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/diy/ - Do It Yourself

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Lye cement is considered superior for some applications, its more malleable, resisting earthquakes/movements not cracking and self repairs. It also breathable and doesn't wick up water and rot wood.

Modern cement (Portland cement) is much stronger, but it cracks under movements, does not self repair, is not breathable, and holds water causing wood rot.

Lye cement consists of lye and an aggregate (sand, clay, etc). Portland cement is composed of mostly lye and an aggregate and a few additives like aluminum oxide.

Why does Portland cement behave like this if its almost identical to lye cement?
>Portland cement is composed of mostly lye

I think you're mixing up the words 'lye' and 'lime'.... Portland cement is made mostly of calcium oxide AKA calcined lime or limestone.
The word 'lye' generally refers to either sodium hydroxide or potassium hydroxide if I'm not mistaken.
Its different substance hence the different behavior.
But both are caustic so wear gloves
So there's a cycle to lime. Baking limestone releases CO2 (a gas) to make quicklime, and adding water creates slaked lime, but there needs to be a carbon donor to turn slaked lime back into limestone. That's where Portland cement comes in; it has additives to aide in converting the hydroxides in slaked lime back into carbonates and water (water being able to release from a casting).
Portland cement should never be used for restoration and repointing old buildings. It is simply too hard and does not expand and contract at the same rate old building materials do. Old hand made bricks will break themselves on Portland cement as they expand and construct with the weather.

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