….. and why lime fell out of favour.
Portland cement
- Made by burning chalk and clay in a series of firings – some types with other additives.
- Burnt at a minimum temperature of 1200°C.
- Invented during experiments to produce extremely eminently hydraulic lime.
- Patented in 1827, but really began to take off towards the end of the 19th century.
- In the 1930s, rotary shaft kilns were developed, with a resulting explosion in the manufacture of cement.
- Called Portland cement because it looks like Portland stone.
- It has great strength, which is sometimes very useful; however, great strentgth is not absolutely necessary on relatively small domestic buildings, and it doesn’t have the advantages of lime mentioned above.
- Stopped being used in most historical buildings after the 70s, when damage that it can cause was realised.
- Sometimes mixed with lime mortars ‘just in case’. This is a bad idea, as you won’t get the strength of cement, but you lose the breathability and flexibility of the lime – so you get the worst of both worlds. Adding cement can also stop the lime setting properly, by preventing it from getting CO2 from the air, and so there will be a loss of strength that the addition of a little cement won’t make up for. Don’t do it.
Gypsum
- The basis of modern plasters.
- Mineral consisting of hydrated calcium sulphate.
- Occurs in sedimentary rocks and clays.
- Formula = CaSO4.2H2O
- Gypsum doesn’t breathe and it’s not flexible, so it doesn’t have the benefits of lime for your building or your health. Neither does it take in CO2 to set, so it doesn’t have the environmental benefits of lime either.
- Gypsum should never be used externally, by the way, as rainwater will dissolve it.
Why did lime fall out of favour?
- Doesn’t suit the modern economy.
- Suits self-build, natural materials, low-rise & traditional building methods.
- Doesn’t suit modern, fast, high-rise buildings / building methods (up fast but also down fast).
- Long drying time / setting time – can’t build too high in one go.
- More labour required, and as labour costs are high in the West, anything that reduces labour will succeed.
- Can’t use in winter because of frost.