The chemistry of lime render

Lime render is made from lime putty and sand.  The render mix we make is three parts sand to one part lime putty with water added, as required, to make the correct consistency.

For the first two coats of render we used washed sand.  For the final coat, we will use fine washed sand.  Ideally, with lime render, you want the strongest coat as the first coat and for subsequent coats to be weaker.  To achieve this, our first coat outside was made with one part red brick sand, two parts washed sand and one part lime putty.  The only problem with this is that the brick sand has a clay component to it which shrinks and swells.  As such, the first coat outside cracked quite considerably.  We've learned over time that it's best to keep water up for at least three days after putting a coat of render on.  This means that the render dries slowly and is less likely to crack.

You can purchase lime putty but it's costly so we make our own.  We purchased two second-hand food grade 220L plastic containers that we used to make lime putty in. We make our lime putty in a ratio of roughly one part hydrated lime to one part water (by weight).  You can get hydrated lime from hardware shops as it is used in making concrete.  Making the lime putty requires a great deal of mixing.  Ineffective mixing leads to hard lumps of lime in the render.

Hydrated lime (calcium hydroxide) is made from limestone.  The limestone (CaCO3) is mined then baked at high temperatures (about 800 degrees Celcuis) to release carbon dioxide (CO2) and leave behind calcium oxide (CaO).  The calcium oxide is then slaked (by adding enough water until the exothermic reaction stops) to produce hydrated lime (Ca(OH)2).  We then take the hydrated lime and add more water to make lime putty.  

The lime render applied to the walls and then reacts with the carbon dioxide in the air and reverts back to calcium carbonate. So essentially our straw bales will be covered in rock!

As the render dries it gets whiter