Lime-bound mortars are vulnerable to damage caused by salt crystallisation, but this can be remedied. PhD researcher Sanne Granneman discovered crystallisation modifiers mixed in with the mortar can prevent them from breaking down. 

Salt crystallisation is a common problem in porous building materials and forms a threat to our built heritage. Lime-cement mortars are particularly vulnerable because of their low mechanical strength. Salt often enters the structure dissolved in moisture; when the moisture evaporates the salt crystallises, causing damage to these mortars. “Adding crystallisation modifiers to the mortars enables them to immediately respond to the penetrating salts,” explains Granneman. “This limits the damage.”

Her research focuses on two modifiers: the anti-caking agent sodium ferrocyanide and the disinfectant borax. Sodium ferrocyanide can ensure that sodium chloride – kitchen salt – remains in solution for longer and is transported to the evaporation surface. Borax – sodium tetraborate – ensures that more salt crystallises on the surface instead of in the material. This modifier is effective against damage by sodium sulphate – Glauber's salt. Moreover, both modifiers alter the final shape of the salt crystals.

Granneman tested her hypothesis in the laboratory. She mixed the modifiers with mortars and once the mortars were set subjected them to an accelerated salt-damage test in which salt solution was drawn into the porous material through capillary action. In this way she simulated a commonly occurring practical situation in which salt enters into mortar through rising damp. The solution and re-crystallisation cycles were then reproduced in a climate cabinet.
The test results were remarkably successful. The standard materials suffered considerable damage from the salt crystallisation, but the mortars with added modifiers remained relatively undamaged.
Ferrocyanide was shown to promote very quick efflorescence, that is to say salt crystallisation on the surface rather than in the mortar. It causes a change in appearance, but does not cause structural damage; the salt crystals can be easily brushed from the surface.

All of the samples contaminated with sodium sulphate showed some surface damage, but the damage was considerably less in the sample containing the modifier. Another positive point was that adding a modifier had no negative effect on the properties of the mortar. Conclusion: the ‘self-healing’ method is eminently suitable for reducing damage caused by salt crystallisation in lime-cement mortars.

Further practical tests are needed before mortars containing ferrocyanide or borax are available in the shops. Granneman is also keen to explore the effect in cementitious mortars. She also wants to see how quickly the moderators leach out of the mortar. In the event of rapid leaching, it might be necessary to add an encapsulating agent. 

Published: March 2019