Hydrated Lime in render?

[-= debully =-]

Can anyone give a precise reason why Hydrated Lime is used in exterior rendering?

Been told to use a 4sand / 1 cement / 1 HL mix, but never come accross HL before.

Rgrds,

debully

[The Natural Philosopher]

At a guess, its to make the mixture less brittle and less prone to cracking.

I did a lot of bricklaying with a 1:1:6 white cement/hydrated lime/sand mix. Nice to work with too.

[N. Thornton]

When cracks form in render the lime reacts with CO2 in the air to grow a new crystal across the crack, thus rebonding it. Consequently suitable mixes with lime are much more crack resistant than cement alone, which has no such mechanism.

HL is sold in bags as 'lime' at any BM.

Regards, NT

[Michael Mcneil]

Lime is calcium oxide and hydrated lime is calcium hydroxide. If you could get hold of lime it would heat up when used. I think that is why in the good old days the stuff was mixed days in advance -no cement being used in the mix. But that was even before my time.

[Anna Kettle]

In a cement render, hydrated lime is there as a plasticiser.

You are talking here about lime mortar where the lime itself sets. A lime mortar contains lime and sand and NO cement. As soon as cement is added, its gets in the way of the lime set so although what you say is true, it is not relevant in this case.

Anna

~~ Anna Kettle, Suffolk, England |""""| ~ Plaster conservation and lime plaster repair / ^^ \ // Freehand modelling in lime: overmantels, pargeting etc |____|

01359 230642