For those not familiar, the secret ingredients are to do with the Lime Cycle [1]:

1) Quicklime/Slakedlime (Calcium Oxide, CaO)

2) Lime (Calcium Hydroxide, CaOH2)

3) Limestone (Calcium Carbonate, CaCO3)

To keep it simple, typically you start with Quicklime (CaO) and after construction you end up with some mix of all three and after hundreds of years, the masonry transitions to mostly Limestone with microscopic traces of the other two. The slow transition of the lime cycle upon exposure of the masonry to both air and water (rain) ends up making the structure "self healing" and "stronger over time".

Fun fact! Lime putty is anti-mold even in humid conditions because upon exposure to moisture, CaOH2 + H20 becomes too basic for mold to grow on.

Cement/Concrete (based on Portland cement) is water proof but Lime by itself is not. But Roman Concrete, made from Lime and mixing with ash or broken pottery or ceramics makes it water proof [2]. I beleive Roman concrete was used whenever contact with water was expected. Both concrete and Roman concrete have the same underlying chemistry (Pozzolanic reaction) to make them water proof [3].

The fascinating thing is that Lime is everywhere in ancient masonry. Lime is more breathable, workable and sustainable. The only thing is, it requires maintainance, which is why Portland cement has taken over the world.

Modern cement/concrete is amazing for large structures. I hope Lime makes a comeback soon at least partly as putty and for building smaller homes.

[1] https://en.wikipedia.org/wiki/Lime_(material)

[2] https://en.wikipedia.org/wiki/Roman_concrete

[3] https://en.wikipedia.org/wiki/Pozzolanic_activity#Reaction

>Fun fact! Lime putty is anti-mold even in humid conditions because upon exposure to moisture, CaOH2 + H20 becomes too basic for mold to grow on.

Can we use this instead of grout in the bathroom somehow?

I forgot another important reason why Portland cement is preferred over lime these days. Lime takes a while to set (a few days?), cement sets faster.

>Cement/Concrete (based on Portland cement) is water proof

That's patently untrue. Otherwise we wouldn't need concrete sealants.