Modern and greener buildings built with lime binders and lime based paints, sealants and coatings
Lime has been the main binder used in construction during the last ten thousand years.
It’s been used since at least 7500 BC in Jordan and can also be found in the walls of early Egyptian tombs. Builders in the Roman Empire used it in many structures, although it wasn’t until the Renaissance that marble dust was first added to lime plaster, allowing for the hard, smooth finish forever associated with Venetian palazzos: Venetian Marmorino.
It is seems magical to see a rock that can produce a paste to be used as a binder for other stones. Over time, sometimes over hundreds of years, this paste becomes chemically the same as the stone it came from. Only now it no longer has the same form, but is everywhere, in the walls or floors as a binder for the various aggregates.
But how does all this happen? Not everybody who has used lime has asked themselves about this, yet it is chemically a very simple process that has been repeated every day for almost ten thousand years.
The process has 4 phases:
- You select a limestone that should be quite pure, and especially should have the least amount of clay possible (less than 5%). A pure carbonate is CaCO3
- The stone is baked at a temperature of close to 900° C (1650° F). This causes one carbon atom and two oxygen atoms, which, in the form of a CO2 molecule is separated and released into the air. What remains is calcium oxide, CaO: CaCO3> CaO + CO2. CaO looks like a white stone in the same shape as the stone that was baked, but weighs about half as much.
- "Slaking", the third step, is what happens when calcium oxide, unstable at room temperature, has water added to it, transforming it into calcium hydroxide. With a little water it becomes a white powder, with a lot of water, it becomes a paste. CaO + H2O -> Ca (OH)2
Slaked lime, in powder or paste, combined with dust and marble chips or quartz is the material that we use every day.
- "Carbonation" is what occurs when the lime, in the form of mortar or grout loses its water content and absorbs carbon dioxide. The loss of water is a relatively quick process, but which is only complete when carbon dioxide (CO2) is absorbed. This occurs in a few hours on the surface, but takes up to dozens of years in the depth of a big wall. Completing the "Cycle of Lime". Stone back to stone.
This is an interesting process because it occurs between a solid Ca(OH)2 and a gas (CO2): Ca(OH)2 + CO2 -> CaCO3 + H2O. The gas makes the solid stronger.
When we combine lime with stone aggregate (marble), we get a very solid, compact structure, that is, something which has the shape of a thin plaster or a big wall, but is chemically just a big mass of stone.
The lime cycle shows us that building with lime does not consume huge reserves of limestone, nor produce a significant amount of carbon dioxide, but causes it all to return in a slow, almost magical process