Portland cement is often used to "fix" old stone walls because it's cheap and familiar — but in historic masonry, it usually accelerates the very decay it was meant to stop.
It seems intuitive: a wall is failing, so repair it with the strongest, most modern material available. For historic stone and laterite masonry, this intuition is almost always wrong, and Portland cement is the most common culprit we see in failed repairs.
Old stone, especially the soft laterite and sandstone common across Kerala, is weaker and more porous than cement. When cement mortar is used to repoint or patch such stone, it forms a hard, impermeable skin. Moisture that would once have evaporated freely through the original lime joint instead gets pushed into the stone itself, carrying dissolved salts with it.
As that moisture evaporates from the stone's surface rather than the joint, it leaves salt crystals behind — a process that physically pushes the stone apart from within. The visible result is spalling: the stone face cracking and flaking away, often years after the "repair" was done, by which point the damage is well underway and harder to reverse.
The fix is not a more aggressive repair — it is a softer, more compatible one. We specify lime-based mortars matched in strength and porosity to the original stone, so moisture continues to move and evaporate the way the building was designed to handle it, not against it.
