A practical introduction to FETLA hydraulic lime — what it is, why it behaves differently from cement, and when it should (and shouldn't) be used in heritage masonry repair.
FETLA — a naturally hydraulic lime — has been the backbone of NDVPL's conservation mortar specifications for over a decade. Unlike Portland cement, which forms a rigid, largely impermeable matrix, hydraulic lime cures more slowly and remains permeable, allowing moisture trapped in old masonry to escape rather than accumulate.
This distinction matters enormously for heritage structures. Many of the failures we are called in to investigate trace back to a well-intentioned but incompatible cement repair carried out decades earlier — repairs that trap moisture against the original stone or brick and accelerate decay rather than preventing it.
Choosing the right mix ratio depends on the substrate: a soft laterite block needs a weaker, more flexible mortar than a dense granite block, for instance. We typically test multiple trial mixes on-site before committing to a specification, comparing colour match, working time, and early-age strength under local humidity conditions.
The result, done correctly, is a repair that is visually close to invisible and structurally compatible for the long term — which is the whole point of conservation-grade material selection.
