and use of different materials.
EC and density data from the Inventory of Carbon and Energy.